+
Finding
The Miss!
+
Ever have a misfire you could feel but there
was no related trouble code set? How about
a cy lind er-specifi c code that proved to be
wrong? These are just a few of the more
troublesom e misfire com plaints professional
techs run into every day!
And that's the topic of this Motor Age/TST
webinar. TIps and tec hniques for locating
the offending cylinder(s) and ultimately - the
sourc e of the mis fire.
Finding The Miss! Misfire codes (P0300 – P0312) are common causes of “Check Engine” light complaints, and their proper diagnosis and repair has been the topic of countless magazine articles and training seminars. So why another one? A few reasons. One – all too often, technicians attempt to correct engine misfires by “shotgunning” parts at the problem until it, or the customer, goes away. Take a look at Internet postings from people looking for the “silver bullet” fix and you’ll see that (more often than not) the first parts to be replaced are ignition components – plugs, coils, plug wires, etc. Typically, the first parts swap doesn’t solve the problem and the customer is forced to come back a second time. It is our hope that the material presented here will encourage technicians to apply a logical troubleshooting process that will help them identify the actual cause and apply the correct repair the first time out. Two -­‐ the severity of the misfire can range from a completely dead cylinder (one that is not contributing anything to the engine’s power output) to that random intermittent that only rears its ugly head under specific engine operating conditions. Sometimes, there will be a misfire that a technician can feel but no related Diagnostic Trouble Code (DTC) is recorded. This makes the diagnosis of the cause more intimidating, but not impossible. In this seminar, we’ll help you understand what a misfire is, how it is detected by the Engine Control Module (ECM) and what the criteria is for the ECM to actually turn on the Malfunction Indicator Lamp (MIL). Three – some seek the latest and greatest tools and techniques to solve misfire complaints, investing hundreds of dollars in equipment they still can’t use effectively because they lack the basic understanding of how computerized engine management works. Most misfire complaints can be solved using the information any good Global OBD II scan tool provides if you know where to look. We’ll discuss briefly what OBD II monitors are and show you how the OBD II modes you need to know. We’ll also give you some background on more advanced techniques and tooling that incorporate the automotive oscilloscope. Let’s get started! OBDII Basics The ECM (Engine Control Module) on an OBDII compliant vehicle (MY 1995 and on) has one primary function…to keep vehicle emissions in line. It does so by controlling and monitoring any system that will have an impact on emissions. Every powertrain related DTC (Diagnostic Trouble Code) is related to emissions in some way. OBDII also corrected some of the problems posed by OBDI. Code format was standardized, and generic code definitions established. The DLC (Data Link Connector) was also standardized. The ECM’s testing ability was enhanced from just basic circuit integrity tests, adding functional and rationality testing of emissions related systems and components. Diagnostic information was made more standard, with common PIDs (Parameter Identification) among manufacturers instead of a smorgasbord of terms for us to learn. Other modes were added to aid technicians not privy to OE level information or scan tools in diagnosing faults discovered by the ECM. Due to the efforts of organizations like the National Automotive Service Task Force, OE information once considered proprietary is now available to anyone. This was not the case at the time OBDII was first implemented. All in all, there are 10 modes available when using the Global OBDII (not to be confused with Generic OBDII) function of your aftermarket scan tool. The majority of “Check Engine” light complaints you solve on a daily basis can be diagnosed and repaired using just the information these 10 modes can provide. Let’s take a look at them… NOTE: You may see the proper designation for OBD II modes on your scan tool – Mode $0x. The “$” sign simply means that the number is a hexadecimal, and it has been removed here for simplicity. Mode 1: Monitor Status and Current Data Mode 1 is where you will find the status of the onboard monitors. Each monitor is associated with a system or component on the car that can impact the emissions it produces. Monitors are classified as either “continuous” or “non-­‐continuous”. The continuous monitors are: the Comprehensive Component monitor, the Misfire monitor, and the Fuel monitor. All the others are non-­‐continuous. Each monitor contains a test, or series of tests, that the ECM uses to gauge the operational health of that system or component. The monitor status screen simply tells us whether or not all the tests contained within that monitor have run to completion. When checked, they should all read “Ready” or “Complete”. If the vehicle you are testing does not use a given monitor, then the message will read “Not Supported” or “Not Available”. Any monitor that reads “Not Ready” or “Not Complete” indicates a few things. Either the codes have been cleared with a scan tool causing the monitors to reset OR battery power has been lost to the ECM. This can be a diagnostic aid. If power was lost due to a wiring fault or ailing battery, the ECM is constantly “rebooting” and this can cause some drivability issues. In addition to the status of the monitors, Mode 1 is home to “Current Data”, also called “Live