EGR Part II
1985-1989 V8 TPI:
These systems use a typical EGR valve with one exception. There is a Diagnostic Temperature Sensor threaded into the base of the EGR Valve. Because these cars are equipped with a MASS Air Flow sensor, it uses this temp sensor to detect changes in temperature. Primarily, it is looking for a sudden rise in temperature when the EGR valve is opened. Everything is the same as mentioned in Part I, with the addition of the temp sender. It is wired directly to the ECM, and serves as an EGR self diagnostic only. Some potential problems that would cause a code on these systems are: Ruptured EGR diaphragm, broken vacuum lines, defective or plugged EGR Solenoid, poor connections to the solenoid or diagnostic switch, or a failed diagnostic switch.
These systems use the same EGR valve for the previous V8's, except the deletion of the Diagnostic Temp Sensor. Because these systems are Speed Density (using a MAP sensor) it can detect EGR operation in another way. Usually the EGR diagnostic is run on a long decell. When the vehicle is in decell, the EGR valve is closed. The ECM begins to monitor MAP sensor voltage very closely, and will cycle the EGR Solenoid to open the valve fully and then closed again and watch for the sudden changes in MAP sensor voltage, due to the introduction of exhaust gasses into the intake. Failures for these cars are similar to those on the previous V8's, except the temp sensor, since it is deleted.
1985-1989 2.8L V6:
Once again, we are dealing with a Mass Air flow system. Because of this, we once again use a diagnostic switch. This one is not like the V8 diagnostic switch. This one is based on assumption, and is not as accurate as the temp switch. The V6 uses a vacuum switch that is basicly "Tee'd" with the EGR valve. When the ECM commands the EGR solenoid to allow vacuum to the valve, it also goes to the diagnostic switch. The ECM assumes that if the diagnostic switch is getting vacuum, then the EGR valve is too. This is usually a correct assumption, but it doesn't mean the Valve is working! Because this is also a diaphragm type EGR valve, it has similar failures encountered as the V8 systems, but with the vacuum switch, you also need to look at it. It is not unusual for the diagnostic switch to go bad. They usually fail always "open" telling the ECM that the valve is never getting vacuum, or they fail "closed" telling the ECM that is is always getting vacuum, either way, it will set a code. Another failure is the Vent on the solenoid/switch assembly. It can get clogged. If it does get clogged, vacuum will be trapped in the switch. This will tell the ECM that the valve is open, and it will be! This can cause rough idle and tip-in hesitation because the solenoid cannot release the vacuum built up in the EGR Valve, and it gets held open.
1990-1992 3.1L V6:
This system is totally different. It does not use a diaphragm type EGR Valve as used on previous engines. It uses a Digital EGR Valve. It has three solenoids, each one mounted above a different size orifice. These solenoids are "on/off" solenoids, so when a different amount of exhaust gas is required, it closes one solenoid, and opens one with a bigger orifice. These cars are once again Speed Density, so it relies on the good 'ol MAP sensor for diagnostics. During the diagnostic, the ECM will cycle each solenoid once, and watched for the MAP sensor voltage to change more and more, since each solenoid lets more exhaust gas to pass through. There are many failures inherent with this system. For one, these solenoids are exposed to hot exhaust gas, and then cooled back down when EGR is no longer needed. This causes excessive thermal cycling and weakens the springs over time. Sometimes the springs can get weak enough to let exhaust backpressure push the solenoids off their seat. Other problems include the solenoid themselves, they can just quit working and never open, or a chunk of carbon can get in there and hold a solenoid open. Another problem is carbon buildup in the Intake Manifold. Carbon gets accumulated in the EGR passage (because hot exhaust gas is allowed to cool in the intake) and restricts it. This restriction will cause only a small drop in MAP sensor voltage, regardless of which solenoid is opened, and will set a code.
1993-1997 5.7L LT1:
The EGR setup on the LT1 is very similar to the late model TPI systems. It uses a vacuum operated EGR valve, EGR solenoid and related vacuum lines. The PCM commands the EGR solenoid on using varying Duty Cycles based on current engine conditions, which causes varying vacuum levels to the EGR valve. Diagnostics are the same as late model TPI engines. One very common failure for the LT1 EGR system, is for the spring inside the EGR valve to become weak, allowing the EGR valve to open prematurely, causing a low speed stumble. Some LT1's had a recall on the EGR valve as a result of this.
1998 & later 5.7 LS1:
The EGR valve used on the LS1 is completely different than all previous versions. The LS1 (up until 2000, 2001-2002 LS1 engines didn't have EGR) uses a Linear EGR valve. Its a pintle attached to a stepper motor and the PCM can command anywhere from 0% to 100% pintle position. Diagnostics are the same, minus the need to inspect a seperate solenoid and vacuum lines, however, one extra feature is built in to the valve, and thats an internal position sensor. The PCM is capable of seeing actual pintle position. Fault codes can store when the PCM commands a certain position, but sees that the actual is somewhere else.
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