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My friend recently bought a 1996 f250 with a 4.9. Really just a great truck, just one problem. The rear tank leaks gas from the cap. Its like it builds up pressure or something. He only uses the front tank and ive suggested he use both to keep them in working condition. If you tighten the cap it stops for awhile and when loosened it hisses like a tank will when its almost empty but there is fuel and it actually leaks from the cap, more than just a little. Any suggestion anyone?
What is a fmu? Doesn't the entire fuel components inside the tank come as one unit on this model? Pump,float, strainer?
Yes the unit is inside the tank, a check valve prevents the fuel from returning to the wrong tank, those valves are a known problem for a few model years.
I didn't think 96 was included in there, corrected the problem but who knows what might have been installed as a replacement part in all this time, old stock for a early year or used out of a bone yard for example.
Need to verify that is what is happening, for example running truck on front tank, rear tank slowly fills and or overflows out filler neck, or vise versa.
To verify after you strongly suspect that is what its doing use something like vise grips to pinch off return line to effected tank. If it doesn't fill/over flow it with it pinched off it would clearly show you that's the problem.
Doubt you'd have to go to that extreme but it would work to verify it.
Yeah it sounds like it has the crossflow problem between the 2 tanks. For additional information try doing an advanced search. There are several detailed threads about it.