I recently picked up a 2007 with 230k miles. Intermittent misfire, typically just on cold startup. Sometimes I notice a slight roughness, and sometimes it feels like one cylinder is missing 100% of the time. The codes I get are P0300 and P0301. I replaced the spark plugs, no difference. Redline, seafoam, no difference. I then moved cylinder 1 fuel injector to cylinder 2, cylinder 1 ignition coil to cylinder 3, and cylinder 1 spark plug to cylinder 4. No difference, still P0300+P0301, and the misfire disappears when I tap the gas. MAF is newer, I cleaned it, non-OEM. Compression readings may be helpful: 120 on 1, 130 on 2, 140 on 3 and 4. So that's a problem. My method for that was to put the car in inspection mode (ready-gas-gas-neutral-gas-gas-park-gas-gas) and then hold start for 5 seconds to crank it. So...what should I be looking for here? Head gasket or worn valves/rings? I have no other symptoms of a bad head gasket. Anyway, if it's at that level I'm just going to keep driving it as-is because it runs fine when I hit the gas. But am I missing something simpler? For instance, I was wondering if the intake gasket could be the problem with a leak around cylinder 1. But the bad compression readings point me in a different direction. Should I replace the intake gasket just in case? Not very hard... Should I put some UV leak tester dye in the oil to see if the head has a micro crack? Any other suggestions? Thanks in advance!
