Strange Idle behavior

[NJSHO]

So I think its a vacuum leak, but I want to see what the forum thinks.

Driving around the car is fine on power, but I get a buck on throttle lift and toe in sometimes when moving at highway speeds. The other problem is if Im rolling up to a stop light and push the clutch in, the rpms drop to ~1000, then rise to 1500 and sit there until I come to a complete stop. Then the car idles normally for 3-4 sec or so then the rpm's drop just a little (50 rpm maybe) and the car kinda chugs. Not like its going to die, but it does get perceptibly rougher and the tach needle bounces ever so slightly.

Whats the diagnosis?

 

[shoclown]

is youor intake manafold clean? maybe the iac is unable to do its job?

 

[sdpatt]

A vacuum leak characteristically causes a high idle speed because of the introduction of unmetered air. The O2 sensors and closed loop operation will add fuel to get the mixture right and the idle will be high. It sounds as if the VSS is sending a signal because the idle speed stays elevated while the car is moving.

You can run the KOEO test to see of the EEC can properly control the idle speed. That would tell if the IAC is working properly. I would concentrate my efforts on the IAC valve. That is, unless the "chugging" is an actual spark misfire. The ignition system would then be the primary suspect.

 

[Devin]

You can run the KOEO test to see of the EEC can properly control the idle speed. That would tell if the IAC is working properly. I would concentrate my efforts on the IAC valve. That is, unless the "chugging" is an actual spark misfire. The ignition system would then be the primary suspect.

How would I run this test?

 

[FiZiX]

It may also be worth checking if your EGR valve or EGR relay is having problems. Usually this will throw a code (but may not have) and can cause idle problems.

 

[SHO GoDz 89]

It may also be worth checking if your EGR valve or EGR relay is having problems. Usually this will throw a code (but may not have) and can cause idle problems.

Normally I would agree, but I doubt he has an EGR. I think only Cali vehicles and ATX's came with EGR's. I could be wrong though.


x2 on the IAC
I had a similar problem with my 89, turned out to be the IAC.

You can pull the codes by finding the "EEC Test" box under your hood (near the passenger side strut tower. There should also be a gray plug with it (or near it).

http://www.shophoenixproject.com/eec/eec.htm

This should help with the KOEO and KOER test. Just remember, any previous codes that the car had might SHOw (haha) up unless you clear them, so a KOER test is usually better as it gives real time problems.

P.S. Don't mean to hijack the thread, NJSHO, where in NJ do you live? I'm in central NJ in the East Brunswick area.