I have lurked a little before but now an official member and posting my first thread Need help for a misfire problem I can't track down. First issue was random cylinder 8 misfire code. Checked everything, nothing obvious. Cleared code, problem stopped and engine ran fine. Then came back, cylinder 8 misfire. Swapped coil with #5 but problem did not return. Since I assumed it would come back, I went ahead and bought Accell supercoils and new Motorcraft plugs and replaced them all (120k miles, no broken plugs ). Truck ran great on the test drive! Then it sits overnight and bam, cylinder 5 misfire as soon as I start it next morning. Then after messing with it I get cylinder 5 bad coil (code 355 I think). Put one of the Ford factory coils back back on #5, no change and still misfiring. New plug, new coil on all cylinders... Why did it move from #8 to #5? If it was an in injector, seems it would still be #8 since I did not mess with those. Stumped.

 

Anybody??? And if it helps, also got a message to "check gas cap" during this time as well.. forgot to mention that. Gas cap was fine though. Really stumped, need to know where to look next!

 

P0355... I tried two of the factory coils resetting code each time. Same code pops right back up, P0355 (PCM not communicating with coil it seems). So, even though the connector wires seem OK at that coil, they do have pretty good kinks in them and have rubbed on that bracket as well. I guess next step is to start looking for broken wires near that connector. Pin fit seems OK. Any comments before I go that route??

 

Wire connections could explain the situation. If you have marginal connections at #5 & 8, it is possible the switch you made could have improved #8 and killed #5, thus the code jump, especially since now consistantly at #5. Wish I knew more to offer.

 

I would swap the #5 and #6 spark plugs and see if the miss follows the plug. I agree with Tooldad that it is more than likely a wire or the connector but it is not unheard of to get a bad spark plug out of the box and this is a quick way to rule it out.

 

I dropped a plug once. Seemed like no big deal, but caused crack that I could not see initially, so installed anyway. Caused misfire that turned into other issues (cat) before I could diagnose.

Awfully coincidental though if you had a misfire in the same cylinder (#5) before and after plug change (if I understand correctly what you did). Could happen though I guess.

 

Interesting... as my 5 year old son dropped one of the new plugs as he was helping me. About the #5, this didn't occur until after the plug change- was #8 before. Now nothing on #8, it's fine. But one other interesting fact- why did the issue on #5 not start until next day? I finished plug change and test drove, it was fine. The next morning was not. Thermal cycling propagating an issue at the plug?? Will try and change it tonight and see what happens, till then it just sits

 

Could be the plug itself or if you got a little heavy with the anti seize there could be a little of that on the electrode. I liberally apply ant seize myself and have not had an issue yet but I have heard of others running into a problem with it. If it is the wiring you may have moved the wire just right or the connector which may have never been removed before didn't want to fully seat or make proper connection.

 

The miss moved to 5 after he changed the plugs if I read and understood correctly. That's what makes me think the plug is a possibility. I never really rule out the connector on the coils myself. After all the broken fuel injector connectors on Windstars I have dealt with over the years I get really suspicious of the locking tabs on old brittle plastic.

 

Verdict is in... was a bad spark plug! I took out the "new" plug in cylinder 5 and then put in one of the old ones; code cleared and ran like a champ. So bought a new Motorcraft plug and put in, drove it 60 or so miles over last two days and going good.. I assume the bad plug was the one my 5 year old dropped on the garage floor.

 

Glad you found it :-) , BEFORE the cat took a dump. That was my result. :-(