My 79 F150 has dual tanks. It's been sitting mostly idle for a couple of years with only some isolated trips to the dump. In the past I'd seen instances where a tank would show empty after filling, to eventually come up to a realistic reading as if the float didn't want to float at first and then did. In the last few days I've been starting it and driving it. When I put gas in it, I switched tanks and the gauge went to empty, when I switched it back, that tank read empty too. I know I have significant amounts of fresh gas in both tanks.

My thought is that old fuel has varnished on the sending mechanism. I put in some fuel conditioner and hope that maybe it might dissolve the varnish and free up the float action after it sits and sloshes around for awhile. Is this a realistic hope, or am I doomed to having to drop the tanks and fix or replace the sending units?

 

mite be time for a new sending unit

 

More likely are bad floats than anything else? They get pinholes and a bit of fuel in them and flop around, and eventually sink.

 

The way to test the sending unit is in here.

https://www.ford-trucks.com/forums/1...then-some.html