I purchased my 4.6L 2000 F-150 with about 65k miles. After about 95-90k miles it suddenly begin to run very rough with loss of power and engine missing. My code was a misfire at #8. I changed the plug and problem was resolved. Same problem at about 115. Replaced plug and again resolved. Problem again at about 134k. Changed plug and problem somewhat resolved although I couldn't get trouble light off. Changed every sensor, filter and anything else suggested. Truck ran okay but couldn't get light off. Suddenly begin missing again with loss of power. Mechanic checked compression at #8 today and said it was low. He said my problem is either burnt valve, valve not seated or cracked valve or valve bent. Said my options are heads or engine. Before forking out major funds is there any other advice, things I should try or tests I should suggest?

 

It has been known for the heater hoses above #8 cylinder to leak and cause a ground in the plug for that cylinder. Sounds more like your problem to me. Did you change the wire for that plug? If not, start there. If so, swap the coil packs around and see if the miss goes to the other side of the motor. If so, bad coil pack. If not, and it is a stuck valve, you can try running a can of Sea-Foam through the oil and the vacuum system. Did your mechanic check any other cylinders? For a motor with that many miles, cylinder pressure will be less than factory. If #8 is within 5-10 lbs of, say 6 & 7, thats not the problem. Did you change all the sensors yourself? If so (don't get offended, I don't know your mechanical ability) is it possible you didn't plug something in put it on quite right? I ask b/c thats a mistake I would make.
Actually, I unplugged the fuel injector for #4 cyl b/c I had a misfire on it (plug) and forgot to plug it back in. Guess what, the misfire didn't go away. After 45 min of scatching my head and cursing, I realized I was an idiot, plugged the injector back in, and was good to go.
Anyway, try what I suggested, and let me know how it comes out. Good luck.

 

Thanks for the suggestions. My brother up North suspected it might be a problem with a clogged egr valve. I tried cleaning the carbon out but I didn't go deep enough. I took it to a Ford dealer yesterday. Like you mentioned pressure will be less than factory. They found the pressure to be fine. Ford removed the intake and did a thorough cleaning of the egr, injectors and throttle body. They also replaced one coil pack and plug. I almost went down the wrong path with a new engine. Although I though $700 was a little steep(I'm a tightwad) it sure beat the heck out of $2k+ for a new engine. My truck is running better than ever. I suspect that the shift problem I've complained about for the past year wasn't related to my transmission as I had been told but to this carbon build up and a vacumn issue.

 

I should have mentioned about cleaning out the throttle body...Sorry. It's actually quite simple. I took mine off and used a drill bit (by hand) to eat the carbon away. Also used 2 cans of carb cleaner. Running much better afterwards. Anyway, I'm glad your back up and running. Anything else, let us know.