6.9 IDI Hard Start/Air Intrusion

[Msch3390]

Hello all,
Working with an 86 6.9 IDI in an F250.

truck suddenly began losing prime about a week ago.
Symptom-
When off for more than 5 minutes, starts immediately, then dies about 5-10 seconds later. Then takes about 2/4 10 second cranks to start. Looked through the forums. It does this on both fuel tanks whether full or 1/2 tank. Once the air is cleared out it runs great. When I changed the fuel filter I noticed it was 1/3 low upon removal( didn’t spill any)

So Far I have Replaced the line from IP to fuel filter (it looked rough and was wet) , replaced all return lines and seals from a kit.
I inspected the lines back to the tank and to the lift pump, everything looks dry…

really just looking for troubleshooting advice of where to go from here. I have heard there is something in the IP to check? Also considering deleting the bowl on the fuel water separator, but am concerned about the wire running from it giving me a dummy light.
I have found trace amounts of diesel there when running, tightened the life out of it, but I have a new bowl on the way.

 

[Msch3390]

Update: I have replaced one fuel cap that looked like fuel was getting around, and the bowl to the fuel water separator with a plastic see through one.
The truck is still doing the exact same thing.

 

[Msch3390]

Update #2
I found this guy, runs from the hardline coming from the tanks, to the hardline going to the mech fuel pump. It’s sucking air. My question is should I be installing a 1 way valve or check valve on this line?

You must be registered for see images attach

 

[sjwelds]

Billet 3/8

The Classic Diesel Design's check valve is compatible with modern diesel fuel, wmo and wvo. This check valve is designed to prevent fuel drain back issues and improve cold startup times. Its a great addition to any electric lift pump conversion and any stock fuel system that suffers from fuel...

classicdieseldesigns.com

 

[TNBrett]

You don’t necessarily need a check valve just before the mechanical lift pump, as there are check valves in it. Just repair the leaking hose and run it like that.

 

[Bio-Bradley]

You don’t necessarily need a check valve just before the mechanical lift pump, as there are check valves in it. Just repair the leaking hose and run it like that.

Agreed. If you're seeing less fuel in the fuel filter, then air is probably being added to the fuel filter in place of the fuel. That being said, air is entering the system prior to the fuel filter. Likely the connection you have in the pic above. I don't believe you need a check valve. Just repair or replace this section of hose and see how the truck responds. Good luck!

 

[BW42]

Had the same problem on my '93 7.3 idi -- replaced the fuel filter with a standard filter, using the OEM 2-piece filter. Leaked big time. Gave up on the two-piece, bought a NAPA 3617 single piece filter, put a new high pressure fuel line from top of filter holder to overflow circuit, same with line from pump to overflow circuit. No leaks, and starts instantly every time. Both lines I replaced were dry and clean.
BTW -- if you replace the overflow circuit for all the injectors, be careful where you buy the kit. I don't remember where I bought the kit about 8 years ago, but I've now replaced most of the 'tween injectors lines with the same high pressure fuel line from Autozone. The cloth-covered lines in the kit began leaking after a few years -- junk lines. BW

 

[Msch3390]

Update #3

So I think I am in the right neighborhood, when I removed the line, almost no diesel came out of the 4 foot 3/8 line.

I replaced the line that was sucking air. I tried it without, and then with the one way check valve. The truck is doing the exact same thing in both scenarios.

When I cut the new line to install the check valve(on the new line), there was no diesel present at the line. So now the question is what “should” stop fuel from back flowing into the tank? Is it the FSV, or should the mech fuel pump “hold” the fuel?

If anything else I will start with the mech fuel pump, as it is 20$, compared to the FSV $$$.

 

[Msch3390]

Final update!

It holds prime now.
It was the Mechanical fuel pump.
I really don’t regret replacing the rest of it, there was air getting in and fuel getting out of some of those other spots.
Thanks for the input everyone!