A rough idle is an engine that shakes, stumbles or hunts up and down in speed while stopped. The most common causes are a misfire from worn spark plugs or a failing ignition coil, a vacuum leak, a dirty throttle body or idle control valve, and fuel delivery problems. If the check engine light is on, read the code first; it usually points straight at the cause.
Rough idle symptoms
- Shaking or vibration felt through the seat and steering wheel at a stop
- RPM that drops, surges or hunts up and down
- Sputtering or stumbling, sometimes followed by stalling
- Worse with the air conditioning on or in gear
Common causes and the codes they set
| Cause | Typical codes | Clues |
|---|---|---|
| Worn spark plugs or a bad ignition coil | P0300, P0301 to P0308, P0351 | Steady shake; worse under load |
| Vacuum leak | P0171, P0174, P0507 | Hissing; high or hunting idle |
| Dirty throttle body or idle control | P0505, P0506 | Low idle; stalls with the AC on |
| Fuel delivery or injectors | P0087, P0201 | Stumble; hard starting |
| Dirty or faulty MAF sensor | P0101 | Hesitation and stalling |
| Cam timing problems | P0016, P0011 | Rattle at start-up; rough when warm |
| Worn engine mounts | None | Engine runs smoothly but vibration is felt in the cabin |
How to diagnose a rough idle
- Read codes and freeze-frame data, even if the light is off; pending codes can be stored before the light comes on.
- Check the misfire counters on a scan tool to see whether one cylinder or several are misfiring.
- Inspect spark plugs and coils, and swap a suspect coil to another cylinder to see if the misfire follows it.
- Listen for hissing and smoke-test the intake for a vacuum leak; check the PCV valve and hoses.
- Clean the throttle body and, on older engines, the idle air control valve.
- Run a compression test if one cylinder keeps misfiring with good spark and fuel.
Shaking at idle but the engine runs smoothly?
If the tachometer is steady, there are no misfires and the shake changes when you shift between drive and neutral, the engine mounts are the likely cause; see engine mount symptoms. A worn harmonic balancer can also cause vibration; see what does a harmonic balancer do?.
Is a rough idle serious?
A mild rough idle from worn plugs is not an emergency, but a misfire that gets worse can overheat the catalytic converter, and stalling at intersections is a safety risk. A rough idle with knocking, low oil pressure or a steady loss of compression points to internal wear; see engine knocking and compare engines by code on our engines page.
More guides: idle air control valve · fuel trim · throttle position sensor · engine sputtering · engine stalling.
Trouble codes: P2176 Idle Position Not Learned · P0508 Idle Air Control Circuit Low · all OBD2 codes.
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