car revs on its own?
 

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?

Re: car revs on its own?
 

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

Re: car revs on its own?
 

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?

Re: car revs on its own?
 

It doesnt hang there. It goes back down.

Re: car revs on its own?
 

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

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

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

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?

Re: car revs on its own?
 

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

Re: car revs on its own?
 

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?

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

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?

Re: car revs on its own?
 

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.

Re: car revs on its own?
 

If he left it like that, he is no mechanic. As Fred suggests, there may be something else that is causing the IAC to react.

Re: car revs on its own?
 

I havent had a chance to take the car in yet. What are the other possibilities that would cause the issue?

Re: car revs on its own?
 

I just helped someone on another site, similar problem (except it was revving up as high as 1,800 RPM). We went through the data logs, couldn't find anything. He gave up, replaced the Opti - problem solved.

Re: car revs on its own?
 

I was afraid you might say that. I had an issue with the car bout 5 years ago, it would hesitate on the highway when I would give gas. I replaced the opti and plugs and wires and all was well. How often do these cars need tune ups?

Re: car revs on its own?
 

Look through these threads - similar problem:

https://www.camaroz28.com/forums/lt1...t-work-879506/

From bad to worse

In the second link, go to post #30 on page 3:

Re: car revs on its own?
 

What brand Opti did you use for the replacement? How many miles on the replacement at this point in time?

Re: car revs on its own?
 

I believe the opti is an AC delco. I put maybe 15000 miles on it.

Re: car revs on its own?
 

OK. The guy who solved his surging problem with an Opti replacement had an MSD Opti when the problem cropped up, and it happened right after MSD had rebuilt the unit due to previous problems. He replaced it with the Delco and the surging disappeared.

Does your tach display a little "twitch" just before the surging starts?

Re: car revs on its own?
 

actually, it does twitch before the surge. It looks like it does a little bounce up and down before it goes up.

Re: car revs on its own?
 

Replaced the IAC. problem still occurs. Guess it's time to order a new opti. What's a good site to order that from?

Re: car revs on its own?
 

UPDATE:

Finally put in new opti and all is well. Thanx for the suggestions.

Re: car revs on its own?
 

Hope you put in an ACDelco or Delphi opti.

Re: car revs on its own?
 

Thanks for the feedback!

This is the same result on three different cases on two sites that I frequent - all three in the last three weeks. The other two were traced to MSD Opti's. Did you verify that your old, problem unit was an AC Delco?

Re: car revs on its own?
 

The old unit was an AC Delco. I dont understand any of this mechanical stuff really, my mechanic said the bearing in the distributor cap wouldnt turn so well. Not sure what that means. Im just happy its all good now!

Re: car revs on its own?
 

The bearing is in the base of the unit. But it supports the drive shaft and the shaft the rotor, and the optical shutter wheel for the cam position sensor spin on.

Re: car revs on its own?
 

Had a similar problem and read through a few posts. Cleaned the iac, cleaned the tps, oiled the throttle blades and didn't get it fixed until I cleaned the pcv valve.

Re: car revs on its own?
 

I noticed last night the rpms were at 1000 when in park or stopped at a light. Is that normal?

Re: car revs on its own?
 

The stock programming for idle on an A4 trans is 550 RPM in drive and 650 RPM in park/neutral. The stock tach typically reads 100-200 RPM higher than actual engine speed. To get accurate engine speed, you have to read the PCM with a scanner.

When you are slowing down, approaching a light, the idle will stay slightly elevated. When the vehicle speed sensor indicates the car has stopped, it should drop to the numbers shown above.

Is it steady at 1,000 RPM, or is it cycling up and down again?

Re: car revs on its own?
 

Its steady at 1,000RPM. I just dont remember it being like that.

Re: car revs on its own?
 

You may have sticky throttle linkage, or blades, or a problem with the IAC valve. The latter problem is where a PCM scan comes in handy, because you can see the exact RPM, and see what the PCM is telling the IAC valve to do to correct an idle that is too high.