If your vehicle is showing rough, shaking, or bouncing 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 incomplete combustion in one or more cylinders, leading to a misfire that feels like a stumble or shake at idle.
A cracked hose or leaking gasket allows unmetered air into the engine, leaning out the air/fuel mixture and causing an erratic, bouncing idle. Hoses are frequently left disconnected after recent engine work.
Carbon buildup in the throttle body or a failing Idle Air Control valve prevents the engine from smoothly regulating airflow at idle, causing RPMs to dip and the engine to shudder.
If the idle RPM is steady but the car vibrates or shakes heavily (especially in gear at a stoplight), the rubber mounts holding the engine or transmission have likely collapsed.
A contaminated MAF sensor incorrectly measures incoming air, causing the engine computer to inject the wrong amount of fuel. This sensor is often accidentally unplugged or damaged during air filter replacement.
Partially clogged fuel injectors or low fuel pressure cause an uneven spray pattern, resulting in a weak combustion cycle and a shaking engine.
Worn piston rings, a blown head gasket, or burnt valves cause a loss of cylinder pressure, resulting in a permanent, heavy misfire and rough idle.