If your vehicle is showing rough, shaky, or erratic engine idle, here are the most likely causes and the parts you'll need to fix each one.
Worn spark plugs or failing ignition coils cause weak sparks, leading to engine misfires that are highly noticeable at idle. Improperly gapped plugs or loose coil connectors from recent maintenance are frequent installation errors.
Carbon buildup in the throttle body or a failing Idle Air Control (IAC) valve restricts and disrupts airflow at idle, causing the engine to stumble, stall, or surge.
Unmetered air entering the engine through a cracked hose or failing intake gasket leans out the air/fuel mixture, causing an erratic idle. Leaving a vacuum hose disconnected after an engine repair is a very common mistake.
If the engine RPM is steady but the vehicle cabin vibrates heavily at idle (especially when stopped in gear), the rubber mounts holding the powertrain have likely collapsed or torn.
The MAF sensor measures incoming air. If it is dirty or failing, the engine computer miscalculates the required fuel, leading to a rough idle and poor overall performance.
A stuck PCV valve or a torn PCV hose acts like a massive vacuum leak, drawing excess air and oil vapor into the intake manifold and disrupting the idle quality.
Injectors that are clogged or leaking deliver an uneven spray of fuel, causing individual cylinders to run poorly at low RPMs. Pinched O-rings during installation can also cause vacuum or fuel leaks.
Sludge buildup or low oil pressure can cause VVT solenoids to stick, advancing or retarding the camshaft timing incorrectly at idle and causing a severe, stalling shake.
Burnt valves, worn piston rings, or a blown head gasket cause a loss of compression in one or more cylinders, resulting in a permanent, severe misfire and rough idle.