Hi all, this is my first post here, thank you for any and all input and advice!

I'm writing on behalf of my dad's '87 F350 6.9 Turbo'd IDI, which for a long time, ran well enough, and recently (when we are in the process of trying to sell it) developed some mysterious air leak somewhere in the fuel system. It would run fine, run fine, and then start chuggling, die, and not start due to loss of prime (on both tanks).

We replaced the injector return line caps, o-rings and fuel return lines with a kit he bought I think from Napa, deleted the rusty water separator unit, changed the fuel filter, and some fuel lines. At this point I'm completely stumped. I used to have a '93 or '94 E350 7.3 IDI N/A that I had a VERY difficult, and actually impossible to find air leak in the system (I ended up selling it with the known issue) so I am familiar to an extent with these motors, and I also have a '97 Cummins, a W123 Benz, and my girlfriends' '03 TDI, so I'm fairly diesel savvy.

That said, I and my dad are out of ideas here. I'm hoping that those of you with experience troubleshooting these fuel systems can chime in, as I know that they are particularly sensitive to air. After above outlined work it did run fine and we thought we were out of the water, and then right when I was showing it to someone, it started having the problem again (of course). Today I pressurized both fuel tanks with air (yes, there is fuel in the tanks ) for like 10 mins in hopes that fuel would slowly start seeping from somewhere, but no luck...

My suspects at this point, and in this order are:
-Return side of the fuel system SOMEWHERE, leaking air back and letting the thing loose prime, though this did not come through on my air test...
-Schrader valve on fuel filter housing??? Do these go? Maybe just seats improperly sometimes?
-Pin hole in hard or soft line.
-Fuel pump.
-Fuel solenoid.
-Lastly, IP.

On my Dodge, there is a pressure regulating spring-check type valve screwed into the IP that releases excess pressure from the mechanical fuel pump via spring loaded banjo type bolt, and when they fail they can loose prime, is there a similar valve on these IP's???

Again, thank you for your help!!!

Hopefully using the word "dodge" doesn't get me kicked off the forum...

 

I read a post here a while back, that refered to the tip of the pick-up tube, in the tank. I haven't gotten into my tanks yet, so...I dont know, but its something to look into.good luck

 

I had an air intrusion problem, came on suddenly one night, started with a plugged fuel filter, ended with me running a new hose off the tank to the engine compartment, into a 10 micron hydraulic filter assembly, across the top of the engine, down to a new fuel pump, and then through a 6.9 fuel filter before my newly replaced injection pump.

Fixed the issues I had that night with a full disassembly and cleaning of stock fuel filter assembly, all new fuel line bypassing tank selector switch to an extra filter assembly (my tank is rusting out). Then to the new fuel pump installed same time and then into the newly cleaned stock fuel filter assembly.

One thing I have installed on other family vehicles, my truck will get it sometime, solid state electric fuel pumps with 80 micron cleanable fuel filter.

 

Orpackrat, so you never quite figured our the source of the problem then? It does seem like the best option, just to entirely re-do the fuel system, but we're trying to get the thing solid to sell, already put a lot of money into it. Hoping someone has some advice on where else to look for problem areas. Thank you!

 

In a sense, I narrowed it down to a highly probable cause.

Before I ended up re-doing it, the fuel lines I replaced by section by section.

The final fix that cured the fuel pump pulling air was replacing the fuel line from the tank to the engine compartment.

Meaning the air intrusion was 1 of 3 parts, the line from the tank to the tank selector valve, the tank selector valve, or the line from the valve to the engine compartment.

For me it was only 2 possibilities as I had already replaced the line from tge tank to the valve.

 

Recently had a similar problem when setting up my mid tank.

What I did was pressurize the fuel line rather than the tank.

Disconnect the line from the tank.

Block this end of the line. Mine has -6AN fittings so was just a matter of putting a plug in the end. But if you've got flexible hose and clamp then put a bolt of the correct size in the hose and tighten up the clamp.

Disconnect the line from the fuel pump.

Now comes the tricky part.

You need to be able to connect a compressed airline to this end of the fuel line. Once again I have -6AN fittings and have a connector with a modified air blow gun to be able to turn on off the compressed air. If you have just flex line and clamp the you need to have a barbed fitting into your flex line and then a airline connector attached to this.

Connect up the airline to this and follow it back to the tank checking all joins. If there's an leak you see/feel it immediately as the air will be jetting out of it.

Mine was in a two piece fuel filter between the selector and tank. It was immediately apparent as diesel could be seen coming out of the joint and air could be felt coming out of the joint.

Replaced the filter and all fine.

 

I'm on my way to the hardware store and curious if anyone knows the size of the inlet fitting on the fuel filter housing? Want to attach a new fitting and hose barb there for some clear fuel line to diagnose air. Thanks.

 

Does anyone know if the fuel return assembly part of the IP has a check valve that can go bad and cause loss of prime?

Connected clear fuel line pre filter housing, and in the return side of things today, found air coming the the fuel supply to the filter, tomorrow gonna just change all fuel lines between tanks and engine compartment, and get rid of all rigid plastic lines. I'm suspect that some of those connections are at fault, being 27 years old at this point...