This is a duplicate post. I didn't know which forum it was most approptiate for. Sorry.

First of all, thanks in advance for your help. This forum and its collective knowledge are priceless.
I’ve done searches on this but I never quite found the same problem/solution.

My 98 Navigator (5.4, 100k miles) gave me a P0304 error the other day. I’ve had some trouble getting to the root cause of the misfire because of the poor access to the #4 cylinder. Normally I’d check the spark and/or compression but as you all know it’s kind of hard to do once you’ve torn it down enough to get at the COP and plugs.

So far I’ve done the following:
Cylinder leak-down (OK)
Primary and secondary coil resistance (OK)

Observations:
Plugs indicate cylinder #4 is running lean; gap had opened up to .065”
Top of COP is coated with “muck”(oil or coolant mixed with dirt?)
Heater hose over #4 has screw-type clamp (originally a spring clamp?).

Coincidently (or not) about 6 months ago I replaced the PCV vacuum line that runs right over cylinder #4 because of the well-known elbow problem.

Seeing as the truck has 100k on it it’s due for some replacement parts but I want to make sure I have the problem solved before buttoning this thing back up. Also these engines with MPFI and COP’s are new to me from a service/repair standpoint.

Based on your collective wisdom what should be my next steps and what parts should I plan on ordering?

 

I had a misfire code on my #1. But the coil wasn't bad or the plug wasn't bad either. After removing them both I noticed the plug wasn't making good contact with the spring (inside the boot). I took the new plug and pressed it into the new coil pack, and you could shake it around & the plug wouldn't fall off. My old one - wouldn't stay on at all.

They sell the boot sets so you don't have to replace the coils too. I'd try a new boot with the spring 1st, with a drop of dielectric grease & go from there.

Good luck

 

Replace cop. On my 98 Expy i had a light miss even after replacing plugs. I replaced the cops and now it runs great. Sometimes antifreeze leaks onto #4 and ruins the cop.
I read other threads that said you can't always get the answer when checking resistance. Good luck.

 

I'd go with what Oldtrucknut7077 said. In Ford's infinite wisdom they ran a coolant/heater line right over the COP with a junction right where it can leak onto the COP if it works loose. I'd make sure that connection is not leaking and replace the COP.