How do I correct a bouncing idle? On a cold start the engine goes immediately to about 1,900 rpm, as soon as it warms up it drops to around 1,200 and then to around 800 and begins to bounce between 800 and around 1,400 rpm. It does not stay at the high or the low end for even a second - just keeps bouncing.

The car ('90 5.3L) has been laid up for months while working on an ABS problem (ABS Warning light continuously on), the only time it has been driven in months has been a couple of test drives to check the brake progress. Only the ABS system has been apart.

I have tried to do some searching in the archives and i have found a lot of information related to the AAV, but that all seems to be about high idle and not "bouncing" idle? Anyone with any ideas appreciated?

Greg L.

 

It's the AAV. Try adjusting the bolt on the side to reduce the amount of air it lets in. When the idle goes too fast there is a fuel cut, the idle drops, the fuel comes back on, the idle goes up and the cycle repeats.

 

What happens is the RPM climbs until the fuel (injector) cut-off kicks in. Then the RPM drops. Then the fuel is turned on again, and the RPM increases....and the cycle repeats and repeats.

The high RPM is caused by extra air coming from....somewhere. A stuck open AAV is common but it could be a vacuum hose having come off, air cleaner housing bolts loose.....

Remove the LH air filter but leave the base in place. Put you thumb over the (approx. 1-inch) hole in base for a few moments. Does the cycling stop? if it does, your AAV is stuck.

Cheers
DD

 

I had similar at one stage and my problem ended up being misadjusted idle microswitch on left hand throttle linkage.

 

Doug and Jagboi64,
thanks for the AAV advice i will try this and reply with the results

 

Thanks Baxtor, this makes me think. As i mentioned, I have been working on an ABS problem during which I replaced the valve body under the reservoir. to do this without removing the reservoir, i had to loosen the throttle linkage on the of rear of the left manifold. When done I simply replaced the clips and the one linkage piece that was taken loose. Could this have caused my problem?
Greg

 

Hunting up/down is usually a vacuum leak, easy to put the AVV out of the equation, when it's doing it block the AVV's air intake, if it still does it then it's a vacuum leak if it stops then it's AVV related.