Horrible gas mileage

[Rc 415]

Hey guys, I am having all kinds of trouble with my fuel system, first off the gauge does not work at all, it always reads full, until it runs out of gas, then sometimes the gauge will drop to empty or close to it. Anyway, I have the dual 16 gallon tanks, and I filled them both up 4 days ago, drove about 60 miles, and now one tank is nearly empty, I know because the truck died on me while going up a hill. Thats 3.75 miles to the gallon! what in the world can be causing this!? Where should I start to fix my problems? My motor is a basic 350, shouldn't drink gas like this! The exhaust does not smell rich either, Spark plugs are not fouled, new air cleaner, O2 sensor, oil change, lots of small stuff. Another thing that I figure I might as well mention, the engine does almost what seems like a misfire, occasionally, but I only notice it when it is in overdrive, not second or third.

[old Rusty C10]

thats bad even by nyc driving standards... do you have black smoke indicitive of too much fuel going into the carb? and are you sure there is no leak anywhere? check the oil to make sure there is no gas in the oil check and recheck the lines
and for fun are you sure someone isnt siphoning your tank at night... and one last stupid question are you sure the tank is taking the full 16 gallonsand not topping out with just a couple
one more dumb idea... are you sure its out of gas? the tank isnt getting its sucking mechanism blocked or some kind of vapor lock?

[Rc 415]

Originally Posted by old Rusty C10

thats bad even by nyc driving standards... do you have black smoke indicitive of too much fuel going into the carb? and are you sure there is no leak anywhere? check the oil to make sure there is no gas in the oil check and recheck the lines
and for fun are you sure someone isnt siphoning your tank at night... and one last stupid question are you sure the tank is taking the full 16 gallonsand not topping out with just a couple
one more dumb idea... are you sure its out of gas? the tank isnt getting its sucking mechanism blocked or some kind of vapor lock?

No smoke at all.
I am not positive there is no leak, but i dont smell or see any fuel when I'm under the truck.
No gas in the oil.
No siphoning, i have lock caps and the truck is just plain in accessible to anyone but me.
tank is definitely taking the 16, I know because my wallet is feeling the pain.
I tapped the side thats empty and it is definitely empty or very low, while the other side is full.

[streetstar]

Did you change out your carb yet? My apologies if thats not you, but i thought you were looking for a new carb last month or something

[old Rusty C10]

the only thing that comes to mind ( but you would smell something) is years back i had an 89 F 250 that has electric fuel pumps and if you filled both tanks and ran it off the back tank first it would pump gas into the front tank ( overfilling it and spilling it) and you didnt see or smell gas.. i had to remember when i filled it up to use the front tank and then the rear and it would run out in like a few hours and then id switch back to the front... Ford ran pumps in both tanks and one on the rail and the one on the rail was the one that was defective.. once changed ( for like $165 parts) i had no problem
i guess your truck is EFI ( like my ford was) could that be a similar issue

[Rc 415]

Originally Posted by streetstar

Did you change out your carb yet? My apologies if thats not you, but i thought you were looking for a new carb last month or something

I didnt, the more I thought about it the more it seemed like a waste of money to get the same carb when everyone was saying the computer system is useless.

[Rc 415]

Originally Posted by old Rusty C10

the only thing that comes to mind ( but you would smell something) is years back i had an 89 F 250 that has electric fuel pumps and if you filled both tanks and ran it off the back tank first it would pump gas into the front tank ( overfilling it and spilling it) and you didnt see or smell gas.. i had to remember when i filled it up to use the front tank and then the rear and it would run out in like a few hours and then id switch back to the front... Ford ran pumps in both tanks and one on the rail and the one on the rail was the one that was defective.. once changed ( for like $165 parts) i had no problem
i guess your truck is EFI ( like my ford was) could that be a similar issue

As far as I know my truck is not EFI, but I am clueless when it comes to engines..

[xtreme80]

You cannot base your fuel mileage on what your tank has or shows. You really need to actually do the math.

Fill the tank till the nozzle shuts off. Don't top it off. Drive it. Record your mileage, and fill it again in a day or two till the nozzle shuts off. Divide how many miles you drove by how many gallons you put in the second time around.

If in fact you're getting mileage that low, you're either running incredibly rich, or you're losing gas somewhere. There's really no other explanation for it. And I honestly don't think that truck would even run at all if it was THAT rich... that would be flooding the engine for sure.

Gas evaporates fast. It may be hard to pinpoint a leak, but I'd bet money there's a leak somewhere. My buddy's 502 race motor gets as good of mileage as that!