erratic idle when slowing down(MTX)?

[JaySHOguy]

Guys, I have a little, uh, problem with my '89 SHO.
Sometimes after coming down to a stop, the idle will bounce between 1,000 and 1,500 RPM's for 20-30 seconds, then even out and drop to about 900.
Any idea what could cause this? Throttle is not sticking, and car runs and drives fine otherwise(besides running pretty rich, but I am not concerned with that).
Thanks for the help. pizza

 

[haydenm315]

I think the bouncing idle can be attributed to a rich running condition. Maybe the computer is making the car rev so it can burn the excess gas instead of stalling? You might as well clean the iac/iab valve. I've seen a number of people have fluctuating idle problems from a dirty one of those. Other than that... I don't know. Good luck with everything. Please let us know of your progress so others will be able to find it in a later search

 

[AutoSHO]

Try cleaning the MAF sensor? and the IAB? I would say that best guess - your IAB is bad. Just a guess, though, and cleaning the above 2 will most likely solve the problem.

 

[sdpatt]

Unless your oxygen sensors are not working your engine cannot run rich. Why do you think it is doing so?

The idle speed is governed by the air that is allowed to enter the intake through the idle air control valve. It could be that the feedback signal from the IAC valve is erroneous and confusing the EEC. This would be an electrical problem due to wear and age and would not be correctable by cleaning the air passage section of the IAC valve. You will need to replace the valve to correct this situation.

Another possibiilty is a vacuum leak, but this would also be apparent in other ways.

 

[JaySHOguy]

Running rich: Belches out clouds of black smoke on hard runs(mixed with some blue oil smoke occassionally) and my muffler tips will be completely carboned up within 3 days of cleaning them. The exhaust also smells stronger than other older vehicles that I have owned.

No codes, so I am assuming the o2 sensors are working.

I cleaned the MAF and the entire intake(including any and all sensors on the intake) about 2 months ago.
I will live with this until it gets to a point that it affects the driveability of the car.
Up until 7,000 miles ago, I have NO idea of the history of the maintenance on this SHO.

 

[PROPHET]

Where is this IAC valve?

 

[AutoSHO]

Circled here:

This picture belongs to Panama Pat. I added the Circle. hail

<small>[ September 04, 2003, 06:25 PM: Message edited by: AutoSHO ]</small>

 

[PROPHET]

Thanks alot Chris couldn't be any clearer. hail

<small>[ September 05, 2003, 03:06 AM: Message edited by: PROPHET ]</small>

 

[PROPHET]

Is the IAC valve the same on all the gen ones and twos are they interchangeable between the ATX and MTX computers?i ask because i have an extra one with an intake just not sure from what year.

 

[NotSoSlowSHO]

PROPHET:
Is the IAC valve the same on all the gen ones and twos are they interchangeable between the ATX and MTX computers?i ask because i have an extra one with an intake just not sure from what year.

no, they are not all the same as the early gen 1's had 'cleanable' AIBs. But I beleive that they are all interchangeable.

FYI, I just replace the AIB in my brothers Exploader and beleive it or not, the AIB looked identical... even had the same plug eek! shrug

 

[AutoSHO]

On second thought, I would check all your vacuum lines very closely. A small leak will only manifest itself under high vacuum, low flow conditions, basically, at idle. I would also check the fuel pressure if you have reason to believe its running rich.

 

[munkee]

It may not be the case with yours but the 91 I am driving right now is low on refrigerant and when I turn the defroster on, the cycling of the compressor will almost make the car stall(needs 60k) then rev to about 1500 at a stop sign.