HELP: 99 S70 Idle RPM oscillates / hunts

Recently my 1999 S70 GLT engine IDLE RPM starts to oscillate/hunt
between around 800 and 1300 and Check Engine light is on. A local
technician said there is a faulty code with speed sensor and it costs
$95 parts + 3 hours of labor. After spending $400, the RPM surge issue
remains. Can someone suggest the possible cause of this problem ?
Also, anyone knows a reputable Volvo shop around San Francisco Bay Area
(I am in Hayward, CA but will drive for a good mechanic

Please help. Thanks !
Ming

 

I assume GLT, that this is a turbo engine.
The throttle plate might be gummed up with oil vapors.
It is like the flue in a fire place. At Idle the plate is almost
closed. The PVC systems sends oil vapors into the engine
past the throttle plate.Hot oil vapors will collect on the
cooler throttle plate. Over time the vapors will build up and
restrict the air flow at idle. Idle will go below spec and the brain
opens up the throttle unit to let more air past. Too much air, or higher
idle the brain closes the t-plate. This is how or why the idle surge
takes place. Cleaning off the oil junk residue from the throttle plate
can restore smooth idle again.
It can take 2 to 3 hours to remove the throttle plate to clean it.
Sometimes cleaning the throttle plate can cause it to fail very soon.
If the throttle unit is origional, it may be starting to fail.
Sometimes updating the software can cause it to fail.
All of the above is because it was about to fail any way.
Something else you can try or have your repair shop try.
This may or may not help.
In the center of the intake manifold, below the fuel rail cover area is a
black vacuum line. Cut off the steel band and carefully remove the
vacuum hose from the brass nipple. If you cut the vacuum line, there
is enough hose to cut the hose end off and put it back on latter.
Get a 3 to 5 inch piece of rubber hose to go over the exposed nipple.
Probably 5/16" or 3/8" hose will work. Push the short hose piece
onto the brass nipple. Get a can of Carb cleaner. Put the plastic
straw on the can place straw into the short hose.
Put a rag around the straw and short hose.
Spray carb cleaner into the intake for about 5 or 10 seconds.
Do not let the straw blow off of the can into the intake.
Let the cleaner set for 2 or 3 minutes. This hose area is almost
top center of the throttle plate area. This will help to disolve
the oil junk on the throttle plate.
Install the removed vacuum line. You can use a regular plastic tie
strap to secure the vacuum line onto the brass nipple.
Do this cleaning outside in an open area.
Hold the gas pedal to the floor and start the car.
It will not even try to start at first. Too much carb cleaner being
sucked into the engine. It might take 10 to 20 seconds of cranking
for the car to try to start up. It will stumble really badly when it does
try to start. Keep the pedal floored untill it does start and then maintain
some rpm's above ldle for a few seconds or so untill it smooths out.
It will blow a lot of white smoke out the tailpipe.
This is the carb cleaner burning off. Try not the breath the vapors.
If the ilde surge is due to a dirty throttle plate the above cleanig method
should help.

BD

 

I would check to make sure the air intake system is sealed up, such as all
air cleaner snaps engages, no loose hose connections. I would look over
every item carefully when the engine is running.

Also, try brickboard or swedespeed for similar posts.

 

That's the first sign of impending Electronic Throttle Module failure.
Click this for lots of interesting reading.
http://groups.google.com/group/alt.autos.volvo/search?hl=en&group=alt.autos.volvo&q=ETM

--
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 had the same problem with my 99 S70 T5. The Electronic Throttle Control
System is the problem. Parts and labor ~ $900. This was done twice in 18
months.