A car that vibrates at idle may have worn spark plugs, bad motor mounts, dirty fuel injectors, or a clogged filter. Have it checked.
Common causes
- Worn or fouled spark plugs and misfires.
- Broken or worn motor mounts.
- Dirty fuel injectors or a clogged air or fuel filter.
- Vacuum leak.
- Low or old engine oil.
- Dirty throttle body.
Quick checks
Does the check engine light show? Does it shake more in Drive than in Park? That points toward the mounts.
What to do
Have a mechanic read the codes. Ignoring a misfire can damage the catalytic converter.
Engine related causes
- Worn or dirty spark plugs, leading to misfires.
- Faulty ignition coils.
- Dirty fuel injectors or fuel filter.
- Vacuum leaks.
- Dirty throttle body or idle air control valve.
- Failing mass airflow sensor.
Motor mounts
Engine mounts hold the engine to the frame and absorb vibration. When rubber cracks or fluid leaks, vibration passes into the cabin, often worse in Drive with the brake pressed.
Exhaust and accessories
- Loose exhaust components or heat shields.
- Worn serpentine belt or pulleys.
- Failing A/C compressor, which can increase vibration when the A/C is on.
Transmission and driveline
Low transmission fluid or worn parts can cause shaking, especially in gear.
Check engine light
A flashing light signals a misfire that can damage the catalytic converter. Get it checked quickly.
Quick checks
- Note when it vibrates: in Park, in Drive, with A/C or after warm up.
- Read the codes with an OBD-II scanner.
- Check oil level and engine air filter.
- Inspect belts and the engine bay for loose parts.
Cost range
Spark plug replacement costs about 100 to 300 dollars. Motor mounts typically cost 200 to 600 dollars each. Check with a trusted mechanic for a quote.
Prevention
Follow the maintenance schedule for plugs, filters and fluids, and use good quality fuel. Avoid driving with a flashing check engine light.
Engine and mount causes
An idling car normally runs at about 600 to 800 rpm, and a rough idle can come from worn spark plugs, failing ignition coils, vacuum leaks, a dirty throttle body, clogged fuel injectors or a faulty mass airflow sensor. A misfire often triggers a flashing check engine light. Worn or broken motor mounts, rubber and fluid parts that hold the engine to the frame, let vibration reach the cabin, and the shaking is often worse in Drive with the brake pressed. Low or dirty oil, an old air filter, or a stuck idle air control valve can also cause trouble.
Costs and checks
A set of spark plugs costs roughly 100 to 300 USD installed on many cars, a throttle body cleaning 75 to 150 USD, and a motor mount replacement about 250 to 700 USD each depending on the model. Quick checks include looking for a check engine light, noticing whether vibration changes in Park versus Drive, listening for hissing from vacuum hoses and having codes read for free at many parts stores. Maintaining plugs and filters at the intervals in the owner’s manual prevents many problems. Stop driving and have the car inspected if vibration comes with a flashing check engine light, smoke or a burning smell.
Based on data from AAA and vehicle service guides
