If the engine shakes at traffic lights, the revs hunt up and down, or the engine tends to stall, you have a rough idle. There are many possible causes, but some are easy to check.
Most common causes
- Air leak: a cracked vacuum hose, leaking intake gasket or a faulty crankcase ventilation (PCV) valve. Often together with P0171/P0174 (lean mixture).
- Dirty throttle body: carbon build-up around the throttle plate changes the idle airflow. Common on petrol engines.
- Spark plugs or coils: one weak cylinder causes vibration, sometimes without a fault code.
- Mass air flow (MAF) sensor: a dirty sensor measures air incorrectly.
- EGR valve stuck open.
- Engine mounts: the engine may run fine but worn mounts transmit vibration into the cabin. You feel it most with a gear engaged.
- Diesels: injectors (back-leak test) or air in the fuel system.
What you can do yourself
- Listen for a hissing sound in the engine bay – it points to an air leak.
- Notice whether the vibration changes when you switch on the air conditioning or engage a gear. If so, the mounts or idle control may be to blame.
- Read the fault codes with an OBD2 reader, though sometimes there are none.
- If the battery was disconnected or replaced, some cars need an idle and throttle adaptation (relearn) procedure.
Diagnosis
At the workshop we look at live data: fuel trims, MAF readings and misfire counters per cylinder. A smoke test finds air leaks quickly, and an injector balance test shows a weak cylinder on diesels.
Don't ignore it
A rough idle often gets worse over time. An unnoticed misfire can damage the catalytic converter, and an air leak makes the engine run lean, which raises combustion temperatures. Catching it early is almost always cheaper.
Describe your symptoms in Diagno's AI diagnosis to narrow down the likely causes, then let a trusted mechanic confirm them with live data.