Data”. This is the screen showing the PID information “live”. Unlike some OE enhanced PIDS, however, Global OBDII PIDs are ACTUAL data. A good example is the ECT (Engine Coolant Temperature) PID. Since this PID is related to fuel control, some OEM’s will substitute a data value if the true ECT reading is suspect, and that is the number you will see in enhanced mode. In Global mode, though, you will see the true ECT reading, whatever it may be. Mode 2: Freeze Frame Freeze frame is a record of the available data PIDs stored by the ECM at the same time it matures a DTC. This can be important information for diagnosis. It helps you recreate the conditions present at the time the trouble code was matured. This is especially valuable when diagnosing codes related to the continuous monitors, since they can be recorded at any load/rpm combination. Freeze frame stored for codes related to the non-­‐continuous monitors is usually just a reflection of the conditions required by the ECM to run the applicable test. Mode 3: Stored Diagnostic Trouble Codes Mode 3 is where you will find a list of any Diagnostic Trouble Code that has matured, and caused the ECM to command the MIL (Malfunction Indicator Lamp) on. These can be the first fault of a “1-­‐trip” code or the second consecutive fault of a “2-­‐trip” code. Even if a code has matured, the ECM keeps testing. If the test(s) that once failed now passes for 3 consecutive tries, the ECM will turn off the MIL. The code, however, will continue to be stored in Mode 3. After a specified number of warm-­‐up cycles with no reoccurrence (typically 40), the code will be erased from memory. This often comes into play when a customer sets an appointment for a Check Engine diagnosis, but by the time he arrives, the MIL is off. Look anyway…the culprit may still be in there somewhere. Mode 4: Clear Codes Once you have completed the repairs, and are ready to verify that repair, use Mode 4 to clear the code(s) from memory. But don’t do it until you are ready to verify the fix…clearing the codes will also reset the monitors and erase any existing test results they contain. Mode 5: Oxygen Sensor Monitor Test Results Mode 5 contains the tests and their results used by the ECM to verify the proper operation of the oxygen sensors. This can help when faced with certain diagnostic challenges, like chasing down the cause of a catalytic converter efficiency code. Since these tests usually use information from the oxygen sensors, a faulty sensor will affect the results and may be the root cause of the DTC. Checking Mode 5 test results, if available, can help verify the health of the oxygen sensors. If Mode 5 isn’t available for the vehicle you are testing, the answer will lie in the next mode on the list. Mode 6: Non-­‐continuous Monitor Test Results Mode 6 is unique in that it lists the individual tests and their results for every non-­‐
continuous monitor. In the past, this information was hidden…that is, the data displayed had to be translated and converted before a tech had real information he could use. Today, though, most aftermarket service information systems list the test identifications and descriptions, making Mode 6 friendlier…and more valuable. Early Ford Mode 6 also included misfire monitor test results, even though this monitor is classified as continuous. All vehicles using CAN (Controller Area Network) protocol have the misfire monitor results as well. In addition, CAN vehicles have improved Mode 6…with data screens already translated and converted to real data. Mode 7: Continuous Monitor Test Results Many aftermarket scan tools list Mode 7 as “Pending Codes”. Here is where you will find record of any 2-­‐trip code related to the continuous monitors that the ECM has failed once already. Mode 7 can be used to test your repairs of these codes by clearing the ECM, test driving the car under the same conditions recorded in the original Freeze Frame, and checking to see if the code reappears here. Some later year OBDII vehicles, and all CAN vehicles, will also record the first fault of non-­‐continuous monitor related codes in Mode 7. Mode 8: Request Control Of Onboard Systems Currently, only the EVAP system is available here and then, only on some vehicles. Mode 8, if available, will seal the EVAP system by closing the canister vent valve, allowing you to then vacuum or pressure test the system for leaks. Mode 9: Vehicle Information Mode 9 contains the VIN (Vehicle Identification Number), and the ECM’s calibration information. Since many drivability issues are related to programming, this information can help you determine whether or not the module requires a reflash. And I have been burned once by missing a mismatched VIN, due to a junkyard ECM being installed in the car! Most aftermarket scan tools require all sorts of information be input before being able to connect in enhanced modes. Global OBDII hook-­‐up is generally faster, with no such need. Now that you know all 9 modes, and what they have to offer, you’ll be able to diagnose even more powertrain related codes using it. Mode 10 ($0A): Permanent Codes Mode 10 was added to the Global OBD II modes to thwart those who would bypass I/M testing requirements by turning off the Malfunction Indicator Lamp just before sending their vehicle in form inspection. Codes stored in this mode cannot be cleared using Mode 4. These codes will only clear after the associated monitor has run to completion and the Engine Control Module determines that the fault that originally caused the DTC to be set has been corrected. Implementation of Mode 10 began in 2010 and has been required on all vehicles governed by OBD II standards since 