2000 Grand Prix SE. 3100 engine. 60,000 km. Two problems that may or may not be related.
One is rough idle especially when engine warm -- I think it's running lean. Rough idle is worse in Park or Neutral than in Drive. When on level ground in Drive, removing foot from the brake pedal results in surging while the car moves forward very slowly, but when pressing the accelerator the engine smooths out. Occasionally when stopped at a light, and the engine is first warming up after running for 3-5 minutes, the engine will bounce around and the whole car will shake for about 5 seconds while the RPMs jump around. Then it stabilizes.
No warning lights displayed on dash. Dealer has scanned for codes more than once but none found.
2nd problem is difficulty starting the engine. More often happens when engine is warm. Engine will start but then immediately stall. If I hold the key in the "start" position, it'll usually start after the 2nd "try" i.e RPM will drop really low after the initial start attempt, then it cranks some more and starts successfully. This has been getting gradually worse over time. But about half the time the engine starts perfectly on the first try with no problems -- RPMs stay up and slowly drop to idle level.
Tried the following already: Replaced fuel filter -- seemed to smooth out the idle for a short time only. Replaced spark plugs. Replaced PCV valve. Replaced battery. Fuel system cleaned by the dealer. Transmission flushed by dealer.
Any suggestions would be appreciated!