4th Gen F-body Exhaust Gas Recirculation (EGR) System


The EGR system is used to lower NOx (oxides of nitrogen) emissions caused by high combustion temperature and excessive oxygen. Adding exhaust gases back into the intake, displaces oxygen and decreases combustion temperatures.

A pipe from the RH exhaust manifold feeds exhaust gas to a port at the back of the intake manifold. An internal passage in the intake manifold feeds over to where the EGR valve mounts (lower, round hole). The EGR valve mounted on the back of the intake manifold is used to meter small amounts of exhaust gas (via upper, square-ish hole) back into the intake and on to the combustion chambers . Flow diagram


Vacuum is used to operate the EGR valve. Only a small amount of exhaust gas is allowed to pass through the valve. Too much exhaust gas can hinder combustion. The valve operates when the engine is warm and in the 900-2200 RPM range during crusing. Scan tools or programs will usually show when the valve is commanded open by the PCM.

EGR Control

Vacuum to the EGR valve is controlled by a solenoid valve that is pulse width modulated by the PCM. This modulation of ON and OFF many times per second controls the amount of time vacuum is applied to the EGR valve.

The PCM uses RPM and info from the following sensors to regulate the valve:

  • Engine Coolant Temperature (ECT) sensor
  • Intake Air Temperature (IAT) sensor
  • Throttle Position Sensor (TPS)
  • Manifold Absolute Pressure (MAP) sensor
  • Park/Neutral Position (PNP) switch
  • Vehicle Speed Sensor (VSS)

    For testing purposes, grounding the DLC output/field service enable terminal (1994-up), with the key ON and the engine not running, will operate the solenoid and allow vacuum to pass to the EGR valve.

    Negative Backpressure EGR Valve

    The 4th Gen F-body uses a negative backpressure EGR valve. The amount of exhaust gas is varied, depending on the amount of manifold vacuum and exhaust backpressure. This is why it is typical to get an EGR diagnostic code when the exhaust system is altered. Adding headers or removing the catalytic converter can create changes in backpressure. OBD-II has higher sensitivity to this and will "throw a code" more often than OBD-I will.

    The diaphragm on the EGR valve has an internal vacuum bleed hole which is held closed by a small spring when there is no exhaust backpressure. The PCM driven EGR solenoid controls vacuum to the valve.

    Engine vacuum opens the EGR valve against the pressure of a large spring. When vacuum combines with negative exhaust pressure, the vacuum bleed hole opens and the EGR valve closes.

    EGR Valve Identification

  • Negative backpressure EGR valves will have a "N" stamped on the top side of the valve after the part number.
  • Positive backpressure EGR valves will have a "P" stamped on the top side of the valve after the part number.
  • Port EGR valves have no identification stamped after the part number. If you have to replace a valve, compare the stampings to be sure you have the right one.

    Results of incorrect operation

    Too much EGR flow will dilute the a/f mixture and make the engine run rough or stall. Excess flow weakens combustion and may result in the following conditions:

  • Engine stops after cold start
  • Engine stops at idle after deceleration
  • Vehicle surges during cruise
  • Rough idle

    Too little or no EGR flow can allow combustion temps to get too high during acceleration and load conditions. This could cause:

  • Spark knock (detonation)
  • Engine overheating
  • Emission test failure

    Functional Checking

    With engine idling, opening the EGR valve should cause the engine to run rough or die. On the forward side of the valve there are openings where you can get your finger or thumb (or carefully with a screwdriver) in to press the diaphragm toward the back (opening the valve). Be careful. The area could be hot. If there is no change in engine rpm, the passages in the manifold may be clogged. This does not appear to happen very often.

    Next, (and if you cannot get in there to push on the diaphragm) you will want to check the valve with vacuum. You can use a hand vacuum pump (like a Mityvac or just your lungs) connected to the EGR valve to open it. The valve should also hold vacuum for at least 20 seconds, which should prove diaphragm integrity. A bypass hose from the EGR vacuum port to the valve could also be used as a vacuum source (engine would have to running). Again, the engine would run rough or stall if the valve opens.

    You can check that the solenoid is getting adequate vacuum by unplugging the vacuum supply hose at the solenoid and putting a vacuum gauge on it. There should be at least 7" Hg of vacuum at 2000 rpm. If not, make sure the hose has no leaks and check the vacuum at the manifold fitting.

    The following will test whether vacuum will pass to the EGR valve when the solenoid is operated:

    To check the solenoid, remove the vacuum harness, rotate it and reinstall so that only the EGR valve side is connected to the solenoid. Unplug the vacuum hose at the EGR valve and install a vacuum gauge in it's place. Install a hand held vacuum pump (ex. Mityvac) to the manifold side of the EGR solenoid. Jumper pins 5 and 6 of the DLC and turn ignition to ON (don't start). This will put the PCM in field service mode and energize the solenoid. Apply 10" Hg of vacuum with the pump and watch the gauge on the EGR valve side of the solenoid. It should read the same vacuum that you are applying. If not, you should check the hose from the solenoid to EGR valve for leaks or your solenoid could be bad.

    If your vacuum reads like it should, turn the key OFF. Vacuum at the gauge at the EGR valve end should bleed off (the pump gauge may/may not bleed off-not a problem).

    If you did not see the same vacuum at the gauge as on the pump, connect the pump to the EGR valve side of the harness. Apply vacuum and observe the gauge. The gauge should read the same as the pump gauge. If it does, your solenoid or hose connection is bad.

    The EGR valve can be removed and checked for excessive deposits that might hinder operation. Any particles that are dislodged should be removed, so they do not get into the engine or clog up the EGR valve.
    You can use a wire brush or wheel to clean the surfaces of the valve and manifold. If there are deposits in the orifices, you can use a screwdriver to remove them. A shop vac could come in handy.

    Fastener specifications can be found in this table.

    Keep or Remove?

    As previously mentioned, the EGR system can help control combustion and engine temperatures, reducing the chance of detonation. It does not make the engine run hotter because it is adding hot exhaust gases. The PCM will retard spark timing when enough detonation (spark knock) is detected. Therefore, it would seemingly be considered wise to allow the EGR system to work and try to prevent detonation from even happening in the first place. This is beneficial for the high compression LT1.

    Because most of it is at the back of the engine, it does not take up much room and can barely be seen. Removing it to "clean up the engine bay" hardly seems worth it. It does not operate at WOT (Wide Open Throttle), so there is no real performance enhancement for removing it, either.

    Some have speculated that the EGR pipe's proximity to the back of the intake manifold seal contributes to the infamous intake manifold leak. Excess heat there certainly does not help matters, but the pipe can be re-bent in some cases to increase the distance. Some heat wrap could also be used, but has potential as a fire hazard if not kept maintained.

    If you decide to remove it, both the pipe and EGR valve ports can be blocked off with plates. GM p/n 10054880 (known as the LT4 block-off plate) can be used to block off the ports where the EGR valve is removed. If you want to block the EGR pipe entry, you will have to get that from another source (there are several on the internet or you can make it yourself---late note: the GM plate is now discontinued). An EGR valve that is sealing properly and has the vacuum supply hose removed can function as a block-off plate. Just remember to plug the vacuum hose in the supply direction.
    Note: LT4 engines did not require the EGR system due to the cam design used.

    OBD-II cars usually do not like removal of the EGR system and will result in trouble codes. PCM re-programming to disable it's detection will take care of it. I have also heard of a couple other more elaborate ways to trick the PCM into thinking it is working.

    edited 5/8/2014

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