Low Idle Revs? What can I do about it? Is it a problem?

I have noticed the last few weeks that my car revs are going up and
down alot.

When I switch the engine on of my 98 S40 (157,000 miles!) The revs are
at about 1,200RPM but as soon as I stop for whatever reasons my revs
drop down to around 500RPM, they go up and down and I can feel some
shaking coming from the car and it greatly concerns me.

I am worried my engine is going to stall or won't start or anything
else that I can think of will go wrong!

What could be causing the problem and how can I sort it out - bearing
in mind I have no practical mechanic knowledge so I would have to go
to a garage but would like to know whats going on and what to do to
sort it.

Thanks.

 

Did the problem occur suddenly? Could be as simple as a vacuum hose that
cracked or fell off. Dirty throttle body, misadjusted or bad throttle
position sensor are also possibilities. I doubt it's anything major.

 

To be honest I am not sure when it started - I had some engine trouble
with the cylinders about a month ago which turned
out to be down to the coils - which were both changed.

Since then I have been noticing my revs moving about alot more than I
did before - so it may have been happening for
quite a while I just never paid attention to it before.

So I am much more worried when my car starts shuddering abit although
it may have been doing it for ages I just never knew!

I have run alot of miles so would a dirty throttle body be likely? I
am guessing its down to wear and tear that it gets dirty and the
higher the milege
the dirtier it could be? (although I am not even sure what a dirty
throttle body is!)

what is the worse that would happen if it was left alone? and how long
and how much would it cost to fix/check those possible causes you
mentioned? Thanks.

 

Well the throttle body gets icky buildup from the crankcase ventilation
system and over time that tends to clog up the small passage that lets
air by during idle. It's also possible that the throttle position sensor
is bad which could confuse it into thinking the engine is not at idle
thereby disengaging the idle speed regulation. I'm not sure how much it
would cost to have everything checked out but I always recommend having
things looked at at the first sign of trouble, it may be no big deal but
then again it may be something that will cost you a lot more down the
road if ignored. Little problems pile up when ignored and before you
know it a car is a beater.

 

I called Volvo and they told me it sounded like a dirty throttle body
and that it would
take 1 hour to clean and cost about £80! ($160!) which seems more than
it should be!

I have noticed over the last two days that my revs have NOT dropped
below
1000RPM when stationary, however, I have only driven 2 or 3 miles over
the
last few days and so it may not be a fair indication of if the problem
is still there!

I do wonder though does a dirty throttle body account for the RPM
moving about
alot some days and not at all on other days? or would that be
something else!?