1996 F 150 w/ EFI (straight 6) 300 cid w/5 Speed Trans.

I've run into a few issues with the 300 six. First is the intake/exhaust manifold gasket. Because there is so much weight hanging off of one side of the motor and because the two manifolds run at different temperatures along side of each other, the nuts can loosen, manifolds warp, gaskets fail and vacuum leaks occur. In my opinion: those Manifold studs and nuts should be checked every year. More often if you haul or tow. The other big concern I've seen is a failing Ignition Module. Their faults are heat related. Especially when you allow it to cool off and it starts up and runs fine. Realistically, this is a big 6, I hope you mean worse gas mileage than usual because it doesn't get great mileage to begin with. It's likely that the parts stores don't have or don't know how to read codes from your truck because it's an older OBD1 system and, lets face it, most of the people in parts stores are Retail Clerks, not mechanics. So, there's a few things to look at but you may consider calling around to a few shops (yeah, the dealer too) and pricing a diagnosis - some shops will soak up the cost of a diagnostic in to the repair. You're right though, guessing gets expensive.

Thank You...Tracy, I think I will try a shop and let them run diagnostic. In the long run this may be cheaper. Thank You again my Friend !

You are very welcome.

Check to make sure that the heat exchanger is in good working order, you will find it located where the intake & exhaust manifold join together, they have a tendency to seize up causing exsesive heat to return though intake ,that in turn causes the head to overheat ,witch will cause push rods to get bent not allowing valves to open all the way..hence ruff idle and poor power. Pull the side cover and look into the oil gallery,check for bent push rods or bad lifters... I’ll bet dollars to donuts that is where your problem is