2012. How The ECM Thinks Let’s start first with understanding the role of the ECM (Engine Control Module). The ECM has one primary function. It is responsible for managing the various vehicle systems related to emissions performance. In keeping with this responsibility, the ECM performs tests on these systems to insure their proper performance. If a problem is found, the ECM can then warn the driver by illuminating the MIL (Malfunction Indicator Lamp). The result is a powertrain diagnostic trouble code, or “P” code. These tests fall into three basic categories: electrical tests, rationality tests, and functional tests. Electrical tests are tests for open circuits, and short circuits to power or ground. Rationality tests are tests that compare ECM inputs and to see if the inputs make sense. An example would be a comparison of MAF (Mass Airflow sensor) to the TPS (Throttle Position sensor). If the MAF is reporting an airflow rate that the ECM knows is contrary to how much air should be flowing at the reported throttle opening, it will assume that one of the two is reporting incorrectly. Functional tests are performance tests of entire systems or individual components. These tests can be done passively or intrusively by the ECM. A passive test is one where the ECM simply watches while the system or component functions normally, looking for verification of that function from another input. An intrusive test is one where the ECM actually changes the normal state of a system or component and verifies that change with an independent input. Now, with all that being said, what is a monitor? A monitor is a grouping of these tests into an individual system function. Any particular monitor can be comprised of just one test, or of several tests. The tests themselves often require that certain conditions be present before the tests will be performed, or “run”. These conditions are called “enabling criteria”. Enabling criteria involves engine/vehicle operation as well as the health of other systems the ECM may need to perform the test. Vehicle/engine operating conditions can include engine temperature, time since start-­‐up, vehicle speed, and whether or not the vehicle is accelerating or decelerating, just to name a few. These operating conditions are called “drive cycles”, or “trips” and are unique to each monitor. Often, a general “drive cycle” is described, designed to meet the needs of all the vehicle’s individual monitors when followed. This allows you to perform a specific test drive that will allow the ECM to recheck any repairs that you’ve made. Because some tests rely on information from a component that may be related to a separate monitor, the ECM will suspend testing if a fault is first found in that component. When reviewing the individual code enabling criteria in your service information system, you will often find a listing of DTCs (Diagnostic Trouble Codes) that will cause this suspension. One example is the Catalytic Converter Monitor. Most of these monitors rely on input from the oxygen sensors, and if a DTC is already recorded for a problem with one of the sensors, the catalytic converter monitor will be suspended until this fault is repaired. Being suspicious of what the affected oxygen sensor is telling it, the ECM can’t correctly judge the health of the converter. Once the enabling criteria of an individual monitor are met, the ECM will perform the test(s) associated with that monitor. If a fault is found, and it is one that will have an immediate impact on emissions, the ECM will set the DTC, turn on the MIL and record the operating conditions present at the time of failure in Freeze Frame. These are the “1-­‐trip” codes. If the fault is not deemed as an immediate threat, the ECM will record the DTC as “pending” and wait to see if the same fault occurs on the next consecutive test before maturing the code and illuminating the MIL. These are the “2-­‐trip” codes. Codes related to the Misfire monitor and Fuel monitor have to repeat under the same load/rpm conditions as the first failure in order to mature, and this information is stored in what is often referred to as a “Similar Conditions Window”. If the second test passes on a “2-­‐trip” code, the code will be erased from “pending”. If a matured code (one that has turned on the MIL) is retested and passes three times in succession, the MIL will turn off. However, the code will remain in memory until a specific number of “warm up” cycles are completed without another failure. Monitors are further identified as being either “continuous” or “non-­‐
continuous”. Continuous monitors are the Misfire monitor, the Fuel monitor and the Comprehensive Component monitor. Continuous monitors run as the name implies…continuously. Every time the enabling criteria are met, from key on to key off, these tests will be performed. Non-­‐continuous monitors run only once after the enabling criteria are met, regardless of how long the vehicle is operated or how often the enabling criteria are met during any one key on/key off cycle. All the other monitors listed on your scan tool fall into this category. Mode $01 lists something called “Readiness Monitors”. A more accurate description would be “Monitor Status”, and is a listing of whether or not the individual monitors have completed their testing. You should see all monitors listed as “ready” or “complete” when you check this screen. Monitors listed as “not available” or “not supported” simply means that these monitors are not applicable to the vehicle you are testing. Monitors listed as “not ready” or “not complete” means that the monitor has not finished testing since the last time it was cleared. Monitor status is reset to “not ready” (and all related testing information that may have been stored in Mode $06 is lost) whenever the codes or Keep Alive memory is cleared. Status is also reset whenever battery power is lost, whether due to a normal battery changeover or a problem in the wiring to the ECM. If cleared for whatever reason, a fault detected in a completed monitor may prevent the “not ready” monitors from running and changing status. Without being reset, the monitor status will changeover to “ready” after the monitor’s first completion. Monitors may list as “ready” even if the ECM detected a failure during the testing…so it doesn’t mean that no problem was found. It also means that we don’t know the last time the monitor completed successfully. Back to our oxygen sensor failure and it’s affect on the catalytic converter monitor. The monitors may all read “ready” even after the sensor failure was recorded, but since the converter monitor uses the sensors for its tests, this monitor will be temporarily suspended. This monitor is a non-­‐continuous monitor, so any test results listed in Mode $06 are also suspect. Monitor status only tells us whether or not the tests have EVER been run since the vehicle first left the dealer’s lot. For this reason, it is good practice when repairing any powertrain DTC to operate the vehicle until all the monitors have rerun and switched to “ready”, and then recheck for any other codes that may have been recorded. Continuous monitors will list the first recorded failure of any “2-­‐trip” codes in Mode $07 (Test Results For Continuous Monitors), making this an easy check. To double-­‐check the test results for non-­‐continuous monitors, though, you’ll need to check Mode $06 (Test Results For Non-­‐continuous Monitors). Different than Mode $07, Mode $06 is a listing by monitor of the actual, individual test results. Often, these test results are unidentified in your scan tool and list results that, at first glance, mean nothing to you as a tech. Today, most service information systems provide more detail on what these tests are and how to translate them. Some scan tools now do this for you, and save you the trouble. But when verifying a repair, your primary interest is in any of these individual tests that show as “failed”, or have test results that are close to exceeding the maximum or minimum limits as shown on the scan tool screen. If your first scan of the results shows all “passed” with results well within limits, you can be pretty sure there are no immediate problems you need to address. If not, then it’s time to dive into the service information to determine what system monitor is being tested and how. ECM Misfire Strategy There is one common troubleshooting tip that we’ve stressed at Motor Age over and over. To properly diagnose any system on any car, it pays to first understand how that system works and how the ECM (Engine Control Module) performs its own tests of that system. After all, the ECM decides whether or not the MIL (Malfunction Indicator Lamp) is turned on and if your repair doesn’t meet its standards, that little light will come back on and your customer will just plain come back. In the case of misfire detection, the most common method in use is the monitoring of engine rpm via the CKP (Crankshaft Position sensor) signal to the ECM. When a cylinder fails to produce its fair share of power, the engine momentarily slows down. The ECM monitors these speed fluctuations, and records them over a number of engine revolutions as a percentage. If this percentage of “misfires” exceeds the programmed thresholds, the ECM will flag the event officially, logging the rpm and load conditions present at the time and storing the information as a pending code. If the ECM sees the same thing happen, under the same conditions and on the next consecutive test run; it will mature the code, store the freeze frame information and turn on the MIL. If the percentage is sufficiently high enough to pose an immediate threat to the catalytic converter, the ECM will “flash” the MIL light, bypass the need for a confirmation and immediately set a hard code. Take a moment to read over that rather generalized description of a misfire detection strategy. What did you learn about the troubleshooting process just from that paragraph? The most obvious lesson is that detection is based on variations in engine speed. So as a professional diagnostic technician, you’d ask yourself “What can cause that variation?” Certainly, any factor that impacts the ability of a cylinder to produce its share of power would be suspect; including a strong spark from the ignition system, the correct fuel amount as supplied by the fuel system, and the full compression and retention of the air/fuel charge in the cylinder. All this has to happen in the right amount AND the right time. Is that all? Could a faulty CKP signal trigger misfire detection? Sure it could and while not common, it should be included in your original list of suspects as a possibility. How about driving down a rough road? Yep, that’s another possibility on some older OBD II vehicles that should not be ignored when chasing down those nasty intermittents. Is it possible to have a misfire and no MIL on or code stored? According to what we learned in the description, it is. If the misfire is not occurring often enough to meet the test threshold, the ECM will not officially recognize it. Some manufacturers have some pretty high thresholds, as most of you already know! Does that mean there is no useful information in the ECM? Not at all! For a mild misfire to turn on the MIL, it has to occur under the same rpm and load