I noticed recently when stopped at a light or in park, the tach goes up to about 1000rpm all by itself. Just wondering what thats all about. My last tuneup was maybe 5 years ago. Time for another one?

 

Could be a sign of a vacuum leak. Start checking hoses.

 

Does it go to 1,000 RPM and hang there, or does it cycle back and forth between the programmed idle (A4 - 550 RPM in gear) and 1,000 RPM?

 

It doesnt hang there. It goes back down.

 

Cycling back and forth repeatedly, or up once, down once?

Have you checked the throttle cable? Is it possible the throttle blades are sticking?

 

Up once, down once. Is say about every thirty seconds or so. I feel like when the car is in reverse, it happens more often.

 

Idle air control sticking? Any codes displayed? Try taking the air intake hose off and see if you still have the same idle?

94 Z28 A4 known mods... reworked ported heads/intake. Cam, 3" cat back flowmaster. unknown PCM tune. still researching car. ...

1990 RS HO 305, TH350(shift kit). 4 barrel carb, Cam, Long tube headers.

 

Might be a good idea to run a data log to see how the PCM is responding to the change in idle speed, or if there is erratic input from a sensor or actuator that is driving the RPM increase. Look at the Scan9495 data logging thread in the "stickies" at the top of the "Computer Diagnostics....." forum.

 

Haven't gotten a chance to check anything yet. I also noticed lately when cruising on the highway at 60, when I want to pass a car, I give some gas and the car shakes and hesitates. I ease off the gas and try again and it works ok. What's that all about?

 

Sounds like a vacuum leak but like Fred said, data log will tell a lot.

Old school thought - If you are good with a vacuum gauge, that might help too but remember, a modified engine's vacuum MAY differ from the normal readings from a unmodified engine. My engine pulls 10" of vacuum at idle (1000rpm) but is steady. Check this site out. Might help now or in the future but good info.

How to Use and Interpret a Vacuum Gauge

 

Looks like its the idle control. New part has been ordered. Would that also correct the issue with the car hesitating at times on the highway when giving some gas?

 

IAC? I don't believe so since this maintains an idle and makes adjustments when there is a load such as the A/C turning on but I am not positive. How did you troubleshoot it to the idle air circuit? They can get clogged and can be cleaned out. There is also a way to reset of the IAC that can clear up issues too.

 

Not sure how these things work. I brought the car into my mechanic. He said he disconnected the idle control and the car was idling fine after that.

 

Hmm, never tried that. Can someone chime in on the IAC valve going bad? I've had them gunked up before but always managed to clean out the passages and the pintle.

Did you test drive it with the IAC valve disconnected?

 

The IAC motor has coils that move the pintle in and out. Shoebox has a test:

http://shbox.com/1/iac2.jpg

.... and a reset procedure:

4th Gen LT1 F-body Tech Articles

The IAC only moves if the PCM tells it to move. There is no feedback to indicate to the PCM what the actual position of the valve is. A data log may show the PCM input that is causing it to reposition the pintle. Unhooking the IAC is just a band-aid to cover up a problem.