My 94 F150xl has a problem I have not seen much information on. I have the dual tank system and what happens is that I can fill up the front tank, drive on it for awhile, and the gas just seems to disappear. Seeming like poor gas mileage, I would switch to the rear tank, which had not been filled for some time, and it miraculously has fuel in it, most likely from the front tank. Anyone else had this problem or know how to fix it?

 

Have you replaced that valve recently? Maybe the line crossed?

 

You have a bad FDM in the rear tank. You need to replace it.
It has a bad HP valve or shuttle valve in it.

The FDM:




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Thanks guys! Helped a lot.