The Wet Test Says Rings, the Leakdown Says Valve
A 2008 2.4 L four-cylinder with 142,000 miles comes in with a flashing MIL and P0302. The misfire counter shows cylinder 2 misfiring heavily at idle and light cruise, but the count stops above about 2,500 rpm. Dry compression is 155, 118, 152, 150 psi. Wet compression, after adding oil, is 158, 148, 155, 153. Cylinder leakdown at TDC is 28 percent on cylinder 2 with air clearly heard at the throttle body; the other cylinders are 6 to 9 percent with air heard at the oil filler. Cranking vacuum is 12 in-Hg against a 16 to 20 spec. Running compression at idle is 55 psi on cylinder 2 and 75 psi on the others. Valve clearance is not adjustable. Plugs and coils have already been swapped with no change.
1. The wet compression test rose 30 psi on cylinder 2, which usually points to rings. Does that rule out the leakdown result? Explain your reasoning.
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No. The two tests answer different questions.
- Leakdown is path-specific. Air at the throttle body on the intake stroke side means the intake valve/seat is not sealing. Air at the oil filler would have meant rings. 28 percent is well past the usual 15 to 20 percent condemnation point.
- The wet test is not path-specific. Oil temporarily seals the top ring, so a rise proves only that the rings can seal when oil is present. This cylinder has been misfiring, so raw fuel has washed the oil film off the walls. Adding oil restores that film and the number jumps even though the rings are serviceable.
- The rise to 148 psi is still below the other cylinders and occurred only after oil was added, so it does not establish a ring problem.
Follow the localization test: intake valve leakage on cylinder 2. The low cranking vacuum and low running compression agree.
2. Why does this misfire disappear above roughly 2,500 rpm if the valve really is leaking?
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Leakage is time-based and proportional to trapped charge.
- At idle the manifold pressure is low, the trapped charge is small, and the engine spends a long time per cycle, so the fraction of charge lost past the seat is large. That dilution is enough to misfire.
- As rpm rises, the same leak has less time per cycle to flow, and the cylinder is filling with a much larger air mass, so the lost fraction falls. The mixture stays ignitable and the misfire counter stops.
- This is the classic signature of a mechanical leak: worst at idle and light load, improves with speed and airflow. It is also why a lean-running or ignition misfire is usually worse under load, not better.
A misfire that improves with rpm should make you suspect a mechanical leak, not a coil.
3. The shop wants to replace the head based on the leakdown alone. What would you do to confirm before authorizing that repair?
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Confirm the leak path and rule out the valvetrain before removing the head.
- Repeat leakdown at TDC and again slightly past TDC. If the hiss at the throttle body persists through piston movement, the seat is the problem. If it changes dramatically, suspect a cam or lifter issue holding the valve open.
- Borescope the cylinder through the plug hole and look at the intake valve face and seat for a burned margin, carbon tracking, or a tuliped valve.
- Check cam lobe lift and valve spring on that cylinder. A wiped lobe or broken spring produces the same leakdown and running-compression result without a bad seat.
- Only after the valve is confirmed should the head come off.
Cost matters: a head job on a 142,000-mile engine is a big authorization. Confirm the path first.