So now armed with this info the possible causes could be:
- failed spark plug, cylinder #7
- failed spark plug boot, cylinder #7 (this car uses a coil on plug setup so it doesn't have plug wires)
- failed ignition coil, cylinder #7
- Failed fuel injector, cylinder #7 (stuck open or leaks like a sieve)
- Failed head gasket at cylinder #7
- Burned exhaust valve on cylinder #7
- Cracked head on cylinder #7
- Cracked block on cylinder #7
At this point in my head I start fearing that it is a failed head gasket as I am a pessimist and the failure happened when I was accelerating hard (blocks and heads don't tend to crack any more unless you are doing really stupid stuff). So now to start trying to better diagnose the problem. Always start with the easiest things so I played musical coil packs and switched the one on cylinder 7 with the one on cylinder 6. No I turned the key to the run position (everything has power but the car isn't running) and used the scan tool to clear all the codes. I am really hoping to see a P0306, but a P0307 wouldn't necessarily be bad at this point either. So I start the car and let it run and then press the re-scan button. Low and behold I get only 2 codes, P0420, and P0306. Now this could have been better as now I know I need to replace the coil pack that is now on cylinder #6 as the car is now shooting cylinder #7. At this point I can pretty much rule out everything else as the misfire moved with the coil pack. Granted coil packs are more expensive than spark plug boots and spark plugs but are cheaper than a head gasket replacement, valve replacement, head replacement, or engine replacement. They are also a lot easier to replace than fuel injectors even if the cost is about the same.
I went and talked with my mechanic neighbor and he suggested replacing all the coils as well as boots and plugs since I am in there anyway and the Bosch coil packs aren't exactly know for their reliability so might as well do all of them instead of potentially having this problem in the future again. This is what I as planning on doing anyway as once you are in there might as well do everything so you don't have future problems. This is still cheaper than any of the catastrophic failures, and the remaining 7 reasonably good coils can be kept as spares in case one of the new ones fail some time down the road, I do the same thing with belts too.
Also for piece of mind (that OCD thing I got going on) I also did the following:
- checked to see if the oil was milky. It will look like chocolate milk if you get coolant in your oil and they get hot and mixed up real good. This is a sign of a problem with the head or block, usually a head gasket.
- Checked to see if there was an oil film in the coolant, oil leaking into the coolant. This is a sign of a problem with the head or block, usually a head gasket.
- Checked to see if there were bubbles in the coolant, combustion gases in the coolant. This is a sign of a problem with the head or block, usually a head gasket.
- Checked the oil filler cap for moisture, some cars like my wife's VW have a problem with this but my BMW doesn't so I didn't want to see any. Often this is a PCV problem but it could also be a small bit of coolant mixing with the oil, evaporating, and then condensing on the oil filler cap. Might be a sing of a small head gasket leak.
- Did a compression test on cylinder #7. This rules out cracks in heads, cracks blocks, head gasket failures, and burned valves.
As all of these checks passed so it is definitely not one of the catastrophic failures which is good. So now I was just waiting for my parts that were on order from the auto parts store. Once received I went and change out the old spark plugs, spark plug boots and coil packs. I cleared the codes and the car fired right up and the sluggishness and misfire has gone away. It was a good idea to also change out the plugs as all but the one from cylinder #7 showed signs of lean burning. This wasn't really a surprise as the emissions system detected unburden fuel (the misfire) and to try to correct it leaned out the fuel air mixture. So with new plugs, boots and coils the care is running great again.