I've got a 1990 F-150 4x4 w/351W. It used to get around 15-16 mpg on the highway up until around 80,000 miles. Then it dropped to around 8-10 mpg. Well the truck now has 99,700 miles on it. The engine now has been rebuilt and has a little over 5,000 miles on and a new heavy duty torque converter in the AOD-E. The truck still only gets around 8-10 mpg. What all would need to be replaced on a 100,000 mile truck to regain mpg's back? I'm thinking I probably need to replace the O2's, cats, plug wires (new plugs went in after the rebuild) should I get 9mm wires?, maybe fuel pump (fuel filter is new), it has K&N filter. This is speed density motor, too, not mass air. Dual throttle body probably needs to be cleaned too? What else do you think should be done? What do you think about the one's i listed? Thanks.

 

I'd try the basic tune up items first. Wires, distributor cap, rotor. Check the plugs while you are at it. If the components weren't in good shape the plugs could suffer.
The O2 sensor would be my second try. I've seen posts that seem to indicate that new O2 sensors helped mileage, even when the old one was functioning well enough to not set any computer codes. These are fairly inexpensive, and I think your truck only has one.
General clean up of the throttle bodies and sensors surely couldn't hurt. You might want to check out the TPS also. These seem to be a source of fine tuning for many.

 

MO4x4 - Also check and make sure you have a clean air filter. Check for low tire pressure. Both will cause low mpg.

Also, a truck that age, check exhaust restrictions including catalic converter. Any restrictions will hurt your MPG.

 

What does the TPS deal that you are talking about consist of doing? I know that is the Throttle Position Sensor but what needs to be performed to it in this case.
Thanks for the help

 

you might also check the fuel injection system. I've seen fuel pressure regulators go bad on these engines causing the fuel pressure to be way too high. normal fuel pressure should be around 30-35 psi. Too high fuel pressure will cause poor fuel mileage.I'd also clean the fuel injectors & the throttle body.

Jay

 

[quote]What does the TPS deal that you are talking about consist of doing? I know that is the Throttle Position Sensor but what needs to be performed to it in this case.
Thanks for the help[/unquote]

What I was referring to was adjusting the TPS to factory specs. It is actually just "fine tuning" the position of the TPS. I have seen posts on this and other boards that state that this makes a difference you can feel seat of the pants. My assumption was that if it makes a power difference you can feel, it would probably have an effect on fuel economy as well.
The procedure itself is easy, and is in most repair manuals. If you don't have a book let me know and I'll quote mine....

 

Thanks for the info. I'm plannin' on getting a manual for it, just haven't takin' the time to. I'll check it out.