i have a 94 f150 5.0 with a strange misfire at idle. it feels like a spark plug shuts off for a second on only 1 cylinder when the truck is warm at idle in 1, 2, D, and R with the heater on. i happens fairly consistantly once or twice every couple seconds. i watch the battery guage move down when this happens and the idle drops maybe 50rpm. truck runs perfect other than this. popped the hood and heard what sounds like a spark arcing but i cant find the source but i do know its on the passenger side of the engine. i cant see any worn spost on the plug wires. any ideas??

 

Ground out each spark plug one at a time have someone in the car watching the rpm. If the rpm goes down when you ground out said plug wire this is good showing the cylinder is firing. If you ground a wire and the engine continues to idle like it was before you grounded said plug wire then there is a problem. Once you have isolated the problem inspect the wire closely for burns cuts also measure with an ohm meter for proper resistance. Remove plug inspect it. If these do not clear up problem, do a compression test. Also inspect the injector harness connection and listen to injector while running with a mechanics stethoscope should hear a click indicating proper injector function.

 

Have you checked to make sure all the plug wires are tightly connected on the plugs and cap? Are the wires routed correctly? If you hear arcing, open the hood in the dark and you may be able to see it.

 

well i had a feeling it was something with the spark plugs so instead of grounding each plug like Piston said, i found my older wires and replaced the 2 that sounded like where the bad ones. took the truck for a quick drive and i seem to have solved my problem . i looked closer at the wires i took off and saw that on one there was a gnarly split on both sides of the boot that covers the plug. probable cause?!