Car · Problems

Rough or unstable idle

An engine that shakes or whose rev needle hunts at idle has an uneven air-fuel mix or one cylinder doing less work.

UrgencyLow
Can I drive?Yes, carefully
Likely causesIntake air leak

Likely causes

  1. Intake air leak25%

    A hiss at idle with an unstable idle often means a split hose or intake gasket.

  2. Failing ignition coil20%

    A misfire on one cylinder that worsens under load or in damp weather is typical of a coil.

    Garage price: Ignition coil £50 – £140
  3. Worn spark plugs20%
    Garage price: Spark plugs £55 – £190Fix it: Replace your spark plugs
  4. Clogged EGR valve15%
    Garage price: EGR valve £275 – £740
  5. Dirty mass air flow sensor10%
    Garage price: Mass air flow sensor £90 – £255
  6. Dirty or faulty injector10%
    Garage price: Diesel injector £245 – £650

What you can check yourself

  1. Listen for a hiss around the intake manifold: it is an air leak.

Fault codes often linked

P0068 MAP/MAF and throttle position correlationP0101 Mass air flow sensor range or performanceP0106 Manifold absolute pressure sensor range or performanceP0121 Throttle position sensor range or performanceP0170 Fuel trim malfunction (bank 1)P0171 System too lean (bank 1)P0174 System too lean (bank 2)P0300 Random or multiple cylinder misfire

Frequently asked questions

Rough or unstable idle: what causes it?

Likely causes: intake air leak (25%), failing ignition coil (20%), worn spark plugs (20%).

Can I drive?

Yes, carefully. Urgency: low.

Indicative estimate. MecaScan cannot see your vehicle: always have a diagnosis confirmed by a professional, and never work on brakes, steering or airbags without the skills.