conditions as the first detection, and it must occur on the next consecutive test to turn on the MIL. This is the standard definition of a “two trip” code. And while the code may not be mature, it may be stored as a pending code and may even have related Mode $06 information that can be helpful in our troubleshooting. Identifying The Weak Cylinder(s) Identifying the weak cylinders is normally easy enough. The MIL light is on, and misfire codes (P0300-­‐P0312) are stored, making cylinder identification simple. But what if there is no MIL on or codes stored? It still pays to connect your scan tool, starting in Generic or Global OBD II mode. Your first stop is Mode $03, or currently stored codes. Even if the MIL light is off, it is possible to find stored trouble codes still alive in the ECM’s memory. The ECM will turn off the MIL all by itself if the problem it originally recorded goes away for at least three consecutive runs. The code, however, will still be listed for quite a while longer. If there are no codes stored here, move over to Mode $07. Likely listed on your scan tool as “pending codes”, this is a list of test results for the three continuously monitored systems under the ECM’s control; misfire detection, fuel trim, and those components included in the comprehensive component monitor. If the misfire is intermittent, you just may find it here. Still no luck? There is one more place you can look using the Global OBD II mode of your scan tool, provided the car you’re working on is a CAN (Controller Area Network) vehicle. Choose Mode $06 to access the latest test results for all the non-­‐
continuous monitors and misfire detection monitor. Scroll down the test results, looking for test identifications starting with “A2”. That’s the misfire results for the cylinder #1, and typically includes two results. The first is the average misfire count over the last ten tests, and the second is the misfire counts for the last test run. Remaining cylinders are in numerical order; cylinder #2 is “A3”, cylinder #3 is “A4”, and so on. These are not “active” tests, so you can’t watch them change as you run the engine. But you can use them to see what cylinders have recorded misfires that may or may not have been enough to set a related code. If the vehicle is a pre-­‐CAN Ford, you can also find misfire test results in Mode $06, but the test identification numbers and counting methods are a bit different (see next section). Of course, if your scan tool has enhanced capabilities, check the data stream for misfire PIDs (Parameter Identifiers) and/or built-­‐in tests you can use to isolate the weaklings. Own a scope? The Pico scope has an available diagnostic software package that can help isolate weak cylinders. There are also aftermarket products available that can be used for misfire isolation and troubleshooting, like the ACE Misfire Detective (for the Pico) and the EMissFire Detector (for the ATS EScope). Own none of the above and still haven’t isolated the weak link? If you can consistently duplicate the misfire, try performing a manual power balance test. Start by disabling each cylinder one at a time with the misfire occurring and the engine held at a steady rpm. Note the corresponding rpm drop associated with each disabled hole. Rpm loss will be greater when a strong cylinder is lost than when the weak one(s) is lost. If the miss is at idle, you’ll have to bypass the idle control in order to see the rpm drop before the ECM does. Isolating The Cause(s) – Basic Techniques Diagnosing the reason for the misfire becomes easier when you know what cylinder(s) is failing and when. If the problem is limited to one cylinder, it has to be caused by a fault that affects only that cylinder. If more than one cylinder is involved, consider what the cylinders have in common with one another. Are they all on the same bank? Do they share a coil? Are they side-­‐by-­‐side? The first stop to look for help is the associated Freeze Frame for any stored code(s) you’ve identified. Freeze frame is a record of engine operating parameters that were in place at the time the code as set by the ECM. It paints a picture of how the engine was running at the time of the failure. Keep in mind that this is only an indication, and the engine could be accelerating, decelerating or holding steady at the time. Keep in mind, also, that the misfire monitor has to see the same failure (under the same load and rpm conditions) twice in succession to set a non-­‐catalyst damaging miss before the MIL is turned on. If you have access to a scope, the very first test you should perform is a relative compression test. This test can be done in a few different ways, but the most common uses starter current draw as the means to quickly check for compression/sealing issues in the engine. A strong cylinder requires more current to turn through than a weak one, and this can be seen on the scope pattern. No scope? Grab your manifold vacuum gauge. If you see the needle bouncing or get a lower than normal vacuum reading, you may be dealing with a mechanical cause. Check the misfiring cylinder(s) compression and leak down rate. A word of caution when assessing the mechanical health of the engine. More and more, techs are running into intermittent valve sealing issues resulting in intermittent misfires. Valves rotate slightly as the engine runs. Because of this, a valve with heavy carbon deposits or even one that is mildly bent, can seal perfectly for a time and leak like a sieve the next. In these cases, it is very possible for a cylinder to pass a traditional compression/leak down test. If the misfire comes and goes with no apparent reasoning behind it, this