I want to try doing a vacuum test on my fuel system, I'm still getting air in the fuel filter when it sits for more than a day. I have deleted the fuel heater and put a 1/2 inch NPT plug in it. There is no fuel leakage anywhere and it does not stall out or sputter when it does get running. Fuel pump is new and I have pickup tubes going to the bottom of the tanks and are cut at 45 degrees. I'm thinking I should isolate the return system by unhooking the return line coming from the IP and sealing it off and also sealing off the return line right before the tank. Has anyone tried this before? 1992 7.3 idi. I could also do this to other lines in the fuel system. I would just have to drain the fuel out of the lines I'm vacuum testing though.

 

that is the easiest way to do it. cap off line at tank, put vacuum to line at pump.

 

If you don't find a leak in the supply line:

Do you have a functioning check valve from the fuel filter header to the #1 injector return cap? If it's OEM(or otherwise unknown) it's probably toast and needs replacement. You can also delete that line(even temporarily) and if the hard start issues are gone then that was the problem. New check valves are available through Ford/online but I don't have the part # handy.

There was a TSB early on to delete that line, but without an E-pump I've found that to only be a so-so fix. I *have* an e-pump, but bleeding air through the return line is always better than through the IP. Works good mostly but if you are late switching tanks you're still as boned as ever and have to bleed the injectors the hard way. I think with the check valve constantly bypassing air this would be much easier/faster.

 

I seem to remember a rubber fitting on the end of my return line, inside the tank. The fitting looked like a molded check valve. When I recently replaced my return hoses, I noticed the fitting to the rear of the engine loose. It had one of those rubber grommets. Where else would the check be located? Nevermind.

 

Fuel header w/labels

It's the brass peice between the header and the return line. There's a rubber check valve inside that deteriorates.

 

Here's the problem:
In my mind, "air intrusion" in the return line system is only an issue when you have the ability for fuel in the supply side to flow backwards and back to the tank. If you have a working check valve on the supply side(and no leaks, say in the fuel filter), the return side could be left open and you would have no issues whatsoever.

Typically, the times that blocking off the return check valve are "needed" are times that it won't help, because you actually have some other issue - air leakage into the supply line(faulty seals at the tank, hole in one of the lines, FSV leaking internally, etc.

The system is designed from the factory to deal with some air while it is running, so as long as the air leak(on the supply side) is small, the engine will still run. It's only when it starts sucking a /lot/ of air that you notice it.

Personally, I would replace the lift pump(It's a check valve itself, or is supposed to be), and pressure-test the supply line back to the tank if you can.
On a couple of rigs, I've added a squeeze-bulb primer after the lift pump, before the fuel filter. This acts as a check valve(preventing supply back flow), but on one of my trucks, it didn't help at all.
I found that the issue there is it is pulling so much air with the fuel that it barely ran as it was(thought it was the IP having issues).It was pulling enough fuel with the air that it ran decently(though not well), but still had starting issues because the fuel filter /always/ had some air in it, even when running!

 

I'll look into that check valve, I have had the truck for 13 years now and I have never replaced it, I think it is original. Plus I know on both my tanks the OEM pickup screens are rotted off, I just pulled the rear tank today and confirmed and the front tank about 4 years ago I put a piece of 5/16 hose where the screen originally was. The filter head does leak a little but just a drip when the engine is running. Usually replacing the filter helps that, but I dunno it may be pulling air in there when its off.

 

I do have a new lift pump, but the OEM pickup screens/check valves in the tank are gone. Does that mean that fuel can flow backwards after the engine is off? I'm replacing those currently so I'll try the pressure test when i get them installed.

 

What does that fuel vacuum sensor do? I have never seen an indicator light for fuel pressure. I never knew what that thing was.

 

It senses a vacuum at the fuel filter, and lights the 'change filter' light. Because it means that the IP is sucking fuel through the fuel filter hard enough to pull a vacuum.

 

Anyplace you have fuel leaking out (even a little) is a place that air will come in once the system is off. Air is far thinner than diesel!
Get that leak fixed and you might just be in business.

 

I have not had any problems since I installed hoses in my tank. I still am prepared to install a check valve prior to my original-but modded water separator if problems should occur in the future. This position will provide enough fuel to start and run while new fuel is drawn from either tank.