I'll explain the scenario, and see if you can figure this out. I was getting a lean fuel error code on the check engine light. My front tank was leaking very slightly, and when I tried to start it with front tank selected, it would sputter and spit. When I switched to the rear tank and reset the code, the light stayed off, and it started just fine. So I figured I just had a leaking front tank and I'd fix it later. I still had a full front tank, so to empty it, I would switch it to the front while I was on the highway, because it ran fine then (it only sputtered when I stopped). The fuel gauge showed full (like way full) as I drove. So, today, I notice fuel dripping from the door on the front fuel tank. I started to take the cap off, and fuel started spewing from the top of the filler tube. I'm starting to wonder if it's not the tank and something else. What would cause this issue?
BTW, I have another issue with 4x4 that I posted in another thread if your interested.

 

was a little confused by the sequence of events as you listed them - but will say as I just said in another post - there is a fairly common problem with the dual tanks where if you run off the REAR tank - the return gas can end up dumping into the FRONT tank - possibly making it overflow if it's full.

Not sure if that explains what you're seeing?

 

Well, if fuel is being pumped from rear to front under pressure, it would explain the fuel gauge showing "way" full, it would explain the fuel spilling over, and it would explain something I didn't mention: rotten mileage. I checked the mileage by filling the rear tank, running on rear only until near empty, and refilling rear again. The numbers said i was only getting 8 mpg. Terrible for a 300 straight six. I didn't tow anything and only went 65 mph on highway.
So, anyone know how or why fuel gets pumped from rear to front??

 

The check valve is stuck open in the fuel delivery module.. Not the pump, but the part the pump attaches to....


Just do a search on fuel transfer in these year trucks, there will be MANY pages of reading....

 

OK, I'll search. Is the fuel delivery module inside the tank - like the sender, pump, and FDM all in one piece?

 

Yep... Front tank has the bad valve.. since it is the one filling all the time...

Here is a drawing to give you an idea what I'm talking about....