may be your problem. Dealing with a single cylinder miss? Parts swapping is a perfectly acceptable diagnostic process. If the engine uses a COP (Coil On Plug) ignition system, try switching coils and plugs to see if the misfire moves with the swap. Take a good look at the plug while its out. Excessive gap creates excessive demand on the coil, and is a leading cause of COP coil failure. But don’t let that alone lead you to the replacement of parts. If the miss didn’t move, there is another culprit to be found. Recommend the plugs for maintenance, not for the repair. Did you know you could verify a weak DIS (Distributorless Ignition System) coil by swapping the companion cylinders? A DIS coil fires both cylinders at the same time but if the coil is weak, the positive side cylinder won’t fire under compression. Swap the secondary leads for the companion cylinders at the coil. If the misfire changes places, the coil is no good. If the single cylinder miss doesn’t go away, the only thing left is the fuel delivery to the cylinder. Swapping the injectors is little more time consuming on most engines. Instead, use your scan tool or manual injector tester to perform an injector balance test. This test records the amount of pressure drop in the fuel rail as each individual injector is activated. Higher than average drops can indicate a leaking injector and lower than average drops point to a restriction. You can perform the same test even faster using a pressure transducer or sensor mounted in place of your fuel gauge, and a scope. Dealing with multiple cylinder misfires will involve testing of these same basic systems, only in a different light. Fuel delivery issues (low pressure/volume) will impact all the cylinders, for example. Leaking head gaskets typically affects two cylinders at a time. Cam timing problems can be across the board or only on one bank, depending on the engine design. The key is to take a step-­‐by-­‐step, logical approach to the problem at hand and testing to confirm your observations. Understand how the systems work and interact, take the time to learn how the ECM is pinpointing a problem, and use that information to plan your diagnostic attack. Above all, verify your finished repair and your customer will come back because he wants to, not because he has to. Understanding Fuel Trim And Applying Trims To Misfire Faults Fuel trims can provide clues to the cause of nearly any drivability problem, and misfire troubleshooting is no exception. But to understand what the trims are trying to tell you, you first need to understand what the ECM is doing. The engine management system, led by the ECM (Engine Control Module), has one primary task: protect the catalytic converter. If the feed gasses entering the converter from the engine contain too much air or unburned fuel, cat temperature will begin to increase with the potential of permanently damaging the substrate. Secondary to protecting the converter from damage is the monitoring of all systems that could potentially cause emissions to increase should they not function as designed. Any problem the ECM discovers that could lead to more (than expected) trash escaping the exhaust pipe will trigger the MIL (Malfunction Indicator Lamp). The ECM keeps an eye on its various subordinates by testing them, and looking for a passing grade. Groupings of these tests, usually related to a system or component, are called monitors. Monitors are further divided into two types: continuous and non-­‐continuous. Continuous monitors are the misfire monitor, the fuel system monitor, and the comprehensive component monitor. As the name implies, the continuous monitors run all the time, repeating over and over as long as the engine is running. All three keep an eye on engine systems and functions that could result in severe catalytic converter damage if a problem arises. For now, let’s focus on the fuel system monitor. The fuel system monitor doesn’t monitor components like the fuel pump or fuel injectors as the name may lead you to believe. It monitors the feed gasses going to the converter by monitoring the amount of fuel fed to the engine. If the ECM detects that too much or too little fuel has been added, and correcting the situation is beyond its abilities, it will flag the appropriate “system lean” or “system rich” DTC (Diagnostic Trouble Code). Every tech learns early on about proper air/fuel ratio, that magic number where the engine works at its best. This ratio is expressed in terms of air mass (weight) to fuel mass, and for a gasoline engine, that magic number is 14.7 pounds of air for every pound of fuel. This is called the stoichiometric ratio and is different for different fuels (10% ethanol fuel, for example, is 13.85:1). It is the ratio that provides just the right amount of fuel to utilize all of the air drawn into the combustion chamber. The actual air supplied versus the theoretical supply needed for stoichiometric is called lambda (also known as the excess air factor), and is represented as 1.0 (meaning 1:1). A lambda less than 1 indicates that too little air was supplied, and the mixture is rich. A lambda greater than 1 indicates the opposite...too much air was supplied, and the mixture is lean. A gasoline engine produces its best power when lambda is 0.85-­‐0.95 but its best fuel economy at 1.10-­‐1.20 (on manifold injection systems). That seems like a fairly wide range, doesn’t it? We still have to consider emissions. To keep most 3-­‐way catalytic converters happy we need feed gasses in a much narrower lambda range, more like 1.000 +/-­‐ 0.005. Outside of this range, NOx emissions begin to increase and further variance can lead