Misfire That Only Appears Under Load
A 2016 Class 7 truck with a spark-ignition CNG engine arrives with a complaint of a flashing check engine lamp and power loss when climbing grades loaded. At idle and light cruise the engine runs smoothly. The scan tool shows a P0300 plus pending P0304 (cylinder 4 misfire) that sets only above about 60% load. Fuel trim at idle is normal; under load long-term trim goes to +18% on both banks. Spark plugs have 30,000 miles, coils are original, and the engine has 210,000 miles. A compression test is 150-160 psi on all cylinders when cold and dry. The technician swaps the cylinder 4 coil and plug with cylinder 2 and the misfire stays on cylinder 4.
1. The coil and plug swap did not move the misfire. Interpret that result and describe what it does and does not rule out.
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Because the misfire stayed on cylinder 4 after swapping the coil and plug, the ignition components are not the primary cause on that cylinder, but the swap only proves the coil and plug are not uniquely faulty. It does not rule out:
- A harness or connector fault specific to cylinder 4 (chafed wire, pushed pin, high resistance under vibration).
- A coil driver fault in the ECM for that channel.
- A mechanical or fuel-delivery problem on cylinder 4.
- A base ignition problem that affects all cylinders but is worst on 4 under load (plug gap erosion, weak coil output at high cylinder pressure).
The positive fuel trim under load (+18%) is a strong clue that the engine is lean at high demand, which can cause load-only misfire on the weakest cylinder. That points toward fuel delivery or air measurement, not ignition.
2. Given +18% long-term trim under load and a load-only misfire on one cylinder, outline a diagnostic sequence that separates a global lean condition from a single-cylinder fuel or mechanical fault.
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Sequence:
- Verify fuel pressure and volume under load, not at idle. A weak pump or restricted filter shows normal idle pressure and drops under demand. Check pressure at the rail while loading the engine on a dyno or with a road test and a remote gauge.
- Check for intake leaks that open under boost/load: charge-air piping, intercooler, and manifold gaskets. A leak that only opens under pressure causes load-only lean trim.
- Evaluate mass air/fuel metering: compare commanded vs actual fuel, check the MAP/MAF and intake air temp sensors for rationality at load. A skewed sensor causes global lean trim.
- Isolate cylinder 4: perform a running compression test and a cylinder leak-down at operating temperature. A valve or head gasket issue that only shows hot/loaded will not appear on a cold dry test.
- Swap the cylinder 4 injector (or CNG injector) with another cylinder and see if the misfire moves. If it moves, the injector is the cause; if not, check the driver circuit and ECM channel.
- Inspect the cylinder 4 harness and connector for resistance and wiggle-test under load.
Global lean trim plus one weak cylinder usually means a delivery problem that the weakest cylinder reveals first, so fix delivery before condemning the cylinder.