Rough Idle or Hesitation: Dirty Injectors or Carbon Build-Up?

Short answer: A rough idle or a hesitation when you accelerate is often caused by dirty injectors, carbon on the intake valves or carbon in the combustion chamber. Worn spark plugs and ignition coils, a dirty throttle body, vacuum leaks and worn engine mounts can feel similar. A flashing check-engine light means a misfire: see a workshop first.

What it feels like

• The engine shakes or the rpm needle wobbles at traffic lights.
• A pause or stumble when you press the accelerator, then the car catches up.
• Less power than before, sometimes with higher fuel use.

Why deposits build up

Short trips, stop-and-go traffic and lower-quality fuel leave carbon and gum on the injectors, intake valves and pistons. Dirty injectors spray fuel unevenly, so some cylinders burn better than others and the engine runs rough. Direct-injection (GDI) engines are especially prone to carbon on the intake valves.

Check these first

• Spark plugs and coils. Check the service record. Worn plugs are a common, cheap cause.
• Air filter and throttle body. A dirty throttle body often causes an unstable idle.
• Engine mounts. If the shaking is felt in the cabin but the rpm is steady, the mounts may be worn.
• Warning lights. A flashing check-engine light means a misfire. Stop and see a workshop.

How to clean it

• KM+ Injector Cleaner cleans and lubricates the fuel pump, injectors and intake valves. Pour the whole bottle into the tank, ideally with only 20 to 30 litres of fuel in it.
• KM+ Nano Decarbonizer deep-cleans the injectors, combustion chambers, exhaust and catalytic converter over one tank, with no dismantling. Add one 350 ml bottle, then fill 40 to 60 litres of fuel.
• For a faster clean, a workshop can do a carbon-cleaning service with KM+ Direct Decarbonizer.
• Afterwards, KM+ Advanced Fuel Additives at every refill helps stop deposits coming back, and KM+ REVO Nano in the engine oil gives a smoother, steadier idle.

Common questions

How often should I clean the fuel system?

A deep clean every 10,000 km or at each service suits most cars, more often if you mainly do short trips.

Will this fix a misfire?

Not if the cause is a failed coil, plug or sensor. Those parts need replacing. Cleaning helps when deposits are the cause.

Not sure which applies to your car?

Send KM+ your car make, model, year, mileage and what you notice. We will tell you honestly whether a KM+ product will help, or whether you need a workshop first.