Volvo 850
 

When the engine gets to running temperature, the revs are erratic. It starts and runs well until it gets hot. I've had the air filter, sensor & the pipe work replaced. 2 mechanics are at a loss & the A.A. couldn't find a code to identify the problem. The warning light on the dashboard is on. This has happened twice in the last 6 months, the first time it backfired and I then couldn't maintain the revs at all. I've had the car serviced and the spark plugs replaced, as far as I know there's no dampness in the system. I'm not mechanically minded, a pensioner, & I need the car as I have mobility issues. I would be grateful for any suuggestions as to the potential problems, any suggestions gratefully received.

well this is an early OBD1 or OBD2 car so there's not the same level of electronic monitoring as a newer car. The common sense starting point is to check out things that vary with temperature or control the fuel mix. I'd start by measuring the ECT sensor cold and then again when warm (or just do the simple thing and replace the sensor and thermostat as preventative maintenance). Did they check for other temp controlled devices like the intake air preheater door? What codes came up when you last scanned for the check engine light? Cold engines prefer a rich mixture but that will foul a warm engine so look for things that may mess with the mix such as the front O2 sensor. Has the MAF sensor been serviced? You can clean it with some MAF cleaner (don't use carb cleaner as its too harsh) to see if that helps. Any signs the MAF has been replaced already? Its possible a PO installed the wrong part so if it looks new, check the part number/type.

Curious - is this a turbo or NA engine? You may want to visit Volvotips.com for some trouble shooting guides.

Makes you wonder if the car has a big purge system leak. The purge system turns on and off, so that might cause part-time symptoms like you are seeing. The rubber hoses in the purge system, pcv, and whatever vacuum users are pretty bad after 25 years. The vinyl tubing elbows seem like goo to me.