G
arbage in, garbage out.
Funny, but that old
computer adage applies
directly to repairing today’s cars. Changes in
national air quality
standards have caused many areas to be
declared in nonattainment, meaning
stricter automotive emissions standards
are required to clean up the air. In response, the states are implementing
new, tougher emissions programs to
meet the tightened specs. Like it or not,
these new programs are here to stay,
and they’re pure opportunity for those
who want and can do the work.
The two charts shown on page 46 illustrate the relationship of the exhaust
gases on a perfectly running engine.
Only the air/fuel ratio has been
changed. Fig. 1 illustrates the precatalyst gases with no air injection, no air
leaks and no misfires. If a misfire were
present, you could have high carbon
monoxide and high oxygen at the same
time, but as you can see, this condition
never shows in the chart. Fig. 2 shows
the exhaust gases after the catalyst has
had a chance to clean things up. With
those few points out of the way, let’s
take a closer look at those so-called
garbage-out gases.
Carbon Monoxide (CO)
The addition of NOX as an exhaust
pollutant, combined with newer smog
programs, has signaled the dawn of
a new era in emissions diagnosis.
Carbon monoxide is formed in the
combustion chamber when there is insufficient oxygen to complete the combustion process. For diagnostic purposes, CO is an indicator of richness
(air/fuel ratios less than 14.7:1). In Fig.
1, notice that CO is over 15% with the
air/fuel ratio at 8:1. It then drops to
about .3% at 14.7:1 (stoichiometry) and
stays at that level past 18:1. For this
reason, CO can never be considered a
leanness indicator.
The red lines in Fig. 2 are postcatalyst CO percentages. There are two
lines here, for systems with and without air injection (AIR). In shop-testing
my van with a new catalytic converter,
I found that the cat was capable of
converting 8% CO to .10% without
overheating. This van uses an AIR system that pumps air directly into the
converter; newer vehicles without an
air pump are less likely to convert this
much CO because of the lack of oxy-
gen necessary to complete the job.
Without an air pump, a 2% drop in CO
is about the maximum you can expect
from the converter.
CO failures are usually due to too
much fuel (bad sensor input, high fuel
pressure, evap failure, etc.) or too little
air (dirty air filter, blocked intake, etc).
Remember, however, that the whole
emissions system is designed as a package, so a CO failure might not necessarily be a CO problem. For instance, some
vehicles with air injection systems are
designed to run at 2% CO at idle (preconverter), with the converter cleaning
up the remaining CO without overheating because of the low volume of air introduced at idle. Systems with pulse air
setups (one-way reed valves that open
when the exhaust pressure goes negative) have a tendency for the valves to
rust shut. Without the extra oxygen to
catalyze the CO, tailpipe emissions will
42
May 1998
fail at that 2% CO level. The obvious fix
is to repair the air injection system.
Always consider driver habits in your
diagnosis. Many of my elderly customers
have older cars without heated O2 sensors. They don’t do a lot of driving, so
these feedback systems rarely achieve
closed-loop. Result: These cars fail their
smog test every year. All we need to do
to get them to pass is change the oil and
heat up the converter to burn off the
excess carbon accumulation.
After repairing a CO problem, make
sure you change the oil. Oil will become
contaminated with fuel when a CO
problem exists. Not changing it may
cause the failure to persist, at least until
the PCV system sucks the fuel out of
the oil. Speaking of the PCV, it doesn’t
hurt to disable the system and look at
the changes in the gases whenever you
encounter an emissions test failure. On
mass airflow (MAF) systems, you’ll
BY MARK WARREN
need to clamp off or block hoses. If you
just pull the PCV valve out, the MAF
sensor will incorrectly think too little air
is entering the engine.
Hydrocarbons (HC)
Gasoline is a mixture of many different
hydrocarbons. So it doesn’t take a genius to figure out that HC in the exhaust is indicative of unburned fuel.
Some amount of HC will always be
present in the exhaust due to fuel too
close to a cool combustion chamber
wall and large fuel droplets that burn
on the outside only. Lots of HC means
incomplete combustion due to engine
misfire.
Looking at Fig. 1, you can see that
even at better than 15% CO (very rich),
HC is only about 500 ppm (although
there is no scale for ppm on this chart,
HC goes from a high of 500 ppm to a
low of 50 ppm). When an engine misfires, however, HC skyrockets (usually
to about 2000 ppm per cylinder). I’ve
seen converters handle as much as 4000
ppm HC with the emissions relatively
clean at the tailpipe, but the cat’s operating temperature will go way too high
and damage will result fairly quickly.
