Several fairly simple tests can be done by any repair shop to verify head or head gasket issues:
1: basic compression test. If one cylinder is markedly lower than others that indicates a mechanical problem. A leak-down test can narrow the possibilities. Usually though significant labor is involved in the repair.
2: cylinder leak-down test: like a compression test but done slower. Air is pumped into the
spark plug hole while the piston is at Top Dead Center (TDC) - i.e. at the very top of the compression stroke and ready for the spark to start the combustion stroke. The rate the air leaks out indicates if there are problems. If the leaking air comes out the tailpipe, that indicates a bad or poor-sealing exhaust valve. Air coming out the oil fill port indicates piston ring issues or leaks in the head gasket. Air bubbles appearing in the radiator indicate cracks in the head or a leaking head gasket. Air coming out the throttle inlet points to intake valve issues. A cylinder with poor leak-down results indicates mechanical failures that, again, are labor intensive to repair.
3: cooling system pressure test: pressurize the cooling system and see if it holds pressure... if not, where is it going? Into the engine via cracks? The leak could dump coolant into the oil which will kill an engine quickly. The leak could be into one cylinder... that should result in a really clean (steam-cleaned)
spark plug. If either is true, it again indicates mechanical failures and costly repairs.
Valve spring issues are fairly obvious once the valve covers are removed. That's not too much work to do/to check and would be part of repairs for the above tests anyways.
Without doing those tests, anything else is guesswork. Other things that can trigger a misfire OBD-II error code are:
1:
spark plug wire or wire feeding the coil bad.
2: coil pack or ignition coil for that cylinder. Replaced already yet problem persists? Swap with another cylinder to see if the error code follows. If so, it's the swapped parts. If not, it's something else. New parts can be defective too. Rare but it does happen, or they could have been improperly installed.
3: poor electrical connections at injectors, at the PCM (engine computer), at the coils, etc. Just re-seating connectors often helps but really eyeballing them for corrosion is better. Verify the metal parts have enough "spring" to grip tightly too. Bad crimps between the individual wires and the connector parts are difficult to locate and are not uncommon.
4: failing PCM. My 2009 V8/Tech just experienced this. Intermittent misfires and stalling when cold. Then a mega-rich event (filled my garage with black smoke). It took a couple weeks to even locate a new PCM which,. for a 10 year old car, is not encouraging. So far (two days now) my car has been okay again. Dirty contacts on the PCM itself can lead to issues as well. I'm wondering if lousy solder joints (cold solder joints) and/or vibration may have caused my PCM issue. Each fuel injector and ignition coil connects to a "driver" circuit in the PCM; the "computer" part of the PCM can be 100% healthy but a bad driver will result in one problematic cylinder. The PCM can't detect if the misfire is a PCM-issue or an issue in the injectors, ignition coil, wiring, head gasket, etc. It can only detect "something ain't right with cylinder x."
mike c.
