Flashing P0301 After Parts Swaps: Compression or Ignition?
2016 compact sedan, 1.4 L turbocharged petrol, 96,000 km, arrives with a flashing MIL and P0301 (cylinder 1 misfire) under acceleration. Spark plugs were replaced 10,000 km ago; the coil and injector from cylinder 1 have already been swapped to cylinder 4 by another shop, and the misfire stayed on cylinder 1. Hot idle is slightly rough; fuel trims are +4% long-term, coolant temperature normal. A dry compression test gives 148 psi on cylinder 1 versus 168–172 psi on the others. Adding 5 mL of oil raises cylinder 1 to 196 psi. A cylinder leak-down test at TDC shows 22% leakage with audible air at the oil filler neck; cylinders 2–4 leak 6–9%. The customer wants the cheapest possible repair and asks whether a fuel additive or new plugs will solve it.
1. Using the compression and leak-down results, what is the most likely cause of the cylinder 1 misfire, and what single additional test would you perform before authorizing engine repair?
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The wet compression rise from 148 to 196 psi (48 psi) and the 22% leak-down with air at the oil filler point to sealing loss past the piston rings or cylinder wall, not a valve or head gasket. Air at the intake would suggest an intake valve; at the exhaust, an exhaust valve; bubbles in coolant, a head gasket. The 22% figure is above a typical 15–20% condemnation point but is not conclusive by itself. A misfire under load fits a cylinder that cannot hold pressure. Before engine repair, borescope cylinder 1 through the plug hole and look for scoring, crosshatch loss, or a washed piston crown; also check crankcase pressure/PCV and oil consumption. Verify leak-down was at true TDC on compression with both valves closed. If the bore is scored or ring leakage is confirmed, the cheapest correct repair is not ignition parts.
2. The customer asks for a fuel additive and new plugs, and a technician suggests replacing the coil and injector again. Evaluate both proposals.
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The coil and injector were already moved to cylinder 4 and the misfire stayed on cylinder 1. That is a controlled substitution test: the fault did not follow the parts, so they are not the cause. Plugs that are 10,000 km old are unlikely to fix a 48 psi wet-compression gain; a plug may be oil-fouled as a result, but replacing it treats the symptom. Fuel additives can address deposit-related injector imbalance, not lost compression. Spending on them delays the real repair and may worsen catalyst damage from raw fuel. The correct next step is the borescope and leak confirmation. If the customer insists on a low-cost step, inspect the cylinder 1 plug for oil fouling and do a running compression test; neither is a repair. Explain that a flashing MIL means catalyst-damaging misfire and should not be driven.
3. What result on the leak-down tester would make you suspect a burned exhaust valve instead, and how would that change the repair estimate?
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If leak-down air is heard at the exhaust pipe or air cleaner rather than the oil filler, the loss is past a valve. Exhaust valve leakage is more likely if air exits the tailpipe and the wet compression test does not raise compression much. Intake valve leakage would be heard at the throttle body. In that case, cylinder head removal and a valve job is the path, not piston/ring work; the estimate changes from lower-end repair to head removal, gasket set, timing belt/chain, and possibly a valve. A head gasket leak would show bubbles in coolant or air at the radiator. The 22% leak-down plus oil-filler air is the key differentiator, but confirm with the borescope. Also make sure the piston is at TDC firing; if it is 180° off, the leak path is misleading.