I have a 2000 F-150 XLT Lariet 4X4 with the 4.6 liter motor. Recently it started missing when placed under load. It never would set a trouble code, though. Finally last night, it started missing while lightly accelerating. I was on a long stretch of road so I just kept accelerating hoping a code would finally be set. Just as I was about to back off because I was going too fast, the check engine light started flashing. My scan tool says "Cylinder number six is misfiring".

The truck has 64,000 miles so I doubt it's anything mechanical. It's probably either the spark plug, the coil on spark plug (COSP) or the injector. So, what'd think? There's really no way to test the COSP or the injector. I'll have to make an educated guess. If the spark plug looks normal should I just buy a COSP ($88) or do you think it's the injector?

Incidentally, cylinder six is on the passenger side second from the firewall, right?

 

Swap COP and see if the problem moves with it. If it does, replace the COP. Inexpensive way to figure it out. Honestly, I would suspect it is the COP according to your description. Aftermarket COP's are just as good as OEM since I believe they are all made by the same company and branded. Should be about $60 and take a few minutes to change.

 

Back of engine

4 8
3 7
2 6
1 5

Front of Engine

 

Are you sure about that cylinder numbering?

I thought it was:

rear of engine
5 4
6 3
7 2
8 1
front of engine


The reason I ask is I went through high water once and the computer set "cylinder number eight is misfiring". I replaced the spark plug and then the COSP on the number eight cylinder as shown and the problem went away.

 

Hey Greentree.
Your at the magic number on the mileage for the coils. Thats the point we see coil failure, like you. You can do a coil swap and see if the problem follows the coil as johns 9163 said. The cyl-numbers are passenger side from front of motor to rear are 1-2-3-4. Drivers side of engine, front to rear are 5-6-7-8. I would shoot for the coil pack, or cop coil on plug. Good luck.

Dave P

 

Shucks, I must've replaced the wrong plug and COP when I went through the high water. I thought number eight was passenger side front. Guess the real number eight dried out on its own and I just thought I had fixed it. I had saved the old COP, but threw it out a few weeks ago when I cleaned out the garage. It was probably good. Oh well.

I didn't know an aftermarket COP was available. I think I'll run down to local parts house and see about it.

 

Yes, there are aftermarket COPs....all 8 of mine are aftermarket and have not had a failure yet. Once I had three of them go, one right after the other within a few months, I just changed them all so i wouldn't have to even think about it again..

 

My understanding of COP failure is that the coil isnt bad, the connection or boot goes out. I saw some replacement boots for about $29@set on the board a few days ago.

 

The boots do get dried out at times, not always, but with a misfire, it is best to replace the COP and yes, COPs do go bad.

 

I've got some good news and some bad news.

I replaced the COP on number six cylinder and it had no effect. So I went to the parts house to get a fuel injector. They were out and no other parts houses had it so I went to the Ford dealer.

While I was at the dealer I bought a rear differential speed sensor to fix my ABS light and one of those cruise control switches (to replace the one that catches on fire). He wouldn't give it to me free --had to be replaced by one of their mechanics. But I'll spring for $19 just to save the hassle.

I carefully removed the number six fuel injector and put in the new one. I replaced the cruise control switch and the rear differential speed sensor, then hooked up the battery. The ABS light went out and stayed out! That thing had been on for months.

I started the engine and let it come up to temperature. It sounded good. I backed out of the driveway to take it for a test drive. At first it seemed to run pretty smooth. Son-of-gun, I've fixed it. But once I got out on the freeway it was same-old, same-old. I kind of think that computer is giving me a bum steer.

Since I have a COP that is good and a fuel injector that is good, I think I'll start swapping COPs --replacing the spark plugs as I go along. Eventually I'll have to stumble onto the cylinder that's giving me so much pain.

But at least I got the ABS light out so I did accomplish something. I'll keep you posted.

 

You might have had a dead #6, cop. But you might have a weak cop on another cyl. I would do a plug change while i was in there.

 

Since I had replaced the #6 COP and fuel injector to no avail, I had to decide what to do next. I bought eight new spark plugs and decided that since replacing the #6 COP had no effect, the original must still be good. I checked the #6 spark plug and found nothing unusual except it had worn to about .058 from the nominal .054 gap. I put in a new spark plug and wondered which cylinder to try next.

As every motorhead knows, the cylinder that's giving you trouble is the one that's hardest to work on. I decided on cylinder #5. You have to move the power steering resevoir and bracket to even get to it. I replaced the spark plug with a new one and replaced the #5 COP with the one from #6. I buttoned everything up and took it out for a drive --not really expecting any change. The engine miss had cleared up!! Didn't matter, heavy or light acceleration, there was no miss at all!!

I drove back home at looked at the old #5 spark plug. It was really worn compared to #6. The gap was .062. Then, as I was measuring it, a small piece of ash broke off the side of the "hook" part of the gap. I'll bet that's what was causing the problem all along. It could have been the #5 COP, but I wonder.

Tomorrow I'll replace the rest of the plugs. Lessons learned: the trouble probably isn't as bad as you think and don't rely too heavily on the error code given by the scan tool. It this case it faked me out entirely.

I suppose one could argue that I wasted almost $200 buying parts that weren't needed, but did I really? What would a dealer have charged me to solve this problem?

Cheers.