88 coupe, L98, no engine mods

Purchased about 4 months ago and have been fighting a rough idle. Had it smoked and found two vac leaks. One was at the rear of the plenum where the booster line attaches. Tightened it up and that took care of that. The other was at the base of the passenger side runner close to the front of the intake. New gasket fixed that one. Still had the rough idle.

Reset min idle and TPS about 1002 times. Used winaldl to adjust for low IAC counts, did this about 957 times. Checked timing about 523 times

Still had a rough idle when stopped at red lights and stop signs. Never could get idle speed to be consistant. (I should have payed attention to this)

Cleaned MAF and throttle body, replaced MAF relays, plugs, wires, dist cap, etc... Checked resistance on all injectors, listened to them all fire in a smooth sequence.

Fuel pressure good. Ran a can of SeaFoam thru the tank.

Nothing made any difference.

What the heck. Lets change the IAC. Changed it, reset min idle, double checked timing, set TPS. Disconnected battery. Had lunch. Hooked battery up and went for a drive.

WOW! It's night and day difference! I'm really impressed.

The old IAC was locked up tight. Winaldl was reporting what the computer was saying, not what the IAC was really doing. Thats why I never could get it stable.

Its not 100% smooth like my wife's Mercedes, but probably as good as a 22 year old car is going to get.

Now to replace the brake hoses. I think the one on the driver's side front has failed internally. Caliper won't release all the way after brakes get used a bit. Let the car sit a while and then rotor is free. Might be crud in caliper. Just gotta take it apart and see.

Expecting some snow this afternoon and tomorrow so won't do it until next week.

Thanks to everyone that participates in this forum. I have learned quite a lot here.




Wayne

 

I'm glad you've finally found the problem.

On the L98 engine one of the First things to check for a rough idle, is the IAC valve and passage way.

 

Glad to hear you got your issue taken care of,

I am fighting a idle issue as we speak, I think it is a vacumm leak somewhere, But I am in the middle of doing my Brakes, Once I am done with the brakes I will continue back at idle issue...


Ant,,,

 

I learned the hard way!!

 

Glad you found the problem.

 

How did you set Minimum Idle when the old one was locked up? You might want to check compression; compare #5 & #7.

 

Good job Sherlock!

 

Right on, I have been preaching the IAC and idle issues for a while. The IAC, through GMs wisdom, will not ever throw a code. The datalogger that is used only shows what the ECM is telling the IAC to do, not what it is actually doing. Why, I dont know, that is the way GM programmed it.
And, if the IAC is faulty, it is a major vacuum leak.
Do a check on IAC this way. Resistance check.
http://shbox.com/1/iac2.jpg

 

Thats just it. I never could get it right. Being a new owner, and not very mechanically inclined, I didn't know if it was me or the car. I just could never get it to do what it was supposed to do. The instructions said that it may be hard to make it idle after unplugging the iac. It always idled with no problem, just rough. I could back the screw out until the plates were completely closed and it still idled around 400. Stupid me couldn't add 2 and 2 and come up with IAC.


Yall are amazing here. You have been a lot of help.

Thanks

 

I have a bonar-fried IAC tester with a IAC noid light.
It checks the computer signals with the noid light, and you can raise or lower the RPM with the up-down buttons to check the IAC operation without removing it....

saweeet!

 

Resistance on A-B 49.5 ohms
Resistance on C-D 51.2 ohms
Open A-C and B-D

The pintle won't move with either 12v applied or physical pressure. It is extended about 1" from gasket mounting surface.

Thanks

Wayne

 

correct me if I am wrong but I understood the ecm only records what the IAC is doing. It doesn't actually tell it to do anything. That's why it doesn't throw a code. because it is not doing anything contrary to the ecm signals. By it only recording what the IAC is doing the rest of the engines reacts to it. If it is stuck closed like mine, it won't allow air in for idle until I open the throttle body with the accelerator.

i'm still trying to learn about these things. Way different than anything I ever worked on.

 

Thanks for this.
Does it apply to an '89 as well?

 

Yes