Fuel guage suddenly reading 1/2 empty with a full tank

[chevymotocross]

My truck in signature below has a new odd issue. Today I was at about 1/4 tank of fuel so I filled up. Started up and drove away. Gauge now only reads 1/2 empty. Gauge always worked properly. Unless maybe I missed the rest of the gas and poured it on the ground.... no that didn't happen.

What are some simple things to check for? I am hoping it is not the sending unit in the tank..... I've had this tank dropped twice for two failed pumps, I'd like to not drop it again. The last pump was replaced months ago so I know it wasn't the pump install that affected the gauge readings.

It's a dual 16gal tank 1987 swb 305

[old Rusty C10]

ground on the tank? does the other side work ok?

[flatblkserria82]

mine is reading right above quarter tank when full but still moves to empty as it uses fuel...

[03cobrakid]

Originally Posted by flatblkserria82

mine is reading right above quarter tank when full but still moves to empty as it uses fuel...

Mine is doing the same thing!!
I put in a new sending unit and made sure ground was good at frame and it is still only reading Quarter tank when full?? I guess i will have to try a new guage

[BigE77']

Originally Posted by old Rusty C10

ground on the tank? does the other side work ok?

Would a bad ground cause the tank to read way past full...both sides do it, they both go back to empty when the key is off. When the key is on they both race up to way past full??

[87Bob]

Guys what you may be seeing is the effect that sulfur in the fuel has on the resistor in the tank. Cheap gas is high in sulfur try adding techron to a few tanks and see if it clears up. Here is a copy of a post from a Corvette Forum where there have been a lot of these issues


#1 cause sulfur build up on the sending unit from using gas with insufficient detergent package....
Use a good fuel system cleaner like Techron fuel system cleaner ( NOT the injector cleaner) , Seafoam.
Berrymans Chemtool B-12, Valvoline Synpower fuel system cleaner for a coule tank fulls and switch to a top tier gasoline www.toptiergas.com

#2. corroded ground for the sending unit
#3. bad sending unit

[joe231]

Originally Posted by BigE77'

Would a bad ground cause the tank to read way past full...both sides do it, they both go back to empty when the key is off. When the key is on they both race up to way past full??

if the circuit is open, broken wire or no ground, it will read way past full like yours is and drop to empty with the key off. If it is the same for both tanks, I would check your tank changing switch.

[BigE77']

Originally Posted by 87Bob

Guys what you may be seeing is the effect that sulfur in the fuel has on the resistor in the tank. Cheap gas is high in sulfur try adding techron to a few tanks and see if it clears up. Here is a copy of a post from a Corvette Forum where there have been a lot of these issues


#1 cause sulfur build up on the sending unit from using gas with insufficient detergent package....
Use a good fuel system cleaner like Techron fuel system cleaner ( NOT the injector cleaner) , Seafoam.
Berrymans Chemtool B-12, Valvoline Synpower fuel system cleaner for a coule tank fulls and switch to a top tier gasoline www.toptiergas.com

#2. corroded ground for the sending unit
#3. bad sending unit

I recently rebuilt the motor and the fuel guages worked prior to pulling the motor. After install they quit working, I must have pinced or moved a wire or something.....I have only been running 93 since the rebuild...Sometimes things show up at the stranges times though it could just be a coincedence, and be residual crud from previous cheap gas..im not leaving anything out of the equation