Misfire Under Load After Head Gasket Replacement
A 2014 four-cylinder engine was returned to your bay two days after a head gasket replacement. The customer reports a flashing check-engine light and a rough idle that smooths out at cruise. You retrieve P0302 (cylinder 2 misfire) and a pending P0300. A compression test gives 165, 160, 120, 162 psi on cylinders 1 through 4, and a cylinder leak-down test on cylinder 2 shows 35 percent leakage with air heard escaping from the adjacent cylinder's spark plug hole. Coolant level is stable, no bubbles appear in the radiator, and the oil looks clean. Spark plugs, coils, and injectors were swapped between cylinders with no change in which cylinder misfires. The vehicle is under the shop's 90-day workmanship warranty, and the service writer wants an answer before the customer arrives in an hour.
1. Interpret the test results and identify the most probable root cause. Explain why each finding points where it does and rule out the alternatives the service writer will ask about.
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Cylinder 2 is low on compression (120 psi versus roughly 160 psi on the others) and leaks down 35 percent, with air escaping into the neighboring cylinder. Air migrating to an adjacent cylinder during leak-down points to a breach in the head gasket's fire ring between cylinders 2 and 3, not to a valve or ring problem. A leaking intake or exhaust valve would send air out the throttle body or tailpipe, and worn rings would send air into the crankcase (oil filler or dipstick tube). Stable coolant, no radiator bubbles, and clean oil make a coolant-jacket breach unlikely, which is consistent with a compression-only fire-ring failure between two bores.
Swapping plugs, coils, and injectors without changing the misfire cylinder rules out ignition and fuel delivery as the cause. The misfire worsens under load because cylinder pressure and the escaping charge increase with throttle, which explains the flashing light at acceleration and the smoother cruise.
Conclusion: the head gasket did not seal between cylinders 2 and 3, most likely from a warped or improperly cleaned deck surface, a gasket installed dry or misaligned, or head bolt torque/sequence error. This is a workmanship issue covered by the warranty, and the repair is to remove the head, check both decks for flatness with a straightedge and feeler gauge, verify head bolt threads and torque procedure, and replace the gasket.
2. The service writer asks whether the vehicle can be released to the customer today if the misfire is 'only at idle.' Give your recommendation and the reasoning, including what you would document.
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Do not release the vehicle. A flashing check-engine light means the misfire is severe enough to dump unburned fuel into the exhaust, which can overheat and destroy the catalytic converter, a repair far more expensive than the gasket job. A compression-only breach between two cylinders also means combustion gases cross between bores, so the engine is operating with abnormal pressure and heat at that deck surface. Driving it risks worsening the head or block deck and turning a gasket replacement into a head or block replacement.
My recommendation is to keep the vehicle, explain the compression and leak-down results to the customer in plain language, and confirm the repair is covered under the 90-day workmanship warranty. Document the compression readings for all four cylinders, the 35 percent leak-down with the escape path noted, the swap test results, and the stored and pending codes. Photograph the deck surfaces once the head is off. If the customer insists on taking the vehicle, have them sign an acknowledgment that the vehicle is not roadworthy and that the shop advised against release, and note that the warning was given.