I have a miss on my new to me 1991 F-250 7.5L 4x4. I pulled the plugs the other day, and found one carbon fouled, safe to bet that's the missing cylinder. Pulled the plug again today, and its wet (oil) so all sorts of bad thoughts start going through my head. I cant find my nice vacuum gauge, so I head to Harbor Freight and pick one up. Hook it up and have nice beautiful vacuum with no fluctuation even when it misses. I'm starting to be less worried. I grab my compression tester, (an OEM brand, Part #27138) hook it up to the offending cylinder, pull the coil wire, and crank her over. I test three times, all three showing 125#. I'm feeling even better! Starting to think maybe I just need new valve stem seals to stop the oil eating. I go to take the tester out of the hole, annnd its stuck. The hose just spins on the fitting. I forgot to put anti-seize on the fitting. Which is round, so I cant put a socket on it. Trying to put some tension on while twisting it I pulled the hose off the fitting. Ugh, now its dark and tomorrow I'll have to try and get it out with some needle nose vise grips which is going to be a PITA...

 

Annnnnd before someone says anything, I'll check compression on the other 7 cylinders at some point, just didn't want to/have time tonight.

 

Maybe it's just a bad plug wire causing the miss?

 

It's likely you are getting oil into the intake via the PCV valve... this is common on these Fords and on the small V8's I know the location of PCV hose at the rear of the intake results in fouling of the rear plugs.

 

That could be, but I don't think so. The wires and fairly new acells. Its an intermittent miss, I think I'm going to grab an ICM from the local upullit if its cheap enough and start from there.

 

This is the #2 plug, and only that plug that's fouling. I haven't checked the PVC yet tho