I have a 1999 F-150 4.2L V6. It had a misfire in cylinder 2, so I replaced the coil pack, spark plugs, and spark plug wires. After that not only did I still have the #2 misfire, but #5 started misfiring. The truck has 280000 miles on it if that helps anyone. But just yesterday I replaced the fuel filter, which looked like it had never been replaced, and the misfires still persist. Any ideas on what else I could look at or fix? I'm really trying to get this working so I can pass emissions.

 

You need to do either a leak-down or compression test on those cylinders.

 

no other codes ??? Could be a sticking injector causing loss of fuel in one cylinder........... also, I don't remember, but does the 4.2 use a coil pack were #2 and #5 share the same coil? That might be telling you something.

 

Hmm, I'm not sure if they do or not. How would I figure out if they do? There are 6 things to plug into the pack, so idk if they share. I'm not too great with cars, just been researching a lot lately and learning as I'm trying to fix my truck on the cheap :P

 

Your ignition;
There are 3 coils in the pack.
cylinders 1 and 5 are paired.
2 and 6 are paired,
3 and 4 are paired.
.
Misfire detection;
The PCM looks at crank rotation time for every cylinder fired. The crank sensor is used for this timing.
If the rotation time is too slow out of limits, the program sets a code for misfire no matter what the cause (on a rotation time basis).
These are the 301 to 306 codes for a V6 engine.
You have something going on that has not been found.
Coil terminal leakage,
Plug wire issues. Recheck the fire order vs coil plugup.
Fuel injector issues, check the plugups for seating
Cylinder mechanics,
Possible EGR issue because they are individual ports inside the intake manifold for 'each' cylinder and not just ports from the EGR valve. This is an older engine design retrofitted for modern fuel injection.
NOTE; your misfires are in two different cylinder pairs.
Go for the plug wire vs fire order first.
The fire order is 1,4,2,5,3,6.
Good luck.