Originally Posted by Torque454
I'm preparing to work on my 87 F-150 with a 5.0 SBF. It seems to run kinda rough especially at idle and kinda acts like it might have one cylinder thats not up to par with the rest of them, but not so bad that its misfiring completely. So far I did a compression test on the drivers side and ALL of the readings were 150 PSI or higher. All the plugs looked good EXCEPT the one in the very back. it was black and shiny with some crud built up on it. When I saw that I was sure it was going to have low compression, but it did not. In fact it was one of two that were actually 165psi.
What can cause this problem? Valve seals/guides?
Also got to find the source of the roughtness, but i suspect i will find low-ish compression on one cylinder. Hoping its a head problem and not a piston/ring problem.
Check and see if the PCV valve vacuum hose connects at or very close to that intake runner. If the valve is bad or there's a lot of blow-by, that'll oil up that plug and combustion chamber pretty good. I'd replace the PCV valve and see if there's an improvement.
A new set of plugs would be nice, but just swapping out the oily one for a different plug will let you know if there's an improvement. Clean the oily plug w/carb cleaner if you have any on hand. Otherwise, using it in a different cylinder should burn the oil off if it's not too bad.