Engine idles rough or shakes at a stop

What this usually means

At idle the engine should feel smooth and hold steady RPM. A rough idle — vibration through the seat, RPM that bounces up and down, or a stumble that almost stalls — means one or more cylinders aren’t burning fuel cleanly, or the engine isn’t getting the steady air it expects. Because idle is when the engine is most sensitive to small problems, a rough idle is often the first warning sign of a fault that gets worse under load.

Most likely causes

• high
  Worn spark plugs or ignition coilsThe most common cause. A weak spark makes a cylinder misfire, which is most noticeable at low RPM.
• high
  Vacuum leakA cracked hose or failing gasket lets in unmetered air, upsetting the air-fuel mix and causing a high or unstable idle.
• medium
  Dirty throttle body or idle air controlCarbon buildup restricts the small airflow the engine uses at idle, causing surging or stalling.
• medium
  Clogged or failing fuel injectorA partly blocked injector starves one cylinder, producing a steady miss at idle.
• low
  Dirty mass airflow sensorA contaminated MAF mis-reads incoming air and can make the idle hunt up and down.

Is it safe to drive?

Typical fix & cost

Most rough idles are cured by a tune-up — new spark plugs and, if needed, coils — or by finding and sealing a vacuum leak. Throttle-body cleaning is cheap and often effective. Confirming which cylinder or system is at fault first avoids replacing a full set of parts when one component is to blame.

Related OBD-II codes

If your car has stored a trouble code, these often accompany this symptom:

Related symptoms