Cylinder 2 Misfire: Top End or Bottom End?
A 2016 1.4 L four-cylinder petrol hatchback, 96,000 km, arrives with the MIL on, code P0302 (cylinder 2 misfire), jerking under acceleration, and oil consumption of about 0.5 L per 1,000 km. The cylinder 2 plug is dark and oily but not fouled; the other three are light brown. Coils and plugs are swapped between cylinders 2 and 3 and the misfire stays on cylinder 2. Compression test, warm engine, throttle wide open: cyl 1 = 13.2 bar, cyl 2 = 9.6 bar, cyl 3 = 13.0 bar, cyl 4 = 12.9 bar. Adding 5 mL of oil to cylinder 2 raises it to 12.6 bar; the others change by less than 0.3 bar. Leak-down on cylinder 2 at TDC shows 38% loss with air heard at the oil filler neck; there are no bubbles in the coolant and no hiss at the throttle body or exhaust. Valve clearances are within specification. The customer wants a firm quote and the car back in three days.
1. Interpret the test data: which components are eliminated and is the fault in the top end or the bottom end?
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Eliminated
- Ignition: coils and plugs were swapped and the misfire did not move, so the coil, plug and that part of the harness are not the cause.
- Valve sealing and head gasket: leak-down air appears at the oil filler, not at the throttle (intake valve), exhaust (exhaust valve) or coolant (gasket/crack). Valve clearances are in spec, which also argues against a burnt valve.
Confirmed direction
- Dry compression 9.6 bar against a 12.9–13.2 bar band is roughly 26% low, and misfire under load fits a cylinder that cannot build pressure.
- The wet test rising to 12.6 bar (≈3 bar recovery) points to piston rings/bore, because oil temporarily seals the ring path. A valve or gasket fault would barely change.
- Leak-down of 38% (reject is typically above 20%) with audible crankcase air confirms the same path.
- Oil consumption of 0.5 L/1,000 km and an oily plug are consistent with oil passing the rings rather than the guides.
Conclusion: bottom end - rings, piston or bore on cylinder 2. Head work would not fix this, so the quote must be based on a strip and measure, not on a gasket set.
2. The customer says 'just do the head gasket, it's cheaper'. Explain the reasoning you would give, and what you must measure before you can quote responsibly.
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Why the head gasket is the wrong job The gasket's job is to seal compression, coolant and oil between block and head. The leak-down air is escaping into the crankcase, and the wet compression test recovers pressure - both say the pressure is being lost past the piston, below the head. Replacing the gasket would leave the 38% leak and the oil consumption untouched, and the customer would pay twice.
Measured before quoting
- Bore gauge readings at three heights and two planes on cylinder 2 against the other bores: taper and out-of-round against the wear limit decide hone-versus-rebore.
- Piston-to-bore clearance and skirt condition; check for scoring, scuffing or a broken ring land.
- Crankcase ventilation and air filter: a blocked PCV can pressurise the sump and drive oil past rings, so rule it out before condemning the short block.
- Head flatness and valve seat condition while the head is off, because it will be off anyway.
- Confirm the cause - overheating, oil starvation, detonation or simply wear - so the repair does not repeat.
Practical advice: with three days, only a measured short-block or oversize-piston route is realistic, and parts availability decides it. Quote conditionally, with the strip-and-measure as a separate, chargeable stage.