Vortec 4300 4.3L V6 (LG3/LU3) Truck Engines
2003 Model Year Summary
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RPO L35 Eliminated
Quiet Cam for LU3
Powertrain Control Module and Software with Additional Monitoring Functions
On Road Vapor Recovery (ORVR) for Chevrolet S-10 and GMC Sonoma Crew Cabs
Improved Oxygen Sensors for Chevrolet Astro and GMC Safari
Solid State Oil-Pressure Sensor For Chevrolet Express and Silverado and GMC Savana and
Sierra
Common Accessory Drive Fasteners for Express and Savana
FULL DESCRIPTIONS OF NEW OR CHANGED FEATURES
RPO L35 ELIMINATED
All remaining applications for the Vortec 4300 4.3L V6 (L35) have ended, including: the Chevrolet
S10, Blazer, and Express, the GMC Sonoma, Jimmy and Savana, 49-state Chevrolet Silverados and
GMC Sierras and the P Truck (a square delivery-style truck now manufactured by Workhorse
Custom Chassis). In all cases except the P-Truck (see P-TRUCK APPLICATION ELIMINATED), the
L35 will be replaced by the LU3 variant of the Vortec 4300, which was introduced for model year 2002
in Chevrolet Astro, GMC Safari, and Silverados and Sierras built for sale in California.
The 4.3L was developed for optimum performance with all types of fuel; differences between the LU3
and L35 lie entirely in the fuel system. The LU3 replaces Sequential Central Port Injection (SCPI) with
Multipoint Fuel Injection (MFI), using the same Multec II fuel injectors found in many GM engines.
These state-of-the-art injectors feature a stainless-steel director plate. They reduce fuel leakage 66
percent, maintain more consistent performance under hot-fuel conditions and greatly reduce the
potential for plugging due to contamination. In the 4.3L LU3, electrical connections are extended from
the L35’s centrally located injector drivers to the injectors (operated electrically rather than by a fuel
pressure pulse) at the intake ports. The 4.3L LU3’s intake manifold is identical to that used on the
L35.
The Vortec 4300 requires slight packaging modifications for installation in the redesigned Express
and Savana. The engine is essentially the same as the LU3 used in the Silverado and Sierra, but the
dipstick and oil-fill tubes are extended for application in full-size vans. The oil pan requires a slight
scallop in its sump to accommodate the van’s front suspension.
P-TRUCK APPLICATION ELIMINATED
With production of the heavy-duty 4.3L L35 ending, the Vortec 4300 will no longer be offered in the P
Truck.
QUIET CAM FOR LU3
The 4.3L LU3 will be equipped with a ``quiet cam’’ designed to reduce vibration and mechanical noise
at both idle and high engine speeds. The new camshaft has no effect on engine output or dynamic
performance. Maximum valve lift and duration do not change, but the lobes have been re-profiled to
keep the valve lifters in full contact with the cam as the lobes ramp down. The cam significantly
reduces noise at high rpm and delivers what customers perceive as a smooth, strong, confident
engine sound.
POWERTRAIN CONTROL MODULE AND SOFTWARE WITH ADDITIONAL MONITORING
FUNCTIONS
A new Powertrain Control Module (PCM), called P59, manages the 4.3L LU3. This PCM increases
clock speed from 21 to 24 MhZ and doubles memory (1.1 megabyte) compared to its predecessor. It
also provides one of the most sophisticated engine control systems in the industry. The P59 is
common to the LU3 and all gasoline-powered Vortec V8 truck engines.
Commonality offers the advantage of reducing inventory complexity and increasing efficiency at
various assembly plants. More important, the P59 offers the most precise engine management
possible, optimizing performance according to temperature or operating conditions and virtually
eliminating unintended variation in every function it controls, from ignition timing to fuel delivery to
transmission shift points. It also allows GM Powertrain engineers to monitor more engine operations
and improves the accuracy and robustness of the OBDII (for On-Board Diagnostics) system.
For example, the PCM now measures electrical current flowing to the oxygen (O 2) sensors on
vehicles equipped with the 4.3L LU3 (crucial components of the emissions-control system).
Previously, the O2 sensors were monitored with a time-to-activity algorithm, which required more
measurement latitude to ensure proper operation. The new PCM immediately reports a malfunction
in an O2 sensor with virtually no margin for misreporting. It also allows a new Engine Off Natural
Vacuum diagnostic (EONV) for the On Road Vapor Recovery system (ORVR), which reduces
evaporative emissions by preventing gasoline vapor from escaping the vehicle into the atmosphere.
With EONV, the PCM actually continues to operate when the engine is turned off, monitoring
pressure in the fuel tank and ORVR system. If pressure bleeds off more quickly than ambient
temperature and other conditions indicate, the PCM can determine whether the system has a leak,
even when the vehicle is parked. The new PCM ensures that the LU3 operates according
government emissions regulations. And thanks to its precision, it will also reduce the number of false
alarms-- OBDII ``service engine’’ warning lights--that require dealer intervention.
ON ROAD VAPOR RECOVERY (ORVR) FOR CHEVROLET S-10 AND GMC SONOMA CREW
CABS
S-10 and Sonoma Crew Cabs equipped with the 4.3L LU3 now carry an ORVR system. The system
is virtually identical to that on other trucks powered by the LU3. The evaporative canister has 3.1 liters
volume, with a leak-detection standard of .020 inches. While there is no obvious change for the
customer, ORVR reduces evaporative emissions by preventing gasoline vapor from escaping into the
atmosphere. The LU3’s OBDII system constantly monitors ORVR. Leaks greater than .020 inches in
diameter would be indicated with a warning light in the vehicle’s instrument panel.
IMPROVED OXYGEN SENSORS FOR CHEVROLET ASTRO AND GMC SAFARI
The 4.3L LU3s installed in the Astro and Safari are equipped with new O 2 sensors. The new sensors
are required for the new PCM current-monitoring function. They also heat more quickly, allowing the
exhaust emissions system to achieve “closed loop’’ operation--maximum emissions reduction--in
about half the time.
SOLID-STATE OIL PRESSURE SENSOR FOR CHEVROLET EXPRESS AND SILVERADO AND
GMC SAVANA AND SIERRA
The 4.3L LU3s built for these four applications use a fully electronic, solid-state oil pressure sensor
(other applications have an analog/mechanical sensor). The Express, Silverado, Savana and Sierra
have electronic instrument packages, and the solid-state sensor is required to drive the oil pressure
gauge.
COMMON ACCESSORY DRIVE FASTENERS FOR EXPRESS AND SAVANA
With redesign of the Express and Savana, all accessory drive fasteners on 4.3L LU3s built for this
application match those on Vortec V8s. Nuts and bolts for alternator and AC compressor mounting
brackets, pulleys, etc., will have the same size. This allows assembly associates to install accessories
on various engines without changing tools.
OVERVIEW
The Vortec 4300 remains one of GM Powertrain's highest volume engines. This 4.3-liter V6 was
developed in the mid-1980s by removing two cylinders from the legendary small block V8. Counterrotating balance shafts were introduced to balance primary crank vibration in the 90-degree block.
The low-end torque delivery of an overhead-valve engine matched with continued improvements in
noise, vibration and harshness control and the latest electronic controls have kept the Vortec 4300 at
the forefront of V6 truck engines.
The Vortec 4300 remains one of the best V6 engines available. It delivers excellent torque and good
specific output in an economical, high-value package, and it continues to surpass increasingly
stringent government emissions standards.