I drove my Vette today for the first time in around two weeks. The gas gauge was registering a full tank. While driving I noticed that the fuel info on the spedometer kept blanking out and reading "sys". By the time I got to work I noticed that the gas gauge had not moved at all. Between the distance I traveled since I got gas (last time I drove the car) and now I figure I should have burned through at last a quarter tank. The last time I got gas I did not use the station that I normally use because I was running dangerously low (reserve light was on). Could I have possibly done something to knock out or damage the fuel guage? Has this happened to anyone else? Right now my gut says that it might be the fuel gauge in the tank but have not ruled out a computer problem. If anyone has any insight please help. Thanks!!

_Brian

 

Most of the time it is the fuel tank sending unit, a small potentiometer in the fuel tank.

If you have a multimeter for checking electronics, there's a procedure to use to see if the unit is bad by taking the unit off the tank.

FWIW, GM makes you buy the entire fuel filler weldment assembly when you go to replace, which costs nearly $500 at wholesale prices. You might want to check with junk and salvage yards and websites and ask if they'll sell you *just* the potentiometer for great savings.

 

I agree with Matt that it is probably due to the potentiometer on the sending unit. They wear out and stop working correctly due to water/moisture in the tank causing corrosion or crud to get inside them. Before buying a complete sending unit try pulling your sending unit and fixing the potentiometer yourself. That is what I did when water in my tank wasted my fuel pump and disabled my fuel level potentiometer. Removing the sending unit assembly is pretty easy and takes 15~30 minutes depending on if you have done it before.

I removed the entire sending unit frame assembly from the gas tank and then carefully pried open the small metal tabs on the potentiometer enough to open the metal enclosure. Inside is a series of very fine wire windings and a brass contact on a flexible metal flange. This is connected directly to the float arm. As the float moves it causes the brass contact to rub across the windings changing the resistance of the circuit. The guage in the dash uses this resistance value to decide what fuel level to display.

Using WD40 and 400 grit (extra fine) sandpaper I LIGHTLY sanded the brass tab on the side of it that would contact the windings. There was a wire end sticking out from the windings and contacting the potentiometer casing which I pushed back in where it looked like it belonged so it didn't contact the case incorrectly. I also rubbed the windings with a clean rag and a little WD40 being sure to wipe it clean of WD40 when done.

Using a volt-ohm meter I tested the wires leading to the float/potentiometer unit and saw a varying resistance whree there was no variance before. Varying resistance is GOOD. Something I did solved the problem. I put the potentiometer casing back together rebending the tabs to hold it closed. I attached my new fuel pump and re-installed the sending unit assembly in the tank. Fired it up and gas level displayed correctly.

If you do this be sure to buy a new tank gasket for the sending unit and put it on correctly. The gasket has an orientation notch cut out that needs to be in the upper right corner when installed. It can be installed incorrectly and this causes the gasket to stretch and possibly leak water into the tank. The left side 2 screws are a wee bit farther apart that the right side 2 screws. That is the reason for the orientation notch.

Also be sure to test the potentiometer using a volt-ohm meter connected to the 2 wires for the float by moving the float. If the potentiometer is working correctly, you should see a change in resistance as you move the float. If it isn't working correctly before you put it back in the tank, it is unlikely it will work correctly after you put it in the tank.

 

Thanks for the info!!

 

I firmly agree that it sounds like the sender has gone bad.