I have a 01 F-150 with a 4.2. Please don't laugh too much. It gets consistantly 19.7 mpg on the highway. An odd thing has developed lately. At speeds around 50 mph and up it seems to miss, and sputter like going up hills, and if you have to get off the line it will stumble or sputter. I have replaced the air filter, fuel filter and have run gas treatment and Amoco 93 in it and it still does it. The truck only has 40K on it and the dreaded "Check Engine Light" is not coming on, and it passed emissions with no problem. Any help?

Thanks.

 

Sounds to me like you've got a misfiring spark plug or wire.
Time to get it checked out or just try replacing the plugs.

 

I had the same problem with my 4.2. Try this With someone reving the engine or just at idle. Spray water from a water bottle on the wires and around the spark plugs. Listen for snapping or popping of the arcing wires. I replaced all the wires and replaced the plugs and that cured it. Hope yours is that simple.