DIY: Moderate
Rough Idle: Why Your Engine Shakes or Stumbles at a Stop
When your rough idle happens is the best clue. Cold only, warm at a stoplight, with the A/C on, or with a flashing check engine light each point somewhere different.
Stay safe first
- If the check engine light is flashing, the engine is misfiring badly enough to damage the catalytic converter. Drive gently to a safe stop and get it checked soon.
Rough only when the engine is cold
Likely causes
- Faulty coolant temperature sensor making the mixture too lean or rich
- Dirty fuel injectors
- Small vacuum leak that matters most at low temperatures
What to check
- Note whether it smooths out as the engine warms. If it does, the temperature sensor and vacuum hoses are the first places to look.
- Read stored codes. A coolant temperature code or a lean code narrows it quickly.
Rough when warm and stopped, in Park or at a light
Likely causes
- Vacuum leak (cracked hose, intake boot, PCV hose)
- Dirty throttle body or idle air control
- Dirty mass airflow sensor
- Worn spark plugs or ignition coils
What to check
- Listen for a hiss and look over the vacuum hoses and the intake tube for cracks.
- Clean the mass airflow sensor with a cleaner made for it. Other sprays can damage it.
- Check the age of the spark plugs. Old plugs are a very common cause of a rough idle.
Shaking in gear that smooths out in Park or Neutral
Likely causes
- Worn or broken engine mounts
- Worn or broken transmission mount
What to check
- If it shakes much more in Drive than in Park, and you may feel a clunk when shifting, suspect a worn mount.
- A mechanic can check the mounts by looking for engine movement as the transmission is put in gear.
Rough only when the A/C is on
Likely causes
- Dirty throttle body that cannot make up for the extra load
- Weak idle control
- Overworked A/C compressor
What to check
- Clean the throttle body first, which is a common fix.
- If the compressor makes a loud noise when it engages, mention that to a shop.
Narrow it down for your vehicle
The same symptom can have different likely causes depending on your year, make, model and engine. Describe what you are seeing and CaliperAI will help narrow down the likely causes for your vehicle.
Related guides
General information about this symptom, not a diagnosis of your specific vehicle. Safety-critical work such as brakes, fuel and airbags is best left to a qualified mechanic.