While the engine runs
Rough idle: why the engine shakes or hunts when stationary
A rough idle, with revs rising and falling or the engine shaking while stationary, usually means unmetered air is getting in, the throttle body or EGR valve is dirty, or a cylinder is misfiring. Sometimes it is just the engine mounts.
Check without hurryWhile the engine runs
Reviewed by Daniel Morales, founder of Reprorace SL, on
Is it serious? Can I keep driving?
Usually not urgent, but do not ignore it. You can usually keep driving and have it checked soon. If the engine stalls at junctions or roundabouts, or the engine light comes on, book it sooner: losing the engine at a junction is dangerous.
A careful, general answer: only someone who inspects the car can say whether it is safe to drive. When in doubt, stop somewhere safe and call for assistance.
Most likely causes
- Air leaks: a cracked hose or intake gasket lets in unmetered air.
- Dirty throttle body or idle control: carbon deposits change the air flow at rest.
- Misfires: a weak spark plug or coil shows most at idle.
- EGR valve stuck open: if it does not close fully, the engine breathes exhaust gas at idle.
- Engine mounts: worn mounts pass the vibration on even when the engine runs well.
Listed from more to less common in general; on your car they may be different. A fault-code reading and a check at the garage confirm which one it is.
To understand it better: What does the throttle body do and why does a fault trigger limp mode? What are engine misfires (P0300)? What are fuel trims and how do you read them?
Car system: Fuel and air
What you can check yourself
Notice whether it changes when you switch on the air conditioning, the lights or turn the wheel.
With the engine idling, listen for a hiss of air under the hood, keeping your hands away from belts and fans.
See whether the vibration is stronger in gear (an automatic in D) than in neutral: that points to the mounts.
Read the codes, pending ones included.
Safe checks only: with the engine off and cold whenever you look under the hood, without taking anything apart and without touching hot or moving parts.
Related fault codes
- P0505Idle control system: malfunction
- P0506Idle control system RPM: lower than expected
- P0507Idle control system RPM: higher than expected
- P0300Random or multiple cylinder misfire detected
- P0171Mixture too lean (bank 1)
- P2187System (bank 1): too lean at idle
- P0101Mass or volume air flow sensor A, circuit: range/performance
Generic codes that often come with this symptom. Seeing one does not confirm the cause: it is where the diagnosis starts.
How the ECO-LITE helps diagnose it
Plugged into the OBD socket, the ECO-LITE reads the fault codes of the control units that answer, with the freeze-frame data captured when the fault happened, and shows live data on your phone or computer. With your permission, your garage can see them remotely before you bring the car in.
What to look at here: actual and target idle speed, fuel trims, throttle position, air flow and misfires.
Frequently asked questions
Is it normal for the idle to be higher when cold?
Yes: when cold the engine idles higher for a few minutes and then settles. Hunting or shaking with a warm engine is not normal.
Will cleaning the throttle body fix it?
Sometimes, if dirt is the cause. Afterwards some cars need an idle relearn, done with a diagnostic tool.
Can the battery cause it?
On modern cars, low voltage or a weak alternator can upset the idle when loads switch on. Checking the system voltage rules it out.
Keep reading
- Fuel and air: how it works and what fails
- Car stalls by itself: causes and what to do
- Car jerks when accelerating: causes and what to do
- Engine warning light on and steady: what it means and what to do
- Car symptoms: what they mean and what to do
- Check engine light on: what to do
- How to read and clear your car's fault codes
- OBD fault codes: what each one means