Weird leak driving me nuts...

[mooseracing]

94 idi NA - I have had a slight fuel leak at the rear tank for a year or two now. I assume the tank has a rust hole so I have only been using the front tank.

Well I find it hard to believe the tank that is empty has continued to leak. I always have a wet spot on the ground at the right rear.

I figured I would get to the tank whenever but since it was continuously leaking I figured I would change out the selector valve. I put a replacement OEM unit on and I still have a leaking rear tank even though it is empty and I have not put fuel in it, in two years or so.

It is starting like it has a lot of air in the fuel lines, and takes about 100ft to clear it out when accel'ing.

I'm not understanding why fuel is getting back there, shouldn't the returns send it only to the tank selected?

 

[DrCharles]

I still have a leaking rear tank even though it is empty and I have not put fuel in it, in two years or so.

This is obviously impossible, so if the leak is coming from the tank it is not empty!

So - something must be returning fuel into that tank. Either through the supply or the return line.
If the lines were wrongly connected, the rear tank would fill and eventually overflow. I don't think that would take two years. That only leaves the selector valve, rebuilt or not...

Also, are you positive that the leak is actually from the supposedly empty tank? Could it be a leak from somewhere else that is being blown towards the rear and dripping off the tank?

 

[Thewespaul]

You sure you’re not blowing coolant out your exhaust by the right rear tire?

 

[DrCharles]

Good point... it's impressive how far the wind under the truck can blow fluids leaking from up front, too. But I assumed the OP had identified the spot on the ground as diesel

 

[snicklas]

I'll second what @DrCharles said,

Just because you changed the valve and that tank is "empty" doesn't mean it is.

I learned this lesson after a 2 mile walk. We had an 83 F-150 with dual tanks. I had run the front tank dry like cutting out dry, and switched to the rear (both tanks had been full, so I switched from a completely empty tank, to a full tank). I was headed to pick up my paycheck and go get gas. I had just left the last small town before the city where I worked. Got about 2 miles outside of town the truck sputtered a couple of times and dies. The gauge still read just above empty (yes i knew I was pushing it, but needed my check to have gas money). This was in 1990, so before having a cell phone, I got to walk back to my Grandpa's office and call Dad to bring me gas. We put gas in the rear tank, I drove to get my check and gas, and filled the rear tank. This was on Friday afternoon. I had to work Saturday and Sunday, which was normally less than a quarter tank of use (less than 75 miles), then drove to school on Monday and Tuesday (maybe 5 miles total, the school was just over a mile away). On Wednesday, Dad needed the truck to move something, so we switched vehicles, he drove the truck, and I drove the van. Since he was using part of "my" gas, he was going to fill the truck back up. On Friday when I had run out of gas, he asked me a few times if I was completely out of gas. I assured him yes, as I had used the front tank until it had started to cut out (and the gauge was at or below E), and I switched to the rear. For some reason, on the way to work (which is abut 20 miles), he switched back to the front tank, and found there to be over a quarter tank. He asked me about it again that evening, and assured him again it was empty.

We found the FSV was bad (this was a carb'ed truck, no returns) and was drawing from the rear tank, and allowing fuel into the front tank...... we replaced the FSV and could drain each tank, without partially filling the other......

So the moral to this story, I was sitting on the side of the road, out of gas, and walked 2 miles to get help, with close to a quarter tank in the front "I am absolutely certain is empty" tank.