P0305 Misfire on Cylinder #5

[Merc02]

2002 Mercury Mountaineer 4.6L V8

I've read other posts with this error but have not read any real solutions.

Has anyone had a definite fix after getting the p0305 error.

I'm hoping for real fixes and not guesses like "maybe O2 sensor or maybe the Coil on plug, or fuel filter"

I changed the spark plugs with original Motorcraft plugs about 200 miles ago, and am just now getting a code.

 

[japracer4]

here are the causes of misfires. spark plugs,boots,coils,dirty or faulty fuel injector,internal engine problem,vacuum leak. if you just plugs in make sure that cyl 5 hasn't started backing itself out. i would assume that you also replaced the boots at the same time. if you did not, then shame on you, but if so make sure they are on tight. you could also take the coil and move it to another cylinder and see if it follows. if anyone says a fuel filter or an o2 sensor causes a misfire then they should be banned!! hope this helps

 

[Merc02]

Ah, boots, and plugs backing out. The boots all looked good, but I didn't add dielectric grease.

Are you recommending changing all the boots when new plugs are installed?

These are good points, I'll look into them.

 

[Merc02]

well, I never got back to changing the boots or adding the dielectric grease. However,
What did get done, was replacing the #5 Coil. Proper marine grade grease comes with it. The instructions said to put it on both the boot end and the connector end. Its been running smooth since. No rough idle and no DTC's have come back. Will still try to get back to all the other boots.

I did all this before reading the above recommendations.

 

[viclono]

i'm having similar issuer on 5. Replaced coil with a BWD (second time actually) coil from advance and it did nopt come with dielectric grease which other coils I've purchased have. It was fine for about two weeks then started to misfire horribly again. I pulled it and reseated, ran fine for about ten miles then the same. This did all start after running out of gas, but I disregarded dirty injector because problem went away for three weeks after puttting in coil.
So now I suspect it is either: 1) bad replacement 2) lack of grease 3) wiring harness 4) plug 5) module
I plan on checking plug visually, then swapping cylinders while also swapping coil with another cylinder. I will first buy and apply grease. If these don't cut it should I attempt to diagnose further or cut my loses and take to a shop? any suggestions?
I am editing my post to add the latest:
Just bought and applied grease. When I puuled the coil noticed a fair amount of oil on boot. pulled plug its soaked with oil. went ahead and reinstalled , greased coil wire and contacts to harness. reinstalled. ran fine till I got home. Am now searching for thread on changing plug & valve cover gaskets.