I've been having issues with my 2001 5.4 running rough at speeds around 50-60mph. I took it in to have this addressed along with oil change about a month ago. I had them change spark plugs (they said the coil packs looked ok) as well as a tune up. They said there was a small vacuum leak as well that they fixed. I got it back and it ran great for about two weeks. It all started back up again last week while I was towing my boat. It seems to be only when the truck is at low rpms, 12-1500, and only at higher speeds. There's no fluctuation in rpms that I can tell, just a lot of shaking until the rpms get back above 1500. It also doesn't happen all the time. Looking for any input as to what y'all might think about it.

 

Common misfire, probably a weak COP that's breaking down internally under severe load.

 

Yeah, it's a coil. I have never been able to get the computer to throw a code to tell me which coil. Even if you do find the bad coil, another will will sympathetically fail. Buy all 8. I would say get the Visteon coils, but I don't see them around anymore. Search ebay for DG508 and there will be plenty to choose from and they aren't that expensive.

 

Classic response to new spark plugs.
.
A coil that is marginal for spark will fire that cylinder when a new plug is installed because the new plug tip is sharp and needs less voltage to jump the gap.
As soon as the electrode tips begin to erode the voltage requirement begins to rise again causing the same misfire you originally had.
This shows how fast plug tips begin to wear.
You have one coil with shorted turns that lowers it's output voltage.
Normally coils have a large over voltage capability but a fault of this type has a huge effect on them.
The way to find it is with a Scanner looking at mode 6 and the cylinder misfire monitors.
The one with the high count is the cylinder's coil to replace.
The shop you took it to doesn't know these things in detail.
Good luck.

 

http://www.araparts.com/dg508-style-...-set-of-8.html