Gas Gauge issue

[My78truck]

I had my original sending unit replaced about 3 years ago because I had needle bounce when driving. Everything was fine, at least I thought, until today. I decided to drive my truck around today running as much of the gas out currently in the tank out without running out. I started at 1/2 tank and when I filled it after running around and doing errands ( a total of about 50 miles) the needle was registering at just about 1/8th tank. Went to "fill" my tank and I only had to put 8 1/2 gallons in. I know this is a 15 gallon tank. So if the guage registered 1/8th above empty, shouldn't I have had to put at least 12-13 gallons in my tank before it registered full? Because of how low the gas fill inlet is, I always get some splash back. I was manually holding the trigger and it splashed back. I waited about 15 seconds until things settled and then started pumping again at low volume and again got splash back. I do not!!!! want to take this someplace again and have the tank dropped again. This would explain how I can drive 20 miles and burn over 1/4 tank gas. The gauge is not registering. Is there anyway to calibrate the gas gauge?

 

[Snoots]

Likely a bad ground at the sensor. Check the input at the gauge and you should have about 90 ohms with a full tank, 0 at empty.

 

[Bradsterk3500]

Likely a bad ground at the sensor. Check the input at the gauge and you should have about 90 ohms with a full tank, 0 at empty.

This is great info. My gauge has been wonky ever since I got the truck. I'll do both your suggestions and see what happens

 

[Grit dog]

I thought your truck was a long bed. Never saw a LB with a 16 gal tank. (They are either 16 or 20 only afaik). Did the 8.5 gal peg it at full full?
In the meantime, so you know where you’re at, why not find “empty” then go from there. Pack a jug of gas, run ‘er til she dies and then fill up taking into account what you added from the jug and how far to the gas station. Then run it empty again. Nothing the total miles on that tank. Then you can get a pretty accurate mpg and a miles to empty when you fill up.
Just a suggestion until you figure out your gauge issue.
And fwiw, both my squares 70s and 80s model seem fairly accurate except that there appears to be about 3 gal left when any of the 4 tanks hit E on the gauge and it it stops moving.
From there I can drive quite a ways on E. Think of it as a reserve capacity.
Assuming the gauge cycles fully from F to E as you use a whole tank then you may be chasing something that isn’t wrong. But you won’t know without doing the empty, fill, empty test IMO.
These are old trucks. If the gauges are working that’s a win right there.
Just some suggestions for a workaround or verification.
Fortunately even the good mileage ones ain’t that great so shouldn’t take too long to do the test. And if it’s full or almost full now, top it off and run it dry, track mileage now and measure gallons later.

 

[Ricko1966]

Sounds like the float arm needs bent,just like the old float and arm system in a toilet. It will be a pain in the arse, I wouldn't bother with it,but if you want to know how post back I'll tell you.

 

[Turbo4whl]

I thought your truck was a long bed.

The original post, by Becky was a year ago. So 2 different trucks.

 

[Grit dog]

The original post, by Becky was a year ago. So 2 different trucks.

Oops. My bad

 

[My78truck]

I thought your truck was a long bed. Never saw a LB with a 16 gal tank. (They are either 16 or 20 only afaik). Did the 8.5 gal peg it at full full?
In the meantime, so you know where you’re at, why not find “empty” then go from there. Pack a jug of gas, run ‘er til she dies and then fill up taking into account what you added from the jug and how far to the gas station. Then run it empty again. Nothing the total miles on that tank. Then you can get a pretty accurate mpg and a miles to empty when you fill up.
Just a suggestion until you figure out your gauge issue.
And fwiw, both my squares 70s and 80s model seem fairly accurate except that there appears to be about 3 gal left when any of the 4 tanks hit E on the gauge and it it stops moving.
From there I can drive quite a ways on E. Think of it as a reserve capacity.
Assuming the gauge cycles fully from F to E as you use a whole tank then you may be chasing something that isn’t wrong. But you won’t know without doing the empty, fill, empty test IMO.
These are old trucks. If the gauges are working that’s a win right there.
Just some suggestions for a workaround or verification.
Fortunately even the good mileage ones ain’t that great so shouldn’t take too long to do the test. And if it’s full or almost full now, top it off and run it dry, track mileage now and measure gallons later.

Hello, after a year............

Truck is short bed.
The 8.5 gal pegged it at full.
I have clocked the mileage around town from fill up to fill up and getting about 11-12mpg
I have thought about the run her till she is empty with a can of gas, need to do that.
When I filled it up last time, it was down on 3/4 empty, I filled it and when the key was turned and put in gear it went up to 1/2, when I started moving it went up to "F".
I still need to take Snoots suggestion and check ground, although checking ground AT the sending unit would require dropping the tank.......again since it sits up against the underside of the bed. And no! not cutting a hole in my bed. So will have to check the wire connection on frame.

 

[Grit dog]

^I think you could reach in there to check ground, but sounds like the float may just be a little sticky.
Again, IMO, if it reads full when full and Empty when it’s starts sputtering. It’s not a research project to endeavor unless maybe one is doing a high dollar nut and bolt resto, or similar.

 

[My78truck]

^I think you could reach in there to check ground, but sounds like the float may just be a little sticky.
Again, IMO, if it reads full when full and Empty when it’s starts sputtering. It’s not a research project to endeavor unless maybe one is doing a high dollar nut and bolt resto, or similar.

Thank you. I did restore, but only for my satisfaction and pride. I will fill her up, take an odometer and run her dry, take an odometer and then fill up again and repeat. Of course, the sending unit I bought, although from Summit, is just Chinese junk. Old one was shot, and better quality, but it is what it is without dropping the tank and replacing, which could still happen. Depends on how stuck in my craw it is.

 

[Ricko1966]

Thank you. I did restore, but only for my satisfaction and pride. I will fill her up, take an odometer and run her dry, take an odometer and then fill up again and repeat. Of course, the sending unit I bought, although from Summit, is just Chinese junk. Old one was shot, and better quality, but it is what it is without dropping the tank and replacing, which could still happen. Depends on how stuck in my craw it is.

Why run it dry? Fill it run it 100 miles,fill it. Do the math now you'vegot a baseline I drove 100 miles on 10 gallons of gas so, I get 10 miles per gallon. Do that a couple of times to get a baseline you get 10mpg every time. Okay run it 140 miles that should be 14 gallons. Fill it did it take 14 to fill it,15 to fill it 13 to fill it. That's going to get you pretty good numbers without sucking the crap off the bottom of the tank sucking the lines and carb dry etc. And you aren't standing at the side of the road pouring in gas and trying to prime the carb. Just my .02