The orange line in Fig. 2 shows the converter’s ability to convert HC.
Any condition that can cause a mis-
fire can result in high HC. Things to
look for include ignition problems,
vacuum leaks, compression problems,
injector deposits and out-of-whack
cylinder balance. Keep in mind that
an air pump makes little difference
on a misfire, because the misfire supplies its own oxygen.
Be careful when quoting prices for
emissions repairs. As a case in point, a
customer came to us with a high HC
failure. My tech diagnosed a bad plug
wire. The customer was quoted a price
for diagnostics, a new set of wires and
installation. After the repair, HCs were
much better but still failing. Turns out
the converter was dead from too much
abuse. Now the quote doubled. Care
to hazard a guess how that went over?
When diagnosing emissions failures,
leave yourself some working room and
explain to the customer that repair is a
process, not a single step. Like Yogi
says: “It ain’t over till it’s over!”
Oxygen (O2)
Fig. 1 shows oxygen (blue line) at
about .5% when the air/fuel ratio is
rich. Then at about 13:1, it starts to
curve up to as high as 6% when lean
misfire occurs. O2 is an indicator of
leanness; it gives no indication of how
rich an engine is running below 13:1.
Assuming that the air injection system is disabled, high O2 readings can be
caused by conditions other than lean
mixtures. An oxygen reading of 20%, for
example, is a pretty good indication that
the sampling hose has fallen out of the
tailpipe. This may sound crazy, but I’ve
actually seen more than one tech proclaim a car fixed with a reading like that.
A high O 2 reading may also be
caused by a leak in the analyzer or its
sampling hose. After a number of emissions failure comebacks in my shop, I
had to find out what was going on. The
last car tested had low CO, HC and
CO2, yet O2 was 10%. By the way, this
car ran great. After double-checking
that the air injection was disabled, the
only conclusions I could come up with
were either a large exhaust leak or a
problem with the sampling hose.
I removed the sampling hose from
the exhaust and plugged the holes with
my fingers. After treating my burned
fingers and cooling the hose, I repeated the test. No low-flow indication on
the gas analyzer. Moving back through
the system showed no improvement
even with the hose removed and
plugged at the inlet. Turns out the
problem was a missing O-ring on the
filter housing. Apparently, when we replaced the filter earlier, we had mis-
May 1998
43
5-GAS ANALYSIS
14.7:1
(Stoichiometry)
14.7:1
(Stoichiometry)
CO2
CO (without AIR)
CO
NOX
NOX
CO (with AIR)
HC
Charts: Mark Warren
HC
O2
Fig. 1
Fig. 2
placed the O-ring. Remember, it never
hurts to test the tester!
Always write down the gas readings
before and after disabling the AIR system, then compare them to see if the
AIR system is functioning properly. I
recommend pulling and plugging air
injection lines for testing; switching the
diverter valve can be complicated and
may not give you the results you desire.
For example, I once worked on a Land
Cruiser for three days trying to solve a
marginal CO failure, only to find an old
piece of air pump vane lodged in the
diverter valve. The valve I thought was
closed leaked just enough air onto the
O2 sensor to shift the mixture rich.
Oxygen represents about 20% of the
atmosphere. If a four-cylinder engine
has a complete misfire on one cylinder,
expect the tailpipe O2 level (air injection plugged) to be about 5%. How did
I come up with this percentage? Well,
one cylinder represents 25% of the total number of cylinders, right? So 25%
x 20% O2 = 5%. On a V8, a single cylinder misfire would spew about 2.5% O2
46
May 1998
out the tailpipe (12.5% x 20% O 2 =
2.5%). No need to go into six cylinders;
you get the picture.
Carbon Dioxide (CO2)
We want carbon dioxide as high as possible on a percentage basis (see the section on “Mass vs. Percentage Emissions”
on page 50). CO2 and H2O are the perfect byproducts of combustion from a
carbon-based fuel. You can see in Fig. 1
that the maximum percentage for CO2
(green line) is about 14.7%. You may
encounter higher percentages on your
analyzer. That’s because most analyzers
don’t measure CO2 or O2 nearly as accurately as they do CO, HC and NOX.
