I need help with diagnosing a problem I have with my 1991 F150 5.0. When I start my truck the check engine light isn't on but after driving for a mile or so it turns on and i lose throttle responsiveness. Ive looked it up online and some sites have some symptoms mine has but no exact match. I'm thinking that it might be an O2 sensor or mass air flow but I am no master mechanic. Any help or ideas is well appreciated.

 

I'd say first thing to do if get the code from the CEL. O2 sensors are very easy to replace on these trucks so if it has the original O2 sensor or you've never replaced it in the time that you've owned the truck I would put a new sensor in either way

 

Agreed. Sounds like an 02 sensor. First thought that came to mind anyway.
But read the codes - that's what they're there for.
Unless spending money is easier than learning how to count flashes.