Cause
Carbon deposits on intake valves
Hard carbon baked onto the back of the intake valves, which disrupts airflow and fuel preparation into that cylinder. It is overwhelmingly a direct-injection problem, and the reason is structural rather than a defect: on a port-injected engine petrol sprays across the valve back and washes it continuously, while a direct-injection engine sprays into the cylinder and nothing ever cleans the valve at all.
The deposit is fed by what the valve does see — oil mist from the crankcase breather and recirculated exhaust — so it builds steadily from new and is normal by mid-life. It disturbs cylinders unequally, which is what makes it a misfire and knock cause rather than a general performance one: the cylinder with the worst deposit runs leaner and more turbulently than its neighbours, so a single-cylinder fault appears on an engine where nothing is broken.
It does not respond to fuel additives, whatever the bottle claims, because the fuel never touches the deposit. Removal is mechanical — walnut blasting or manual cleaning with the manifold off. Check it before condemning an injector or a coil on a direct-injection engine with a stubborn single-cylinder misfire, since the deposit is visible with a borescope and costs nothing to look at.
Typically specialist work. Needs equipment or training most home workshops do not have.
Some of these codes mean you should stop driving
1 of the 5 codes this can cause indicate a fault where continuing to drive risks damage or is unsafe. Which one you have matters more than the cause does — read the code's own page before deciding.
Codes this can cause (5)
Codes where this is a likely explanation first, rather than merely a possible one.
- P2338 Cylinder 3 Above Knock/Combustion Vibration Sensor Threshold Likely for this code High
- P0263 Cylinder 1 Contribution/Balance Stop driving High
- P0316 Engine Misfire Detected on Startup (First 1000 Revolutions) Moderate
- P0326 Knock/Combustion Vibration Sensor A Circuit Range/Performance Moderate
- P0331 Knock/Combustion Vibration Sensor B Circuit Range/Performance Moderate
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