I'm trying to troubleshoot an issue with my dad's truck. It's a 2005 F-150 with the 4.6 v8 and about 150,000 miles on the odometer.
Around 2 years ago it started to misfire pretty badly while driving, I wish I could give more details than just that but unfortunately I wasn't able to look at it. He took it to the local mechanic and they said it was an issue with the coil packs and replaced all 8, I'm 90% sure they also replaced the spark plugs. All was well for a while and then it started to idle rough. Once my dad started to drive it the misfire went away. I suspected it could be a clogged fuel injector, so I had him run a few cans of seafoam through the gas tank and that seemed to have worked, as after a while the truck idled fine and had no issues while driving.

Unfortunately, the misfire is back again. From what I've seen it only happens while driving, and even then it isn't constant. It only really happens when the truck is accelerating, once it gets up to speed it's fine for the most part. CEL came on and the code was a Cylinder 3 misfire. I replaced that coil pack which fixed it for about a day. but the misfire is back again with the same misfire code.

Any ideas on what this issue could be? Any tips help, I haven't replaced the fuel injector yet but I don't think a bad fuel injector would be able to "fix" itself for several months at a time before becoming a problem again. I'm also not the most well versed with Fords, so if there's some well known issue with the modular 8 misfiring that I missed I apologize.

 

Change the spark plug yet? Seems the obvious thing to do...

 

who knows what brand plugs and coil packs they installed... need to be motorcraft...

 

Pull #3 coil and plug. Inspect condition and note what parts/models they are. Then swap/replace #3 coil with #2 coil and #3 plug with #1 plug (also take note of those parts/model info)

clear codes

drive, several cycles/days of driving, scan for codes.

Post back with what you find/learn.

 

Clean cop connections esp high volts tab , replace all boots . Any connector that doesn't lock on replace and solder in .
Learn to do a mode six test to see what cyl are about to act up . You may have bad plugs .

 

Inspect plugs ,if carbon coming up thru threads they are not torqued in enough . Blowby will burn plugs up ,those hot gases act just like a cutting torch .Destroys boots which should be changed every 100k anyway or if they get oil,gas or antifreeze on them .

 

I would guess it is the coil pack. Maybe you have a small leak or little water or moisture found its way to the coils and collected in the spark plug holes. Id scan and pull any coils with a code or pending code, clean everything and then put dielectric grease on the end of the boot. This is very important to keep from getting misfires.