The fuel Gauge changes when going from accessory to the engine running. I tested the ohms directly from the tank full and empty and it was 0-91 ohms. perfect. I drained the tank and the gauge also went from full to empty following the correct ohms that the tank was showing. The fuel gauge is autometer set to 4-86 ohms and the tank has a 0-90 ohms sender.

I retested with an almost empty tank with the engine off it was 6.25 ohms and the gauge was one line over empty perfect. After running the engine one or two minutes the gauge goes to full and then to 1/4 and stays at 1/4. I restarted engine with no gauge attached and the tank ohms bounce around and settle at 22.5 ohms which would show 1/4 on gauge.
So it definately a resistor problem when the engine is running. I tried an extra ground strap to the tank but that did not help.

What would cause the resistance to change when started.
Thank for any help

Raising the ground potential on the body. Do you have ground straps from the fender to the battery negative terminal, and straps from the engine to body, and subframe to the body?

The connector in the trunk gets pretty cruddy. You can disconnect it, push the spades out, hit them with some emory cloth, and reassemble with some dielectric grease. Also make sure the female one hasn't lost it's "spring" so that it makes good contact with the male. You can fold it out a little if it is mashed. The only other connection is where the fuel sending line connects to the top of the sending unit. If all that is good then it's most likely the sending unit itself.

Thanks for the help guys.

I should have mentioned every part on the car is new. I have a trunk mounted battery with 00 guage grounded to frame, welded thru pan subframe connectors, grounded engine to the frame, 2 guage ground from starter to battery, grounded front sheet metal to frame and some more grounds. I'm running a lot of electrical equipment like, flaming river ignition, tec full fuel injection wiring, five amps, 10 nexus gauges, complex radio, alarm, gps locator, 160 amp alternator and much much more.

I tried jumping the power to the aeromotive 1000 fuel pump and as soon as it turns the ohms change from 1.6 to 21.0 next I'm removing the return fuel line to test.

bad fuel pump ground all fixed:hurray: