My 99 slowly started getting a misfire but wouldnt throw any codes. It finally got so bad it did all the time instead of intermittent. The cel flashed a few times and I got the reader plugged in enough tine before it went off and it was cylinder 3. I have brand new 02s and cps 49 pounds of fuel pressure. Well last night I cleaned the throttle body and iac and put a fresh plug and wire in cylinder 3. Took it down the road and ran smooth as Joe cool with a Marlboro at the pool hall wearing shades. Then this morning it started the intermentent misfire again and it's slowly going from intermittent to all the time like it did before. Coolant sensor resistance reading came out good so did the injector. So why did a new plug temporarily bandaid? And what could it be I'm stumped?

 

What did the plug look like? I'm wondering if you have a head gasket failure that is flooding that cylinder.

99s have the coil on plug? Or do you have a distributor?

If it's a coil on plug, suspect the wire that fires the coil. If a distributor, suspect the cap and rotor.

 

Distributor and I already put my spare cap and rotor on. I forgot to mention that. No white smoke no coolant smell. The first plug looked a lil rich as far as carbon but not really concerning I'll pull the one I replaced it with after work this evening. If it is a head gasket failure the plug should have been clean looking from the moisture entering the cylinder. Makes no sense that changing one plug corrected the issue temporarily. I'll have a buddy bring his head gasket chemical style tester home and see. I was thinking maybe coolant sensor but that should affect overall timing not just one cylinder and obviously no 0331 head with distributor ignition

 

I have a spare 4.0 and if I don't figure it out by next weekend I swap the damn turd

 

I think I'd be looking at injectors before I swapped an engine. If #3 is spewing too much it will give you problems.