Hi guys... I've been having this pain in the a** problem lately with my 1990 F150.. The #6 cylinder in the engine keeps loosening the spark plug by itself. I've only had this truck for about a month or two now and I've always had this problem with the truck kind of "stuttering" at about 1/4-1/3 throttle, then going away as I give it more gas. I figured it might be spark plug related, so the other day i looked under the hood with the truck running, and see the #6 spark plug vibrating.. I shut the truck off, wait a while for it to cool, then take off the plug wire and unscrew the plug by hand! The plug looked like it was in pretty good shape though, so i put it back in and tighten it as far as it should be tightened, and then take it for a little test drive. I notice that the stuttering has gone away, so I think "ok, great, problem solved." The next time i drive it, i notice the stuttering problem again, so i check the plugs again, and find the #6 one has loosened itself again. What would cause the spark plug to loosen itself??? Any help would be greatly appreciated...

-Sorry for the long post, but i seem to go into great detail in my stories.. :o

 

does it have the correct plugs in it. there are taper seal plugs and washer seal plugs. I'm not sure which you are supposed to have though.

also how tight are you tightening it. snug it up and then give it a 1/4 turn.

what are the threads like? are they damaged or has it been incorrectly repaired before you got it.

try swapping it with #5 plug and see what happens.

 

fords have taper seat plugs..
like lees stated

tighten it "good"
but just a little more..
the 1/4 turn is for the washer seal plugs


and make sure the sealing area is clean on the head..
loose plugs = yuck on the seat sometimes

...zap!