I've noticed that sometimes when in a parking lot with the engine running the engine will rev up a little, the RPM needle will jump from zero to 1, in a rhythym, and when I go to move in to a parking space it will try to lurch forward. What causes this? As it is still under warranty I guess the dealer can fix it. Is it just a minor adjustment? Andrea
If the aircon. is running, the compressor will engage (to cool) and disengage (when it's cool enough) in response to a thermostat. The load slows the engine, so there is (on most air conditioned cars) a system to compensate and make it idle faster when the compressor is engaged. That might be it.
Sorry to tell you, but this sounds like the first symptom of the dreaded electronic throttle module failure. Often cleaning the throttle module seems to fix the problem for a month or two, but it almost always comes back, and slowly the throttle gets more erratic.
I have 2000 S80, 90K miles. I am noticing a bit of idle problems, but in my case the engine RPM goes to about 400, then back to 700 - almost stalling the car. This happens every other day, once or twice. What I think is happening is that the transmission is not disengaging all the way, or starts engaging a little when the car is idling. This causes load on the idling car, taking the RPM down a little, then the engine control sees the low rpm and applies a bit more gas.. the transmission then releases and the rpm goes up to about 900, then back down to 700. I changed the transmission fluid about a year ago but did not flush the complete system.. which I will do soon. Hope this fixes this small problem.
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If it's in any position other than N or P the transmission is engaged, that's why the car will move without the brake on. If it normally idles at
400rpm in gear and 700 in neutral, that's probably too slow on both counts.Sorry, it idles 700-800 in D and in P or N, but sometimes drops to 400 when in D - and it's at that time I think the transmission may be engaging a little.. but just a guess. When this happens,the engine sees the idle is to low and try's to raise it.. the car may briefly want to lunge forward, maybe for 1/2 second, then the transmission disengages and all is OK.. until it happens again.
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They usually idle a little faster in P or N because that's the only time they are actually disengaged, except very briefly during gear changes. It's the torque converter which slips when parked in D etc and allows it to be engaged with the motor running... unless automatics operate differently now.
I'm NOT up to speed on torque converters vs transmissions.. to me they may be the same thing!.. but if I flush my transmission and put in new fluid, would this action perhaps cause the torque converter to NOT engage?
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Maybe, I haven't ever done one - something else always dies first on my cars :). There's more to it than engine oil or a manual g/b. there is fluid in the box and also in the t/converter - that looks like a big donut where the clutch would go. Without enough fluid it won't drive, but that wouldn't explain the revs dropping because it's meant to be trying to drive all the time. It would only do that if the converter was trying to lock solid for some reason rather than freewheel, which is less likely than something causing the engine itself to drop revs. Similarly the transmission itself would not slow the engine (parked in D), anything which might go wrong with that would likely let the motor idle faster. ...unless the converter is siezing, I guess that's possible but I've never encountered it.
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