Hot-Only Misfire With Normal Static Compression
A 2013 four-cylinder with 118,000 miles arrives with a flashing check engine light on light acceleration and stored P0303. Cold start is smooth. The misfire begins once coolant reaches about 180 degrees F, is worst at idle, and largely smooths out above 2,000 rpm. Dry compression: 150, 148, 152, 149 psi against a 140–165 psi spec. Leakdown at TDC: cylinders 1, 2, and 4 at 8–10%; cylinder 3 at 18% with air clearly audible at the throttle body. Swapping the coil and spark plug from cylinder 2 to cylinder 3 leaves the misfire on cylinder 3. Injectors measure 12.1–12.3 ohms and fuel trims are LTFT +6% with STFT swinging plus or minus 4%. Valve clearance spec is 0.15–0.25 mm intake and 0.25–0.35 mm exhaust, checked cold.
1. Interpret the compression and leakdown results. Where is the cylinder 3 leak, and what does that rule out?
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Compression is even and in spec - the 4 psi spread is well inside 10% - so there is no ring, piston, or gross valve failure. The static test is passing because it is done cold and at cranking speed.
Leakdown is the telling test. Air escaping into the intake tract (audible at the throttle body) means the intake valve is not sealing. Air out the dipstick or oil fill would be rings; air out the tailpipe would be exhaust valve; bubbles in the coolant would be a head gasket. Eighteen percent with intake noise is abnormal next to 8–10% on the other cylinders.
What is ruled out by the swap: ignition coil and spark plug, since the misfire stayed on cylinder 3. What is largely ruled out by the injector resistance spread: a dead injector solenoid - resistance is a weak test, but combined with the leakdown finding it is not where you look next.
2. Why does this misfire appear only when warm, and what is the cheapest decisive test?
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The head expands when it reaches operating temperature, and a valve with insufficient clearance gets held off its seat. Cold, the valve seats and the cylinder is normal, which is why cold compression and cold start are fine. Hot, the valve leaks, the cylinder misfires at idle, and the leak improves as rpm rises because there is less time for the cylinder to lose pressure and the manifold pressure changes.
The cheapest decisive test is a valve clearance check, cold, on cylinder 3, before any parts are sold. Two outcomes:
- Clearance is at or below the low limit, or effectively zero. That confirms the mechanism and, if the cam is not badly worn, adjusting the clearance may restore the seal - a shim or adjuster costs far less than a head.
- Clearance is correct and the valve is still leaking hot. Then the valve face or seat is damaged (tuliped valve, receded seat, or a burned margin) and the head comes off.
Running compression and a cylinder leakdown repeated hot are supporting tests if the clearance comes back in spec.
3. The clearance on cylinder 3 exhaust measures 0.03 mm. What do you do, and what is the risk if you just adjust it and hand back the car?
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0.03 mm against a 0.25–0.35 mm spec is essentially a valve held open - that is the misfire.
Action plan:
- Inspect the camshaft lobe and the bucket/shim face for wear or scoring before adjusting. Clearance that closed up on its own usually means the valve face or seat has receded or the valve has tuliped; the tight clearance is a symptom, not the disease.
- Set the clearance to spec, recheck leakdown hot, and recheck the misfire with a cylinder balance or misfire counter.
- If the clearance goes tight again quickly, or the leakdown stays high after adjustment, the head has to come off for a valve job.
The risk of simply adjusting and returning the car: a valve that is not sealing loses heat transfer to the seat and can burn, turning a valve adjustment into a cylinder head and possibly a piston. Also address the root cause - valve lash does not close up without a reason - and warn the customer that a flashing check engine light means raw fuel is reaching the catalyst, so the car should not be driven hard until the misfire is corrected.