Cylinder 3 misfire

[mrb1]

Help! Cylinder 3 mifire on 2001 Protege 2.0 with 170,000 mi. Got it scanned at parts store and had P0303 and another code for O2 sensor. (I'm assuming O2 code is secondary to the misfire problem.) Plug is slightly wet and oily compared to other 3 plugs.Took plug out and and it looked like it was getting good spark, but changed plugs and coil just in case. Still missing on 3. Swapped coil with other coil just to make sure new one wasn't defective...still missing. Took out #3 fuel injector...looked clean. Swapped fuel injector with another one and miss stayed on 3. Checked compression and cyl 3 has about 135 psi while 1, 2, and 4 have about 145...within spec per the shop manual.

Odd thing is that when you start it up sometimes it will intermittently run fine and the miss comes and goes. But then after a short time it misses constantly. Driving it down the road it still has decent power. Could it be a computer problem? Valve? What can I check next?

Not sure if it has anything to do with it but I think the car overheated a few months ago when it lost a belt and the radiator blew...put a new radiator in it and it ran fine for a few months but now has the miss.

Sorry for the long post but wanted to try and explain what I tried so far. Any help is greatly appreciated!

 

[HA-Y-N]

If you didn't change the wires i would suggest that first. if the rubber spark plug boots have a crack or hole in them the spark wont make it to the plug but instead ground out on the engine. if you checked for spark with the plug away from the engine this may explain why the spark seemed "good". I poked a tiiiny little hole with a mechanics pick while removing the boot and it was enough for the spark to get through and ground out on the spark plug tube. Also O2 sensor could definitely cause a misfire but usually a random misfire. i have a sneaking suspicion its your wires or the rubber boots connected to the coil packs. Good luck

 

[mrb1]

HA-Y-N, thanks for the suggestion on the wires. Forgot to mention in the original post that the wires were replaced about a year ago. When I was checking for spark I had the plug out and it was touching the engine for ground. I also swapped the wire with another cylinder and the miss stayed on cylinder 3.