This is a duplicate post. I didn't know which forum would be most appropriate.

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?

 

Have you tried replacing the COP boots. They break down over time and let the spark jump to the heads, thereby causing a misfire. Make sure you use plenty of dielectric grease.