My 97 (4.6) began missing a couple of months ago. I changed the plugs, then the wires and it has just become worse. The check engine light came on the other day. I took it to the dealer and they ran a diagnostic test ($100). They said that it was the #4 cylinder, and said that the heater hose leaked down in there. They told me that I need to change that plug & wire. They also stated that I should have used Ford wires. I did what they recommended, but it still missed. Due some related threads I have read in the forum, I checked the resistance in the coil packs and they appeared to be okay according to the manual(12k ohms). I changed the one that the #4 wire was connected to anyway, but it still is misfiring.

What could be wrong?

 

I had the same thing happen to me. I installed a Jacobs DIS and plug wires. The problem comes from the location of the wires and the inability to push them all the way onto the plugs.
First get some spark plug grease and line the inside of the boots with it. While you are at the store get a plug boot pulling tool. I got one made by KD (part # 135 spark plug terminal pliers). Push the plug wires onto the plugs as far as you can. Then, using the tool, grip the plug at the point where it is flanged (the point were it enters the block). Push it down into the block until the wire snaps onto the plug.
Once you get them on, no more miss fire.
It seems simple and took a long time for me to figure this out but now I can run my plugs gapped at .070 (with Jacobs) and not missfire. I had to move the Power steering pump and work my way around the hoses and wires. Get high over the block and take your time it will be worth it.
GOOD LUCK AND I HOPE THIS WORKS FOR YOU...

 

Follow WLF's advice and put some dielectric grease on those spark plug boots if that wasn't done in installation. I found that it was the only way I could get my Accel wires to fully seat down. Also, the wires are supposed to sit in one direction only. I wouldn't fully listen to your dealer in saying to only use Ford wires. I do agree that the Ford wires are very good and most people under rate them but you can get get use out of other brands. So far I have had Ford wires that were on there when I bought the truck used they went at about 98K, then I put on Xact brand wires and those only lasted about 30K and started cross firing, I have since went to Accel wires and coil packs and have had no troubles which has been about 5-6K miles. Other than checking all that again I would say you've got problems elsewhere.

 

Thanks for the suggestions.

When initially installing the wires (UltraSpark- Double Silicone from AutoZone-$54)and plugs (Bosh .054 Gap), I paced Dielectric grease on the boots (both the sparkplug end and coil end), and sparkplugs (top only). I will go out today and purchase a sparkplug tool and give that a shot.

 

I tried the spark plug tool, and was successful in sealing the boots into the hole. However this still didn't help. Anymore ideas?

 

I had the same problem, too. Mine turned out to be a combination of plugs and wires. The Motorcraft wires that came on the truck lasted 30K, and so did the second set. I found some wires at Pep Boys for $75 that have a lifetime warranty and fit and work well.

You could try decreasing the plug gap, I run mine about .040. Works well for me.

-Jay
98 F150 4x4 SC ORP
33x12.50 Pro Comp AT's, transgo shift kit, K&N filter, RS5000 shocks

 

I maintained a 92 grand marquis for 10 years, you should change the wires every year and use Ford ( I tried auto zone lasted 6 months.) Make sure you use the grease. Check for arcing in complete darkness

 

I was going to say that you should jerk those Bosh(sp) and repalce with something else. But I reread your post and think you were syaing you now have new Motorcraft plugs and wires, right? Do you know exatly what cylinder is misfiring. Maybe you should check or even replace the other coil pack and maybe even the newer one too. Do not worry to much about the plug/wire seat thing. Some of mine are not perfect, just as long as the pop down over the end of the plug. Check for arcing at night too, that's how I found my bad wires.

 

These guys are right check for arcing at nite and listen as well, you can hear the spark jumping around. my v6 in f150 plug wire rubbed on the ground braided wire and would arc to this...Look at all of them as well as #4....

 

I also agree with checking for the arcing at night. Pull the light out from under the hood and have someone rev the engine (a little and repeat several times) so it will put some advance into the timing. Some missfires only show up under load.
Have you changed your pvc lately??

 

Thanks for all your input. I finally got the problem resolved. If you remember earlier in my first post, I stated that I took it to Ford once the check engine light came on, and was charged $98 for them to diagnose the problem. They stated that it was the number 4 cylinder that had the missfire. Well, after focusing all my attention on that cylinder and arriving at dead ends, I took it to a friend of mine at work. He is a mechanic and has an OBD II scanner. He put it on the machine and read the codes. It turned out to be the #3 cylinder, and not the #4 like Ford had told me. He recommended that I replace the Bosh plugs with motorcraft and everything should be fine. I did just that, and when I got the #3 spark plug out, you could definitely tell that the plug was shorting out around the insulator. Once I got everything back together, the miss was gone. Evidently, the water from the heater hose had leaked down inside the hole and caused this. I just hope that Ford really fixed the leak like they said.

Hopefully this will keep someone else from going through the same frustrations by not relying on the dealer. Again, thanks for all of your input.