Hey everyone,
I know this has been beat to death but i just cant find a straight answer anywhere but here is what i got.

I have a 2004 f150 5.4 lariat crew. Just about 100k for miles, I have replaced spark plugs at about 91k. Now i am having a problem with misfires
(what i believe) any time i am driving about 55-65 mph on the freeway it starts to buck and "pop" but if i accelerate harder and it shifts down it goes away, other times if im getting on the on ramp and i floor it it misfires really bad.
This has been going on for a couple months and it NEVER throws a code, and i drive about 100+ miles a day! so this is getting old.
I am just wondering why is wont throw a code? where do i go from here? do i have to replace all the cops? at about 40 dollars a piece i hate to do it.

Any help would be great! Thanks again!!

 

It will throw a code sooner or later. But on a serious note have all the COP's been replaced when the spark plugs were done? If they werent then there is the problem. If they were then it could be something along the lines of a clogged cat, or dirty MAF sensor or dirty throtle body.

 

Did you use Motorcraft plugs? If not, you should have at least replaced the COP boots.

Try to force a CEL by finding the spot where it misses and hold it there.

If you want to give it a try, search Ebay for "DG511" - some sellers have generics for under $100 a set. I also see genuine Motorcrafts for considerably cheaper than $40 each.

Did you use dielectric grease when you changed the plugs?

 

I had a similar issue with my 1997 5.4. I changed the plugs for reg maintanance. Wound up with a misfire. Replaced the COP w/boots. Different cyl misfiring now. Checked the connectors at the Coils and found a bad connector. Replaced it and now it runs great under load and light load. Hope this helps. Tim