Note that the CO2 level will be lower
than 14.7% if an air injection system is
diluting the exhaust gas with air.
Nitrogen Oxides (NOX)
Nitrogen oxides is the new kid on the
block for most state emissions programs. NOX is formed under high-temperature and high-pressure conditions
in the combustion chamber. NOX fail-
ures can be attributed to EGR system
failure, overadvanced ignition timing,
lean-running conditions, a failed reduction portion of the converter, high
compression due to combustion chamber deposits, overheating, overly warm
intake air and valve timing and cylinder balance problems.
The good news from the field is that
most NOX failures are pretty easy to diagnose and fix, although some, such as
valve timing problems, can be pretty
far down the process of elimination.
Look at NOX (black line) in Fig. 1.
While CO and HC are fairly low in the
14.7:1 ideal mixture range, NO X is
quite high. Obviously, the converter
has to work pretty hard to convert NOX
in this range. Notice in Fig. 2 that the
converter is very efficient at converting
NO X at ratios of 14.7:1 and below.
Above stoichiometry (leaner mixtures)
the reduction portion of the converter
stops working on NOX due to the presence of too much O2 (greater than 2%)
in the exhaust. To properly reduce
NOX, the exhaust must be low in O2
5-GAS ANALYSIS
and the converter must have some CO
to work with. Even so, a converter can
handle only about 2000 ppm NOX;
much more than that needs to be addressed by EGR or engine design.
Watch out for a CO to NOX failure!
The typical scenario is that a car comes
in failing CO, you fix the problem and
now it fails with high NOX! What gives?
While it’s difficult to have a NOX failure
with a lot of CO present, high CO also
causes carbon buildup in the combustion chamber and in the reduction portion of the converter. Even if you fix the
CO problem, this carbon buildup remains. With CO low and the carbon
buildup still there, you have the perfect
recipe for NOX formation. It’s interest-
Preconverter O2 Sensor In Closed Loop
Converter
Light-Off
Screen saves: Mark Warren
Postconverter
Revved to
O2 Sensor
Sweeping With 2000 RPM
to Warm
Front Sensor
Converter
Postconverter O2 Will Follow
Front on Rapid Throttle Changes
ing to note that Denver has a CO problem, not a NOX (ozone) problem. But
as their emissions program cleans up
the CO-failing cars, NOX may become
an air quality issue in the future.
When performing a compression
test, don’t overlook cars with higherthan-normal compression. I’ve seen
many 3.8-liter GM engines with a 135psi compression spec check out at 180
psi or higher from combustion chamber deposits. Decarbon that engine or
you’ll have a NOX problem!
Note that some GM engines with taper-seat spark plugs have had problems with carbon buildup around the
plug threads. When you go to spin in
new plugs, the carbon ramps up in
front of the threads and the plug feels
like it’s tightened, although the taper
never landed. Result: The spark plug
overheats, the engine pings and NOX is
created. We have removed these plugs
and have seen no witness mark on the
tapered part of the plug from seating.
Moral: Clean those threads!
Testing Converter
Efficiency
Fig. 3
Preconverter O2 Sensor In Closed Loop
Postconverter O2 Sensor At
Converter Light-Off
Fig. 4
48
May 1998
When you get a car that fails an emissions test, you need to know the general health of the converter. Is it dead,
inefficient or simply trying to handle
too much? The first test I like to run is
called a cold converter test. To do it,
bring the engine to operating temperature and then shut it down for a halfhour. This allows the converter to cool
but still keeps the fuel system pretty
close to closed-loop operation.
In Fig. 3 we’re monitoring the front
O2 sensor (top chart), the postconverter O2 sensor (second chart) and engine
rpm (lower chart) on a 1997 four-cylinder Chevy Lumina. Notice that after
300 seconds at idle, the converter still
didn’t light off. At the 350-second
point, we revved the engine to 2000
rpm and light-off occurred about 50
seconds later. As you can see, the rear
O2 sensor flattens out and quits following the front O2 sensor like it did before light-off. The later movements on
the rear O2 sensor are relative to rapid
throttle changes.
Fig. 4 is the same recording only
zoomed in to get a better look at the
sweep of the O2 sensors and light-off.
This is exactly how OBD II systems test
converter efficiency—by comparing the
pre- and postconverter O2 readings.
