C4 Fuel Sending Unit Repair
Q: My '90 Corvette fuel gauge will display a full tank of gas until the tank is a little less than half empty. Then the gauge appears to read correctly until it is time to fill the tank again. I'm concerned that the gauge may stop reading properly at all and I don't want to run the vehicle out of fuel. I only drive it on nice weekends and sometimes I don't remember exactly how much fuel was left in the car from week to week. Do you think this is a gauge or wiring problem? Thanks.
via question at a Corvette seminar
A: Mike, as we discussed at the seminar, the most common cause for this problem on older vehicles is the fuel-tank sending unit has an internal fault. This is generally caused by certain types of gasoline that can cause deposits to build up on the fuel-sending-unit potentiometer (variably resistor). This can cause the fuel gauge to be erratic or even non-functional. Remember fuels have changed tremendously over the last few years and some components on older vehicles were not designed with today's blends of fuels in mind.
To repair this problem, the fuel-sending unit will need to be removed from the fuel tank. After removal, the resistor and wiper can be accessed by bending back three small metal tabs and removing the cover.
Upon visual inspection you will notice that the small metal wiper slides up and down across a resistor. If you observe wear marks or a buildup of gray residue on the resistor, take some fine Scotch-Brite (or its equivalent) and gently clean the face of the resistor assembly and the face of the wiper contact. Take care not to damage any of the components during cleaning.
Also inspect for broken wires on the resistor, weak resistor-spring tension, or a fuel float that has become heavy due to fuel saturation.
After performing the repair you can check your work with a Digital Volt Ohm Meter (DVOM). With the float resting at the bottom of its travel, you should get a low resistance. Slowly raise the float toward the top of its travel. You should get a steady increase in resistance with no flat spots. Flat spots are locations where the resistance drops to zero momentarily. Once the reading is steady with no flat spots, the repair is complete.
Mike, sometimes these fuel-sending units cannot be repaired and will need to be replaced. Some of them, such as the one in my C4 ZR-1, can be very expensive, so if you can repair it and save a little bit of money, I am all for it. Good luck.