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Car revs up and down while idling

My 2001 Isuzu Rodeo Sport (65k miles, auto transmission) sometimes revs up and down (from 500 to 2000 revs/min) while idling in Park. When it does this, putting it in Drive causes it to shudder as if it's going to stall, but once it gets going, it runs fine.
<br/> This doesn't happen every day, but it doesn't seem to be related to the temperature. And it only happens after the car has sat overnight. I fear that this portends bad things to come. I imagine that the gears may be slipping, but am not sure. I have recently had a transmission flush, but that doesn't seem to have made any difference.
<br/> Any idea as to what might be wrong?


  • edited July 2007
    Sounds more like a vacuum leak or an engine sensor going bad. This doesn't sound like a transmission problem at all.
  • edited July 2007
    I agree. I'd look for a vacuum leak first. The transmission "flush" you had may cause problems down the road, but the transmission is not the cause of this particular problem.
  • edited July 2007
    Thanks for the response. So is there a way to make this happen consistently so I can show it to a mechanic? Otherwise I won't even be able to verify that the repair fixed anything (until some time later by when the mechanic will have absolved himself of all responsibility.)
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