Rough idle: why the engine runs unevenly when stopped

What it means

On nearly every car built in the last two decades there is no separate idle valve. Idle is just a very small throttle opening that the engine computer commands. Carbon building up on the throttle plate changes the airflow at exactly those small openings, and a fixed air leak matters most at idle and fades at speed.

A cylinder that is not sealing properly, an ignition fault, or one injector flowing differently from its neighbours can produce the same uneven feeling, which is why the symptom alone does not name the cause.

Common causes

• A vacuum leak, meaning a cracked or loose hose letting in unmetered air
• Carbon on the throttle body
• An ignition fault, such as a coil or spark plug
• A restricted injector, or injectors flowing unevenly
• Low compression in a cylinder
• Excess EGR flow, or a faulty evaporative purge valve
• A sensor feeding the engine computer incorrect data

What to do now

1. Note when it happens: cold or warm, only at idle or under load as well, and whether it clears once the engine has warmed up.
2. Check whether the check engine light is on, and whether it is steady or flashing.
3. Look at the intake ducting for a split or a loose clamp, and at any connector that was disturbed by recent work.
4. Have any codes and the freeze-frame data read before anything is cleared.
5. Before a part is replaced, ask what was tested, such as a smoke test for air leaks or a compression test.

Rough idle, or something more specific?

• Rough running with a flashing check engine light: commonly a severe misfire, so reduce load and have it seen promptly
• Idle too high, too low or unstable, with code P0505: the idle control, often a dirty throttle body
• Rough idle with a loss of power and harder starting: possibly low compression, and a compression test checks each cylinder
• Direct-injection petrol engine, rough only when cold: possibly intake-valve deposits, among several other things
• Diesel, rough running for the first few minutes of a cold start: points at the glow plugs
• Diesel, idle sitting a little higher with the fan running harder: a particulate filter regeneration, which is normal

What people often get wrong

That the part named in a code, or a failed idle part, is the answer. Most modern cars have no separate idle valve to blame, and in one worked example a lean code set at idle came from a split breather hose, not the airflow sensor the owner had been told to replace. Anyone who names the cause of a rough idle before testing is guessing.

When the car must go to a workshop

Get it checked soon, and have it tested rather than guessed. A technician can compare fuel trims at idle and at higher engine speed, smoke test the intake for air leaks and run a compression test, which separates causes that feel identical from the driver's seat.

Read next

• P0505: unstable idle
• Engine misfiring: rough running and what it means
• Flashing check engine light: slow down, this one is urgent

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.