Cylinder Contribution Test With a Misfire Under Load
A 2014 transit bus with a 6.7 L diesel has a steady misfire and a slight gray haze from the exhaust at idle that clears above 1,200 rpm. The MIL is on with a cylinder balance/contribution fault for cylinder 4. The engine has 210,000 miles. A relative compression test shows cylinder 4 about 8% lower than the average of the other five. A cylinder cutout test shows only a small change in engine speed when cylinder 4 is cut, compared with the others. Fuel rail pressure is within spec, and injector return flow is within spec on all six. Valve lash was adjusted 20,000 miles ago.
1. How do you distinguish a fuel-side fault from a mechanical fault on cylinder 4 using the data given?
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Use the evidence to separate air/fuel delivery from sealing and compression.
- Fuel side: Rail pressure is in spec and return flow is in spec on all six, which argues against a grossly failed injector or a rail-supply problem. However, return flow within spec does not prove the injector is delivering correctly. A cutout that produces little speed change plus a contribution fault can still be an injector that is under-delivering or has a bad spray pattern.
- Mechanical side: The 8% relative compression deficit on cylinder 4 is the stronger signal. A cylinder that is low on compression will misfire and haze at idle and may partially clear as rpm rises and airflow improves. Valve lash was set recently, so rule out a tight valve only if lash is rechecked.
- The combination of low relative compression plus a weak cutout response points to a mechanical sealing problem (rings, valve seat, or head gasket) rather than pure fuel delivery.
Next step: confirm with a manual compression test and a cylinder leak-down test on cylinder 4.
2. A leak-down test on cylinder 4 shows 35% leakage with air escaping into the crankcase. What does this indicate and what is the repair path?
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Leakage into the crankcase (heard at the oil fill or dipstick tube) indicates the pressure is escaping past the piston rings or through a hole in the piston, not past the valves or head gasket. Thirty-five percent is well beyond a healthy cylinder (typically under 10-15% for a diesel).
Repair path:
- Bore-scope the cylinder to check for scoring, a holed piston, or a broken ring land.
- If scoring or a damaged piston is found, the engine needs at least a top-end and likely an in-frame or out-of-frame overhaul depending on the block condition.
- Verify the cause of the damage (overfueling, injector wash-down, overheating, or ingestion) so the repaired engine does not repeat the failure.
- Recheck injector calibration and cooling system health before returning the bus to service.