I just bought a 2005 F150 with a 5.4. It started with a rough idle, then the check engine light came on. The code scanner said misfire #3 cylinder. Replaced the coil, ran fine for ten miles , same problem same code. Put spark plug in old coil no spark, swapped with new coil, no spark, tried old coil again, spark. Truck ran good for about 5miles started with rough idle again. Seems like it fires and misfires whenever it wants too. Looking for ideas? Oh yeah truck has 115k on it!

 

I would inspect the coil connector and wiring harness. Perhaps a rodent has eaten thru a wire or their is a bad connection somewhere.

 

+1 on this ^^^ ! Had ths same thing happen about a week after installing new Granatelli COP's---I was bummed. Discovered the locking tabs on the harness connector had been damaged (perhaps by previous owner?) and it decided to stop making full contact. NAPA EC259 solved this problem, about $20 each sorry to say.

With engine running jostle each COP's connector, looking for frayed wires etc. the misfire should come and go as you manipulate the wiring---might be very easily seen at that point.

Oddly enough though that #3 plug even on my E250 with 5.4 has been problematic on two different vehicles so far---curious this is!

HTH

 

Different engine, but same principal. See if this helps: https://www.ford-trucks.com/forums/1...il-wiring.html