I filled up both my tanks with diesel on my 97 F350 for an extended trip last weekend. Drove 260 miles on the front tank till it was just about empty and switched over to the rear tank.

On the rear tank I only got about 80 miles before it was empty. Thought maybe I was harder on the pedal or something. I stopped because I thought that was weird. I know the rear tank is smaller than the front but not that small. Anyway no leaks anywhere. I change my fuel filter regularly and use the Ford additive or Stanadyne stuff as well.

Here's where it gets weirder. I switch over to the front tank again and now it's reading over half full!! I swear it was empty before. I thought something was wrong with the fuel gauge but I had no choice but to stay on the front tank cause it was a long ways to the next gas station that has diesel. I drove another 50 miles with no problems and filled up the front tank. I will add that the gauge was right because it took a lot less diesel to fill the front tank than the first time I filled it. Seems like fuel is getting transferred from the rear to the front tank. If the gauge was wrong I would have run out of fuel. No problems with power either. I can add that the truck is made in Canada too if that makes any diffrerence.

I really don't know about the fuel system on these trucks other than its a mechanical fuel pump with two supposedly SEPARATE tanks. So when I switched to the rear tank, is it possible for the valve to be screwed up thus pumping fuel from the rear tank to the front tank. I have never come across anything like this. I've never had this problem before either and the tank switching has worked fine up till now.

What's going on here? Should I just fill both tanks and head out next weekend as I have another long trip planned? What am I looking at replacing?
My mechanic friend told me to fill them up and not worry about it if it doesn't happen again. I can add that it was pretty cold outside. Thinking maybe the valve got stuck in a weird spot.

 

Under your seat on the frame is the switching valve that selects which tank to draw from and which tank to return to. Normally it is trouble free and just works. I've never had one apart so don't know the insides but don't think they are serviceable. I would agree with your mechanic and try another trip, just keep an eye on it after switching tanks. Most likely the problem won't reappear however if it does look at replacing the switching valve assembly. That duel tank switch on the dash does does nothing but control a valve assembly solenoid that controls (supply - return fuel lines) and fuel level sensor from each tank.

 

When my selector switch went out, it would read the other tank but still draw from only one, i.e. I would switch from rear to front, it would read the front tank being full but still draw from the rear. Took me awhile to figure out what was going on.

 

The switch switches three things: (1) the fuel feed (valve, tank to pump), (2) the fuel return (valve, pump to tank), (3) the gauge sender (strictly electrical). What's evidently happening is that the feed and the gauge sender (1 and 3) are switching, but the return (2) isn't. So when set on FRONT, it's feeding from, returning to, and gauging the front tank. But when set on REAR, it's feeding from and gauging the rear tank, but returning to the front tank.

If you always run down the front tank first, then switch to the rear and ONLY run the rear tank when the front tank starts out real low, technically, you'd be safe. Your mileage would even be accurate, if you fill both tanks and calculate based on that. The danger comes when you're on the rear tank, but the front tank is close to full. All that return fuel from the rear tank, returning to the front, could overfill it, and AFAIK, there's no check against that.

 

Thanks guys. So it looks like it could be the actual selector switch on the dash or the switching valve at the frame? There any way I could test these out so I don't replace both if I don't have to.

 

sounds like the valve on the frame.

 

The same thing was happening in this thread:

https://www.ford-trucks.com/forums/showthread.php?t=1098215&highlight=Dissapearing+fu e

Some entertaining, and OT, reading to say the least. But i believe he found the solution. Give it a read, maybe it will help!

 

Just a followup with after 2 more trips filling both tanks. Everything works normal now. I was told this can happen but not to do anything unless it happens constantly. So I guess I'm good for now.

 

That's good to know.