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?

 

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

** Posted from http://www.teranews.com **

 

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

 

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

** Posted from http://www.teranews.com **