What Causes Engine Idle Vibration When the Car is Warm?

[DelJohn]

The causes of engine idle vibration when the car is warm include: clogged idle air jets, malfunctioning idle control devices, weak spark in individual cylinder spark plugs, poor valve sealing in some cylinders, intake manifold leaks, incorrect ignition timing (either too advanced or too retarded), and improper idle adjustment. When experiencing unstable engine idle, the first step is to adjust the idle speed. If the issue persists after adjustment, check whether the idle jets and idle air jets are clogged. If the jets are clogged, they can be cleaned using gasoline or acetone and blown through with compressed air. With the engine running at a steady idle speed, listen for air leaks in the intake manifold or carburetor at the lower cylinder sleeve. If leaks are detected, they can often be eliminated by tightening screws or adding/removing gaskets.