Its an '89 Iroc with L98. I was setting the timing and noticed the EGR selonoid was disconnected. I owned the car for 2 years and I know I didn't leave it off. However, since I first got the car it shakes kind of roughly at idle even though the tach only bounces around by 50 RPMs or so. When it gets hot like around 200 degrees it starts to shake even worse and the idle drops some. When I set the timing I noticed that when it shook the timing jumped around a bit. Well I reconnected the selonoid and it didnt help. The funny thing is that I never got a code. I know the SES light works because I had the car in diagnostic mode to pull the codes and all I got was a 12 which is normal. On a side note I had the IAT sensor disconnected to and I got no code but I do get one for bypassing the EST to set timing, MAF, etc.. I am using the stock PROM. I was thinking that the EGR was stuck but the diaphram can be moved but it is tight. I couldn't get my fingers under the plenum to try and move it while the car was together and running though. The vac lines from the TB to the selonoid to the EGR look good and I sprayed all around with carb cleaner and the idle was not affected. The car has fresh plugs, wires, and coil. The original engine only has 60K miles on it and has good power and does not burn oil..no blue puff on startup eighter. It is a summer only car and sits in a garage during the winter. It is started regularly and the stabilizer is added to the gas.
Well my questions are:
1)Why no SES light when the selonoid was disconnected for so long? (Yes I do do a lot of highway driving so the EGR should have been called upon)
2)Why does the car shake at idle?
3)Why does the timing jump when the car shakes?
4)What can be effected by letting the car sit the 4-5 months for winter? Injectors..?
Thanks!!

 

1) I haven't had an EGR or solenoid for nine months, no codes at all.
2) replace your o2 sensor
3) the car is shaking because of the timing, not the other way timing is rpm based
4) only way to tell is to perform a couple of fuel pressure tests, but yes I'd say gas sitting in the injector could erode the internal parts if it goes bad

I'm just guessing on the o2 sensor, but it's about time so that could eliminate one thing and hopefully help how your car runs. While you're at it I'd replace the MAF relays and coolant temp sensors.

 

I replaced the O2 sensor and the fuel pressure is 47psi with vaccume disconnected. I have replaced the relays and got a brand new CTS from GM dealer already installed.

 

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I am looking into the same exact problem and I am going to replace my computer soon because I have tried everything you have and more -- (Iac CTS valves timing fuelpressure ect, ect)
only thing I havent replaced is the computer.

 

bump

 

Try some suggestions found in my post here:
https://www.thirdgen.org/techbb2/sho...threadid=83494
I have many of ur symptoms and I am now leaning toward the ECM. Just waiting for some more feedback.