I had noticed the power was down,engine would "smother" at cold startup(same symptoms as a stuck choke on older models) and fuel mileage was 13 mpg combined highway and city.There was no check engine light or codes.
The air and fuel filters were replaced, the fuel/intake systems were professionally cleaned, and 8 ounces of Lucas fuel treatment were added at 100k miles.
Those repairs helped slightly so I cleaned the mass airflow sensor,replaced the spark plugs with Autolite double platinum APP25 gapped at .054,replaced the plug wires with Motorcraft, and the distributor cap and rotor were replaced with Borg Warner.

I am glad to report the engine runs as good as new and fuel mileage is now 20 mpg combined
highway and city for the first time ever. No jack rabbit starts but running 80 mph on the highway.

Maybe this will help someone with similar problems.

 

You might get a little better gas milage if you change the o2 sensors

 

Sounds like your truck does pretty good for economy. With my 89 it does about 19-21 as a pretty solid average on the highway, and thats with a 5 speed manual.

I havent checked my 02 sensor ever since I bought the truck, about ten years ago. Might be time to get me a new one

 

I guess I have never checked the mpg but my 94 f150 with the 5.0 gets reliably 300 miles to a tank with in couple miles either way. I use the trip meter to decide when to fill the other tank.

 

Changed at 90k and truck has 101k.

 

Green 69 F100, What else have you done to your truck? I have a similar setup and I get an average of about 16 mpg city and highway. You can see a list of all of my mods in my gallery.

Jonpfeifer, How are you getting 300 miles to the tank? Do you run it all the way down below E?

 

any one can point out that lucas oil treatment? i wanna try some.. but i cant find it.. and anyone can tellme wich one to use?

 

It is mostly original however, did change tire size from P235/75/15 to P255/70/15. I hope to set up a gallery soon.

 

I only use the Lucas fuel treatment and it works with diesel or gasoline engines.

 

Man I really need to clean some crap up this weekend. I'm getting around 10-12mpg highway and city. 20 bucks gets me 4-5days of gas and thats only if I go to school and home no side trips to get some

 

The Chevron concentrated Techron stuff works well, so does Berrymans. Berrymans used to have a product called total fuel system I know for a fact that it worked. When I ran it for a while, I could throttle it and black soot and crap would blow out the tail pipe. I have heard great stuff about Seafoam, but never used it. I don't particularly car for Lucas, becuase I have seen that it foams up in the crank case. That being the case, I don't know what it does in the fuel system. I won't use it.

 

I also have 255 70 15 tires. I wonder what you have that I don't. Altough my truck has the 4R70W and your truck has the E40D. I wouldn't think that would be the reason though. If anything I would have thought the the 4R70W would be better for fuel economy. It also doesn't weigh as much.