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EGR operation 95 SL2

I have a question for the super-nerdy, need to know how everything works type. If this is you, read on, perhaps you can shed some light on the situation. I swapped the motor on my 95 SL2, that motor had the electrically operated EGR. The motor I installed is a 94 twincam with vacuum operated EGR. Unfortunately for me, the EGR mounting is different between the two model years. The head for the 95 is junk, so a head swap is at this time not the preferred option. Can someone describe in detail, the EGR process for the electrically operated EGR. The vacuum operated EGR I assume is controlled by the PCM, as it has an electric solenoid to switch the vacuum to the EGR. The check engine light comes on after I drive a short distance, with no egr connected. This is more of a nuisence than anything. Is it possible to "convert" to a vacuum EGR, and eliminate the check engine warning, short of installing a PCM and chassis harness from a MY94?

Reply to
eric.ellsworth
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Speaking in general - the vacuum operated EGR is driven from a vacuum sensing port in the throttle body. This vacuum signal is usually shut off until the engine is at operating temperature. The vacuum signal is also combined in a control valve with an exhaust back pressure measurement. The objective is to get EGR only when engine is warmed up and then at a rate proportional to the intake air mass-flow to the throttle body. Ultimately Exhaust gas recirculation reduces available oxygen (totally counter-intuitive) in the cylinder. That reduces peak temperature which controls pinging and reduces NOx formation.

The Electric EGRs are sometimes modulated solenoids and others are step motors. Some have a feedback device on the pintile position and others are completely open loop. Your '95 ECU is looking to control the EGR and is not seeing anything. Automatic tests that the onboard computers are getting devilishly smart at finding bypassed or defective emission control systems. At a most simple level, the ECU tests the EGR drive coil(s) for opens or shorts. Then it tries to move the EGR pintile (with engine warm) and detects a change in the pintile position feedback (if your motor used that) or looks at the O2 sensor for a corresponding change.

Given all that, probably best to use the EGR that was on the original motor, make an adaptor plate and plumbing adaptation. My guess is that swapping the ECU from the new motor will give you other compatibility issues. I hope someone can prove my assessment wrong.

Oppie

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Reply to
Oppie

Thanks Oppie for the detailed explaination. One more question, will the lack of EGR keep the engine running in open loop? The gas mileage isn't as good as I had expected

Reply to
eric.ellsworth

More than likely, the engine will run somehow in open loop - either rich fuel mixture or retarded ignition timing. Both of these are customary ways of dealing with malfunctioning EGR. Without any EGR, the engine will have a tendency to ping (without any correction like enriching mixture or retarding). The EGR is also considered a calibrated leak and contributes to the air/fuel mixture. Without it functioning, I would expect less than optimal fuel economy. Hope that helps

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Reply to
Oppie

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