I have a 1999 Saturn SL1. Right after a cold start the engine runs OK with no apparent problems or roughness. Once warmed up, the car idles really rough in gear (Drive), RPM really starts to fluctuate between 500 and 700, the car kinda shakes, and the engine is likely to stall if I leave it in gear (and with car not moving) for much longer. If I immediately shift to Neutral at this point (bacially I have to shift to neutral every time the car is stopped) the car would not stall but RPM would surge briefly to 2000+ before going back to normal (900 ish). The problem started suddenly one day after a long (3+ h) drive. Prior to that the car ran smooth. I’ve replaced spark plugs, plug wires, EGR, PCV, IACV, exhaust pipe, catalytic converter, and cleaned the throttle body, without resolving the issue. My code reader sometimes reads a ‘cylinder 4 misfire’ code. What could be the problem?
Right now Im guessing maybe a bad O2 sensor or sensor connection, a vacuum leak, or just really worn-out leaky engine cylinders (less likely). What else can it be? Suggestions appreciated