She just... died....

[RSchanz]

I did the clear line trick on my truck for troubleshooting. It's been several years and I still haven't taken it off.

I agree this is likely an air intrusion issue. A relatively easy way to prove this is to remove the fuel filter after the engine has been been sitting for a while. Say, a day or a few. If the fuel level is down about 1/3 from the top, you know for sure diesel is draining back to the tank (or fuel simply ran out).

There was a time when I had hard starting and it turned out to be a weak lift pump. I figure one of the internal valves failed, which allows fuel to drain back, but also made it nearly impossible to prime after the truck was shut off for any length of time. Worse still, if parked pointing uphill. Once I got it running it was usually fine until the next cold start.

It doesn't leak fuel to the ground so if anything it must be draining back. The lift pump is relatively new like 4 years. I don't see any fuel leaking anywhere so I feel like this may be hard to diagnose. I already bypassed the water separator too. Any idea what some other main causes of this could be?

 

[david85]

Having no visible fuel drips while off is not unusual. Air leaks more easily than diesel, so yes you can have an air leak while shut off, without having a visible fuel leak, while running. Sometimes you can get lucky and see fuel leaking after a long drive while the engine is still running. These leaks can be slow slow that it may not leave a drip on the ground when parked, and they blend in very easily with grease that's often found on the underside.

For what it's worth, the lift pump in my truck was about the same age, with low miles on it. It's possible a piece of dirt got into one of the check valves and allowed it to start draining back, without the pump actually having a mechanical failure. After having issues for a few weeks, I simply swapped the pump.

How good is your hearing? Sometimes you can actually hear a tiny whistle from an air leak near the return line caps. Still happens to me sometimes if I'm near the fuel filter and the nearest injector return line cap gets disturbed. The clear plastic tube confirms the leak by showing bubbles climbing up to the filter head. A few twists of the return line cap is enough to usually stop the leak in my case.

 

[RSchanz]

Reviving this, I got another car that I've been working on so this was pretty far back, sorry.

Yesterday I did a very thorough inspection of the return lines, caps, lift pump, IP, and lines from tank. I got the truck running and it appears that the problem only occurs when I switch to the rear tank. I was watching a video about air leaks and the guy briefly mentioned something about the innards of the tank malfunctioning and causing an air leak(something to do with a shower head... haha didn't make a ton of sense to me). Any ideas or help here?

 

[onetonjohn]

The shower head is a little plastic pickup with screen that draws fuel from the tank. It's attached to a metal line of the fuel sending unit. The sending unit sits in the gas tank like a modern fuel pump. It has float for your fuel gauge and this shower head (both of which go bad). There should be a lot of information (better than I can give) if you do some googling with those terms.

 

[Jim993]

When I bought my '85 F250 6.9 4 x 4 in 1998 the water separator was bypassed. I removed the old junk and simply change fuel filters every 15,000 miles. NO issues over 27 years but it is garaged every night.