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Hey everyone, I have a new to me 79 f250 with a 400 79000 miles.. Completely stock down to the 2 barrel. I tried searching for any like posts before adding anything but I didn’t see anything that really matches what I’m looking for. My truck seems to be using way too much fuel, a 3 mile round trip yesterday used a 1/4 tank. It does have dual tanks, I’m thinking the switch is bad but I don’t know these trucks well enough yet. The truck runs great otherwise, I did a complete tuneup, no vacuum leaks. and even went through the carb. There’s no fuel smell in the oil either. Smog equipment is still on the motor, I just cut the belt do reduce unneeded drag on the motor.
@440sixpack. thanks for the reply. I’ve had the truck since November. It seems to have gotten worse recently. I’m thinking the tank switch is bad and it’s returning to the wrong tank however when I switch to that tank the fuel level doesn’t reflect that. I know the truck overall is going to get terrible mileage, I’m ok with that it just seems excessive and I want to make sure there isn’t something wrong. Thanks!
@440sixpack. thanks for the reply. I’ve had the truck since November. It seems to have gotten worse recently. I’m thinking the tank switch is bad and it’s returning to the wrong tank however when I switch to that tank the fuel level doesn’t reflect that. I know the truck overall is going to get terrible mileage, I’m ok with that it just seems excessive and I want to make sure there isn’t something wrong. Thanks!
You'll want to replace any soft fuel lines with an ethanol safe fuel line and some good stainless hose clamps not Chinese junk, if it hasn't been done.
The fuel tank selector switch is a notorious home for electrical gremlins as well as the fuel tank sending units. Look over the electrical wiring that feeds the tank while you're replacing soft fuel lines.
This is really the only way to get the kind of confidence you want to take the old girl on longer adventures. 🙂😎
Your truck may not even have return lines. If not, then there is no way for the fuel tank selector valve to return fuel to the other tank.
Check that first anyway, because it's easy and free! Just peek under there on the frame and if it's only got three fittings and hoses (2 in from the tanks, and 1 out to the engine) then it's a simple valve and no way to fill the wrong tank.
And don't go by the gauge no matter how well you trust it. At least for discussing fuel usage.
For usage, tell us just how much you're using. Fill the tank, drive until it's at least half empty, then refill it and divide the gallons into the miles.
If it's less than 8mpg, you still have work to do. But if it's more than 9, you're right in the ballpark!
As mentioned, 10mpg on one of these with any around town driving is doing pretty dang good!
But while the gauge is making it sound pretty bad, let's get the real check out of the way to see if you can trust it.
Also, are you running larger tires? Did you bring the speedometer back up so that it's showing accurate mileage?
Your truck may not even have return lines. If not, then there is no way for the fuel tank selector valve to return fuel to the other tank.
Check that first anyway, because it's easy and free! Just peek under there on the frame and if it's only got three fittings and hoses (2 in from the tanks, and 1 out to the engine) then it's a simple valve and no way to fill the wrong tank.
And don't go by the gauge no matter how well you trust it. At least for discussing fuel usage.
For usage, tell us just how much you're using. Fill the tank, drive until it's at least half empty, then refill it and divide the gallons into the miles.
If it's less than 8mpg, you still have work to do. But if it's more than 9, you're right in the ballpark!
As mentioned, 10mpg on one of these with any around town driving is doing pretty dang good!
But while the gauge is making it sound pretty bad, let's get the real check out of the way to see if you can trust it.
Also, are you running larger tires? Did you bring the speedometer back up so that it's showing accurate mileage?
Paul
thanks paul, good info. I checked the speedo against the GPS on my phone and and it was within a mile or 2 per hour. I’ll have to get back to you next time I get gas. In the meantime I’ll run through the items you mentioned. Thank you!
I did not know your trucks had a return fuel system thought that did not start till the 80"s
But yes look at the valve if 3 ports no return.
I would never go bu the gauge as to what the MPG might be as from full to half may take time but from half to empty drop like a rock!
I would also run it down to a 1/4 and fill up at least 3 times before I would say I get XXX for MPG
My 81 F100 has gone from about 14 MPG to a high of 17 MPG over may fill ups but would say avg is 15 MPG
That is with a 300 six, NP453 2.75 rear gear, 235/75/15 tire and advance adapter over drive in a 4x2 flare side.
Some have gotten 20 MPG with the 300 and over drive.
Dave ----
Thanks for all the good info to everyone that responded. It’s been cold and snowy lately so I haven’t gotten under the truck. But I will as soon as I’m able to. It might just be me not being used to the truck I guess. I pulled a plug and it looks to be running really well. I definitely misspoke on the return line, thanks to those that corrected me. If I find anything that might result in an issue I’ll report back. Thanks again!
