Idle drops when stopping;pressing brake in park raises idle

jasnall · April 2012

I searched through the posts with "idle" in the title and have a good general idea of what can cause idle issues now but didn't find a good answer to my issue. My 90 Rt auto starts fine, warms up fine, and doesn't seem to "hunt" for idle. When I come to a stop from speed the tach drops to what looks like 0-200 RPMS and struggles to get back up to the 600-700 range unless I rev it or put it in nuetral. I would say this happens 70% of the time I come to a stop from 30+ mph and the car never stalls out.
Today in the parking lot I was sitting in park with the idle around 600 and noticed when I pressed on the brake the idle jumped to 950ish RPMS and slowly fell back to 600. This happened over and over again everytime I pressed and held the brake while in park. Overall this takes about 4 seconds to quickly jump to 950 RPMS then slowly fall to 600. If I tap the brake it doesn't quite jump to 950 but maybe 800 and falls back to 600 quickly. Any ideas?
I've noticed a rattle of sorts coming from whats seems to be the alternator. From what I've read could be that the bearsing have gone out.
Stock engine with AEM sri, ebay header, new fuel filter, new o2 sensor, recent timing belt and water pump.

Grifty · April 2012

I would check all your ground cables are nice and tight, it also wouldnt hurt to give them a quick sand to guarantee a good connection. Try that and let us know how it goes

driftin90 · April 2012

alternator or vacuum leak.

jasnall · April 2012

Alternator voltage checked out ok, 14.3 + or so at idle. I revved it and did all the tests with the lights on and the guy said the battery and alternator looked fine.
So next I guess I hunt for a vacuum leak or blame it on the idle control stuff.

Jimmy427 · April 2012

clamp off your brake booster vacuum and see if the problem goes away, if it does, replace the booster.

Idle drops when stopping;pressing brake in park raises idle

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