I have a 89 ford f-150 with a 4.9L and it will loose power when i accerlerate. I hear a clicking noise when i get on the gas but i cannot hear it at idle. I get really bad gas milage. I'm not sure but i'm about ready to drop another engine in if i can't figure it out. I think it could be a lifter or valve but I want to make sure before i go to the trouble. I was wandering if anyone could figure it out. THanks

 

was it tuned up recently? (plugs, wires, cap, rotor, fuel filter)
was the o2 sensor ever replaced?
what is the timing set at?
did u perform a compression test?
did u perform a leak down test?

u can also tell the condition of the engine by its intake manifold vacuum. check this link out: http://www.users.bigpond.com/ergoff/vac1.htm
gives a good description on what readings are good, bad and how to perofrm them.