Just purchased my truck. Duel tank, both fuel pumps work but when pulling fuel from rear tank it also fills up front tank. If front tank is full it will continue to fill the tank to the point of over flowing. Pouring fuel out of fuel door and down the side of the body. How can I fix this issue. Tbis is my first post let me know if more info is needed. Thanks

 

Year, engine,
Sounds like a check valve failure. I believe a number of years ago, Ford had a kit for this problem. Consisted of 2 replacement check valves and I believe a pressure regulator. I've never had this problem so I'm really not all that familiar with repair process.

 

The truck is a 1996 F250 with 351w

 

That's the right time frame. Mine is a 1996 F250 Extended Cab PSD. Several years ago I noticed that my front tank was ending up with more diesel than I thought it should have. Fuel mileage on the back tank seemed to be dropping off. Before I could really get into it, hunting season came on and a lot of miles on rough and rugged mountain logging and USFS roads took place. Must have shook something loose because I never noticed it after the hunting season was over with and I never really tore into the problem.

Before tearing into it, you might try filling the front tank and running some fuel system cleaner through it (and the rear), but use the fuel in the front tank before switching to the rear. That way you will have plenty of room for any back flow.....

 

Ill run some cleaner through it....is there a mechanism / valve that closes when switching to the rear tank to keep from back flow? I'll do my homework too. Also report back after run cleaner through it. Thanks fellows!