Low RPM when engine is warm. 2.0L Petrol
 

My Volvo S40 MK1 2.0L Petrol (BS420S engine - year 2000) is having an idling problem. When in a cold start it runs fine and idling RPM reaches usually 800-900 which seems normal.
However after running few miles or if let id idle until engine get warm, RPM drops to 200-400 range. Sometimes engine stops. In traffic lights, its a chaos.

Some steps I've taken:
- No Error codes when scanned.
- Clean and fitted the existing MAF sensor, Idle air controller value
- Checked for Air intake leaks with soap water
- Cleaned the throttle entrance
- Current spark plugs are only 1 years old
- Fuel filter replacement should be done, having an old filter for last 3 years.

Have anyone experienced similar category of problems? Any guidance or clue whats making the idling bad when engine got warm? I'd like to check and report if someone can help me with any proceedure. (Staying home due to social distancing :)

Thanks.

most idle issues have to do with air intake leaks or a MAF / fuel metering/pressure issue. Since it runs ok cold, I'd start by inspecting all vacuum lines (not just the air intake boots) and consider having a smoke test done. Next, considering your car behaves different at temperature is to test or replace the engine coolant temp sensor. Does the temp guage behave erratically or read different from normal? have you measured fuel pressure at the rail? Any check engine codes present?

Really thanks for the reply. Let me check these at the weekend and get back to you. Only difficulty I'm facing now is making a smoker. Anyway I'll look in to this.

Could be the upstream O2 sensor (aka Air/Fuel sensor). When you start the car, the ECM is supplying an extra-rich mix of fuel and air, but after it warms up, the ECM switches over to using the upstream sensor to set the amount of fuel injected. I always consider O2 sensors suspect over 100,000 miles (160,000km).

Thanks for adding a really good clue. I'll add this to the list. First I'll check the existing one for any bad performance when warm.