I have a 2001 Volvo S60 T5 Turbo with 145K on it, lately when at a stop, in Drive, my idle fluctuates, dipping below 1K RPM where it normally is, fluctuating between 500-1K RPM. Note the spark plugs have nearly 100K on them, but otherwise, car runs great (besides lots of oddball suspension noises)
The first thing to check for is a defective Idle Air Control/Air By-Pass valve. This device is what controls the engine idle speed anytime the accelerator is released. These valves can fail where they hunt for the proper position for the idle condition. And this causes the idle speed to fluctuate while sitting at a stop.
This is also what you will get with a small vacuum leak and at the age of this car, any of the vacuum lines would be suspect. You could pop the hood and take a close look around at any bits of small, usually black tubing and plastic bits around the engine compartment. Sometimes you can get lucky and just happen to spot something - disconnect, break, crack or whatever.
Thank you very much, will check these out. After years of listening to Car Talk and hearing about the website, this is the first time I’ve actually spend time here, and its great. Thank you!
If you are running the Volvo spec plug, I believe Volvo recommends new plugs every 30K miles. New spark plugs could be in your future. This motor also requires new plug wires, cap, etc. every 30K miles. Once these items are up to date on maintenance and you have a clean air filter, if the problem persists it could be a dirty MAF sensor.