I've got a '99 Legacy Brighton 2.2L with 155k miles and on my trip home I got a P0304 (misfire cylinder #4) flashing at me whenever I was stopped and idling in gear. It sounded pretty rough, but once it got moving again it felt and sounded like it was hitting all 4 just fine. I pulled the plug and it looked pretty corroded, so I put a new one in and initially it sounded like things were ok, so I eased my way the last 150 miles home and figured I'd take a look when I had a few more resources.
Once I got home and had it idling in park, it started flashing the P0304 again. Checked the coil, no problem. Plug wire to #4 was not conducting correctly at idle or according to the voltmeter, so I replaced it. Restarted the engine, but there was no difference in how rough it was running. Swapped the fuel injectors, no change in cylinder misfire. Took off the valve cover and got appropriate play and gap in both the intake and exhaust valves. Got out the leak-down tester, got #4 to TDC and there was a near 100% leak. Didn't hear it from the intake side, no bubbles in the radiator, felt the slightest puff in the tailpipe. Checked #2 and it had minimal or no leak.
I've not lost any coolant over the last few thousand miles, temp gauge is solid, I have a good deal of oil consumption/loss (I'm thinking that oil separator plate has given in to age, as there's lots of slick stuff from the transaxle back...), and another interesting piece of the puzzle is that there is a non-fouler inserted in the 4th cylinder with no telling how long it's been there.
So now I'm wondering...did that bad plug wire give me a burnt exhaust valve? is there a cracked head? worn out piston ring? head gasket?
I've owned the car for the last 35k miles, and the previous owner had excellent records of his maintenance with no indication of anything done to the pistons or valves aside from a timing belt change and valve cover gasket/spark plug tube seal replacement.
And to make it all the more compelling, I'm in TN, next Monday I'm starting a new job in Oregon, and my family and I need to get out there to where our stuff is already.