Cylinder 3 Misfire Immediately After a Valve Job
A 2011 V6 with 118,000 miles had both heads reconditioned after an overheating event. The shop ground the valve seats, replaced guides and seals, and reused the original valves with new springs. The engine now starts but runs rough and sets a P0303 (cylinder 3 misfire) within 30 seconds. Compression is 155, 150, 158, 88, 152, 149 psi for cylinders 1 through 6. A wet test on cylinder 3 raises it to 92 psi. Leak-down on cylinder 3 at TDC shows 45 percent leakage, with air clearly audible at the throttle body. Valve lash on the cylinder 3 intake valve measures essentially zero with the lifter fully pumped down. The head was milled 0.006 inch and a stock-thickness gasket was used. Cam timing marks are verified correct and the ignition and injector for cylinder 3 have been swapped with known-good parts.
1. Interpret the compression, wet test, and leak-down results together. What is the defect, and why did the wet test rise only 4 psi?
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The three tests agree on a single leaking valve, not rings. A 45 percent leak-down with air at the throttle body puts the leak on the intake side of cylinder 3. A wet test that rises only 4 psi is inside gauge and repeatability noise; the small rise comes from oil briefly bridging the seat, not from sealing worn rings. A genuine ring leak would jump the wet reading toward the other cylinders and would send air out the dipstick.
The zero lash explains the mechanism. Grinding seats sinks the valve into the head, which raises the installed stem height. With a hydraulic lifter already near the end of its travel, the valve is held slightly off its seat at all times. On a valve that was previously fine, near-zero lash after a seat grind is the signature of that geometry change, not a coincidence. The valve cannot seal against combustion pressure.
2. What measurement confirms this before the head comes off, and what is the correct repair?
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Measure valve stem installed height on every valve against the service spec, not just cylinder 3. The seat grind changed stem height on all of them; cylinder 3 is only the one that crossed the line. Also check the valve margin and seat width, since a valve sunk too deep loses margin and can become a hot-spot failure later.
Confirming checks without disassembly: collapse or bleed the lifter and measure actual valve clearance, and back up the leak-down with a cylinder leakage test at both TDC and slightly off TDC to rule out a piston or bore contribution.
Correct repair is to restore stem height. That means new valves or valves with the stem tip corrected per the manufacturer, seat replacement to bring the seat back up rather than grinding deeper, and re-verification of installed height and running lash on all six cylinders. With 0.006 inch milled, also check valve-to-piston clearance before returning the engine.
3. The shop manager says the 0.006-inch head milling caused the misfire. Evaluate that claim.
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The claim does not fit the evidence. Milling changes chamber volume, compression ratio, and, on an overhead-cam engine, cam timing by a small amount, but it does so uniformly across the head. A single-cylinder, intake-only leak with 45 percent leak-down on cylinder 3 and normal compression on the other two cylinders sharing that head is a local defect at one valve, not a head-wide geometry change.
What the milling does deserve is a check of valve-to-piston clearance, especially with a stock gasket and original pistons on a high-mileage engine, and a check of compression ratio shift. Neither explains a hissing intake valve at TDC on one cylinder.
Keep the reasoning disciplined: the defect follows the component that was individually altered, the seat on cylinder 3, not the operation applied equally to the whole deck. Using the mill as the explanation would leave the real defect in place and the engine would come back.