to increased HC and CO emissions as well as potential converter damage. On most vehicles, fuel control is based on feedback from a conventional oxygen sensor. Contrary to the name, though, an oxygen switch is a more accurate description of how these sensors work. When feed gasses passing by the sensor are at lambda = 1, output voltage is about 0.450 volts. Vary just a bit on either side, and the sensor’s voltage shifts dramatically. The ECM makes its base fuel delivery calculations depending on the type of system in use (speed-­‐density or mass airflow), then commands the injectors open. The feed gasses pass into the exhaust, past the sensor, which then shifts high or low depending on the amount of excess air in the exhaust. The resulting feedback voltage from the sensor lets the ECM know whether its initial calculation was too rich or too lean for the current rpm/load conditions. The ECM then applies that information as a modifier to the base fuel calculation and attempts to correct enough to make the oxygen sensor voltage shift to the other extreme. If successful, the ECM will switch direction and if not, the ECM will continue its original correction until the voltage shift does occur. These ECM corrections are what you see on your scan tool as STFT PIDs (Parameter Identifiers), STFT standing for Short Term Fuel Trim. Since the oxygen sensor cannot actually measure the excess oxygen content (or lack thereof), the ECM has to overcorrect in order to know where it is in relation to a lambda = 1 fuel delivery. Typically, STFT will alternate on your scan tool’s screen from +5 to -­‐5. What this means is that the ECM is adding 5% (+5) or subtracting 5% (-­‐5) from that modified base calculation. It also means that the ECM is able to control fuel delivery to the engine, and protect the converter. If you see the STFT trending positive or negative, the ECM is correcting the fuel mixture. It will continue the direction of correction until it sees the oxygen sensor voltage shift in the opposite direction. Things change over time, and the ECM is capable of learning to adapt to normal engine wear and tear. If normal corrections don’t cause the expected shift in sensor voltage, the ECM will continue to correct in the same direction (adding or subtracting fuel) until it does. The excess amount of correction is learned by the ECM, and applied in the base fuel calculation as a more permanent means of controlling the feed gasses. This is the LTFT (Long Term Fuel Trim) PID you see on your scan tool. If the engine is being fed a mixture that is too lean, for example, the first thing you’ll notice on your scan tool’s data screen is a positive trend in STFT. At the same time, you’ll see a positive trend in LTFT. This is the ECM learning how much of a permanent correction it’s going to take to regain control. Keep watching, and you’ll see STFT numbers getting smaller and once again beginning to alternate between -­‐5 and +5 while LTFT begins to stabilize. As with STFT, LTFT numbers represent the percentage correction made. Unlike STFT, these numbers are stored and applied under similar rpm/load conditions. They are a learned correction, and are only modified when they are no longer sufficient enough to allow the ECM to remain in control. So don’t be surprised when you see LTFT numbers in the 10-­‐15 (+ or -­‐) range on your scan tool. As long as STFT is moving back and forth from -­‐5 to +5, the ECM is in command and the cat is getting what it needs to stay healthy. Now let’s apply what we know to diagnosing misfire causes. Let’s use, as an example, a V6 engine equipped with a Mass Airflow (MAF) sensor with a single-­‐
cylinder misfire. Start by adding together the STFT and LTFT for each bank. On a healthy engine, the Total Fuel Trim (TFT) should be relatively equal. Now consider what should happen if a fuel injector on one bank fails to deliver its fuel charge. That bank has three cylinders, right? With a missing cylinder’s fuel charge, the oxygen sensor on that bank will see that unused air pass by and report it as a lean condition to the ECM. The ECM will begin to attempt to correct that lean charge by adding fuel until the oxygen sensor was happy. How much fuel is missing for the total air being taken into the three cylinders? A third of it, right? So if the cause of the misfire was fuel, you could expect to see TFT corrections in the +30 range. Another cause for a completely dead hole is lack of compression. What would happen to fuel trim then? The total amount of air being reported by the MAF sensor would be affected, wouldn’t it? Less air, split between both banks, would result in less overall fuel needed. But the bank with the missing cylinder would show lean (negative) corrections while the bank with three healthy cylinders would show rich (positive) corrections. Why? Put on your thinking caps. Total air charge is lower, but all that loss is occurring on one side. That side would be rich at first because the ECM is looking only at the total air mass being reported by the MAF and then feeding all cylinders the same amount of fuel. The side with all healthy cylinders would be running lean for the same reasons. These cylinders are getting a normal amount of air, but the ECM thinks they are getting less based on the total air charge it’s been told. What about an ignition system-­‐related problem? There is still fuel in the cylinder and some, if not all, will be burned either in the cylinder or in the exhaust as it makes its way out of the tailpipe. Just like the missing fuel scenario we discussed earlier, the oxygen sensor will see the “lean” condition and attempt to correct. Here, though, TFT corrections