Back to the cold converter test. I
record the 5-gas readings before and
after converter light-off. This gives a
good indication of what the converter
is trying to deal with. Although this test
works great for CO and HC, it’s not
nearly as effective for NOX because by
the time you adequately load an engine to test for NOX, the converter will
already be lit off.
Another option is to perform the
dual O2 OBD II test, which checks the
ability of the converter to store O2 and
oxidize HC. Like the cold converter
test, however, this test won’t indicate a
problem in the NOX reduction portion
of the converter.
Okay, so how else can we test converter efficiency? How about measuring the temperature difference at the
front and rear of the converter? While
this may tell you if the converter can
light off, it gives no indication of true
efficiency. Also, some vehicles run so
clean under no load that the converter
may not even light off for lack of
enough to burn.
GM has an excellent but complicated
test to determine converter efficiency.
You first warm up the converter by running the engine at 2000 rpm. Then shut
the engine down and disable the ignition system and injection. Next, connect
propane to the intake with a flow gauge
attached, set the flow to the prescribed
amount from a chart on engine size and
crank the engine at wide-open throttle.
Finally, record the CO 2 percentage
from the tailpipe and compare the reading to the chart. A quicker way is to
warm up the converter, disable the ignition and crank the engine over, checking for CO2 out the tailpipe. This won’t
give you percentage efficiency, but with
practice, you can get some good data.
The best way to know what’s going
on with a converter is to test the gases
before and after it. So drill a small hole
in front of the converter (this can be a
problem on some cars that have the
converter built into the manifold) and
compare the readings. This takes some
time but it’s a sure-fire test. And be-
cause of the inability of the other
methods to check the reduction (NOX)
portion of the converter, it may be your
only hope.
Converters die from overheating,
poisoning from sulfur, lead, zinc, phosphorus, silicone and carbon and physical damage. Don’t replace an overworked converter and expect the new
one to last. Also consider the possibility
of poisoning before completing or
quoting a repair. If a car is a major oil
burner, the new cat won’t have much
of a chance. If you have a blown head
gasket, be sure to warn the customer
of possible converter and O2 sensor
damage. A bunch of O2 sensors and a
new converter may cost almost as
May 1998
49
5-GAS ANALYSIS
much as the cylinder head job itself!
Some converters get coated with
contaminants, yet aren’t dead. Remember the story of the old folks who
never drive long enough to get the
feedback system into closed-loop?
Most shops will take that car on the
freeway for a while to relight and
clean the converter. We’ve had good
results creating a misfire to heat the
converter nice and toasty. This procedure may also revive a converter with
sulfur coating. There is a great amount
of caution and discretion associated
with the misfire rejuvenation procedure. We never take the shell temperature over 800°F, and we usually perform the procedure only on a converter that has been declared dead.
Mass vs. Percentage
Emissions
We were all trained in percentage and
ppm gas readings. This worked great
for years, so why did the government
50
May 1998
switch over to grams-per-mile? Do
they do this stuff to make our lives
harder? Not at all.
The feds need to predict the total
vehicle emissions inventory to calculate air quality. A 2-liter engine at 1%
CO doesn’t spew nearly the same total
emissions as a 7.5-liter engine at the
same CO level. Grams-per-mile is the
only effective way to measure this. Also, keep in mind that grams-per-mile
changes the way some gases appear.
Remember I said we want CO 2 as
high as possible on a percentage basis?
Well, in grams-per-mile, a high CO2
reading may indicate dragging brakes.
In this scenario, the engine could be
in closed-loop and the percentage/
ppm gases could look good, yet fuel
mileage is poor and CO2 emissions excessive.
Using grams-per-mile allows us to
take HC, CO and CO2 readings and
predict fuel mileage. That’s why many
state emissions programs give an esti-
mated mpg rating on the printout. Finally, grams-per-mile readings measure
mass emissions and are affected by air
injection. Percentage readings are diluted by air injection, mass readings
are not.
As you can see, 5-gas analysis isn’t
the easiest thing in the world to comprehend. It’s knowing how each exhaust gas relates to the others. It’s
knowing what catalytic converters can
and can’t do to clean things up at the
tailpipe. It’s knowing how driver habits
can affect emissions. Above all, it’s
putting all these things together in an
effort to achieve the cleaner air we and
our families deserve. Gentlemen, start
your analyzers!
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