Probable cause for 5.4 misfire?
 

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.
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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?).
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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?

Swap the Cop on # 4 with another that's easier to access for a start.Examine the Cop boot for any arcing.

Yea, I thought of that but it sure seems like a lot of messing around. Maybe I don't know the best way to get at the COP's.

Well, you have it down to spark and fuel. At 100k, you really should replace the plugs. My stock spark plugs were shot at 65k. I'd also go ahead and replace that coil as well just to be done with it.

Thanks, since I was this far I was planning on replacing all the plugs. I wouldn't even mind taking the shotgun approach to fixing this (replace all plugs, COPs, injectors) if I was pretty sure it would solve the problem.

The only thing that bothers me about the COP is that I wouldn't expect that it would cause it to run lean on that cylinder. Faulty logic? Like I said I'm venturing into uncharted waters for me with COP's and Multiport FI.

The shotgun approach worked well for me last month, and I picked up some mileage. I just had a random miss though. Sence you have the problem isolated I wouldn't do injectors or cops on the other side unless your using higher performance parts. I'd up the injectors before putting stock back in.

Do you have a scan tool that can read the injectors?

You could have a partially clogged injector. If there is muck on the coil, some of it probably got into the plug pocket and is causing it to arc and misfire. Make sure to use plenty of dielectric grease on the bottom of the boots when reinstalling them.

my 99 ford 150 had the same problem. #4 misfire, bought a new cop from oreilly. put it on and problem solved. wasnt to hard to get to at all on my truck, not sure bout your vehicle. cost was under 20 dollars for the store brand with a lifetime warranty.

cylinder number 4 is directly under the heater hose connection. these tend to leak ever so slightly at higher rpms (when water pump pressure is up) and drips directly onto this coil pack... Look closely for residue under the heater hose.. Ford made a revised hose just for this problem.

I agree with the above I bet the heater hose fried the COP. If you have already pulled the COP to check the plug gap it is no big deal to switch that COP with say #1 and see if the misfire moves to that cylinder after 50-100 miles of driving and resetting the comp.

If you change plugs I'd suggest changing all COP boots, changing all COP's and injectors is major overkill and will get a bit pricey for really no good reason. I'm running over 200K miles on 7 original COP's and 8 original injectors. My #4 COP fried about a year ago and I change all the boots with every plug change.

Just for info, Checking primary and secondaries on the COP's is not a good check and usually will not give you any meaningful info. Not that you shouldn't check, but if all seems well according to the resistance, it doesn't mean all is well. The only true check is either in the engine, or a true simulation. As a matter of fact I have a COP in my tool box that will check fine on the resistance, but will not fire.