will be less than half of what we’d normally see with a fuel problem. Using Mode $06 For Pre-­‐CAN Ford Misfire Diagnosis Ever had a pre-­‐CAN Ford with a miss you could feel but had no code set? There is help in finding this problem if you know what to look for in Global OBD2’s Mode $06. First, Mode $06 (the dollar sign simply denotes a hexadecimal number, and is used by the ECM to ID the test) is a listing of the individual tests and results the ECM uses in running the non-­‐continuous monitors like EGR, EVAP, and catalyst efficiency. Even though the misfire monitor is continuous, Ford thought having access to the individual tests for this monitor would be beneficial to techs, so they included it in their Mode $06 data. The misfire monitor is a two fold monitor that checks for variations in the CKP signal to detect a miss. The first part is the percentage of misfires detected over a 200 revolution period and looks for catalyst damaging misfires, and the second part is the percentage of misfires detected over a 1000 revolution period, looking for emissions related misfires. These misfires are referred to as Type A and Type B, respectively. Emissions related misfires are generally in the range of 1-­‐3% at all engine speeds, and are a 2 trip code. The misfire must either be detected in the first 1000 revolutions at start up or on 4 different 1000 revolution checks while running. Catalyst damaging misfires are anywhere from 4% to 30%, and depends on rpm and load under which the misfire occurs. If a Type A misfire is detected, the MIL will flash and a code will be set on the first trip. Lastly, since there are minor variances in the manufacture of the teeth on the crank reluctor, the ECM must first learn this variance before the misfire monitor will run. This is done during 3 60-­‐40 mph decels with no brake applied. If KAM is cleared by a scan tool or by disconnecting the battery for longer than 5-­‐10 minutes, this profile will have to be relearned to enable the monitor. Mode $06 on most scan tools will look something like this: TID $53 CID $01 Measured 420 Minimum 0 Maximum 20992 “Passed” TID stands for test ID, CID stands for component ID. The values shown are in “computer speak” and have to be converted to a number you can understand. For most of Ford’s misfire tests, you multiply by .0015, and this number is a percentage. For example, if we translate the example above, we get: TID $53 CID $01 Measured .6% Minimum 0 Maximum 31.4% “Passed” In other words, out of the total number of times the plug fired, a misfire was detected on .6% of them. This is below the 1-­‐3% Type B “threshold”, and certainly below the Type A threshold, so the test passed. The maximum tells us that the test level used to pass this test was based on the Type A threshold, and was done at or near idle due to the high percentage “Pass” level. Back to the opening question. If you know what the tests are, you can use them to find the cylinders that are misfiring, even when there is no code stored or pending. You can also use them to verify the repair, as long as the same conditions are present as when the misfire occurred. The following is a listing and description of the Ford Mode $06 misfire tests. TID $50 Total misfire rate in percent as compared to emissions threshold in percent. Multiply values by .0015 to convert to percentages. This test is “active”, and when checked KOEO, the measured value will always show “0”. However, if you access this test with the engine running, you will get a number if there are any misfires occurring. Test results are updated every 1000 revolutions. TID $53 (TID $51 on early OBD2 Fords) Cylinder specific misfire rates in percent and the threshold value used for test “pass or fail”. Probably the most commonly used Mode $06 test, this will list up to 10 separate component IDs. These CIDs correspond to the individual cylinders, with those not used showing inferred test results. For example, only the first 6 tests will be accurate for a 6 cylinder engine. This test updates whenever a higher misfire rate is detected and will usually “freeze” if a code is set or pending. It is useful in IDing the cylinder or cylinder(s) that are contributing to the misfire problem, and by how much. Multiply values by .0015 to convert to percentage. TID $54 Highest catalyst damaging misfire rate calculated and threshold used. Since the Type A threshold value varies from roughly 30% at idle to as low as 4% under high rpm and load conditions, the threshold value shown here may help in identifying the conditions under which the misfire occurred. If a code is set, compare this to freeze frame data. But keep in mind, freeze frame is logged when the code is set, and in the case of a misfire, this isn’t the same as the instant the miss occurred. Multiply values by .0015 to convert. TID $55 Highest emissions related misfire rate calculated and threshold used. Same as TID $54, but based on lower Type B threshold of 1-­‐3%. Multiply by .0015 to convert. TID $56 Cylinder events tested and number needed to complete monitor These test values do not need to be converted. They are simply total counts. The threshold value is the number of cylinders times 500…500 “sparks” for each cylinder for every 1000 engine revolutions. This is also an “active” test, and will show “0” measured if checked KOEO. However, if you look at it KOER, you will get a count. Typically, this measured value will be close to the threshold value, and indicates the monitor is working. Note: It is possible for the misfire monitor status to show “complete” and not be working. Check this test for a “passed” status to make sure the profile has been learned.