What does it mean?
Here the control module sees a peak combustion pressure on cylinder 6 higher than the modelled target for the current fuelling, timing and load. Excessive peak pressure is treated seriously because it stresses pistons, rods and bearings, and it usually means the charge is burning earlier or harder than intended. Over-fuelling from a leaking or over-delivering injector, injection or ignition timing advanced too far, excessive boost, heavy carbon build-up raising effective compression and pre-ignition all push the peak upward. A sensor whose zero point has drifted up, or one clamped at the wrong torque, can also report pressures the cylinder is not really producing. The module stores the code, lights the MIL and typically pulls timing or limits fuelling and boost to protect the engine.
Symptoms
- MIL on together with audible knocking or a hard, sharp combustion note under load
- Reduced power once the module pulls timing or limits boost to protect the engine
- Rough running and vibration, most noticeable when accelerating in a high gear
- Rising coolant temperature or signs of overheating in sustained heavy driving
- Diesel engines running noticeably harder and louder than usual
- In severe cases, engine damage such as a burnt piston or damaged bearings if the condition is ignored
Likely causes
- Injector for cylinder 6 leaking or delivering more fuel than commanded
- Injection or ignition timing advanced beyond specification through a timing chain fault, a mis-set pump or an incorrect calibration
- Excess boost pressure from a stuck wastegate, a jammed variable geometry mechanism or a boost control fault
- Heavy carbon deposits on the piston crown and in the chamber raising effective compression
- Pre-ignition or knock caused by poor fuel quality, a hot spot in the chamber, or a spark plug of the wrong heat range
- EGR not flowing, so peak temperature and pressure rise in that cylinder
- Pressure sensor with a drifted offset, an incorrect sealing washer or an over-tightened mounting reading falsely high
How to fix it
- Read the freeze frame and note boost, fuelling and timing at the moment the peak was recorded, and avoid hard driving until the cause is found
- Compare live pressure traces across all cylinders; one high cylinder points at fuelling or mechanical causes, all cylinders high points at timing or boost
- Test the injector on cylinder 6 for leakage and delivery, checking return quantity and correction values
- Verify actual boost pressure against the commanded value and inspect the wastegate or variable geometry mechanism for sticking
- Check injection or ignition timing and the condition of the timing chain or belt, tensioner included
- Inspect the spark plug and the chamber with a borescope for heavy carbon, hot spots or signs of pre-ignition, and confirm the correct plug specification
- Remove the pressure sensor and check its seating, sealing washer and torque, replacing it only once the fuelling, boost and timing sides check out