engine problems?
 

my buddy is looking to buy an '85 6.2L of another friend. for some reason when the truck is warmed up, and should be running better, it lags and will only do 5-15 mph... is this a major problem or an easy fix???? :humm:

this sounds more like a transmission problem then a engine problem, does it shift. do a compression test. whats the fuel flow like?

Does the truck sputter when it is doing 5-15? Sounds more like a fuel flow problem to me. Unless the engine needs completely rebuilt there isn't much that can be a major problem.