My S80's engine rev's up while idling

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
http://www.andrearogers.com

 

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.

--
Mike F.
Thornhill (near Toronto), Ont.

Replace tt with t (twice!) and remove parentheses to email me directly.
(But I check the newsgroup more often than this email address.)

 

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.

sc

 

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.

sc

 

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?

sc

 

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.