Fass Losing Prime...

[Snoop]

I finally installed my FASS thats been sitting in the box for way too long. This is after going through 3 Air Dog pumps... no surpise.

Install went well without any issues. Only thing is it loses prime after only a few minutes after having the truck shut off. I cannot hear or see any fuel/air leaks anywhere.

One thing that I think I probably did wrong was utilize the existing Air Dog Return Line. Currently my new fass is setup with a supply from my sump, then sends to heads and regulated return, then return comes down and Ts where 1 way is coming from heads, 1 way is the return feed from Fass and other way is going to the tank (this is the Air dog fuel setup). Did I plumb that incorrectly? Should the return from the heads go directly to tank, and the return out of Fass go directly to tank (in filler neck) so they don't share pressurized lines? Is this causing my pump to lose prime and creating a syphon?

Ugh... Always wrenchin on the 6.oh

 

[Snoop]

Update :

The correct plumbing is and has been in place after discussing with Dennis.

Sump to shutoff valve to pump. Pump supplies heads to regulated return. Pump sends regulated return down to my 5/8" pickup in tank.

Pump has 75# PSI spring in it. Pump only makes 70#s. I have the heads regulated at ~64PSI.

I have re done all of the fuel lines, still cannot detect any air nor fuel leaks.

I have also primed and let the truck sit with both the fuel cap on and off, and it still loses prime.

Prime is lost within 5 minutes of the truck sitting on level ground.

Any suggestions?

 

[Eamon]

Did you ever get this figured out? I’m having the same problem with my 7.3

 

[Mhatlen]

I have a FASS 125GPH and run a 5/8 draw tube and 1/2” up to filter housing as well as return to tank. The 5/8 supply hose comes over top of tank and drops into the pump inlet.

By using the draw tube supply once fuel is primed to the pump every time trucks shut off that 5/8 supply hose is full of fuel above the FASS pump. So it always has a load of fuel at the pump for each consecutive startup.

Now if you are running a sump in your truck you want to run your supply hose up to the bottom of the bed and strap hose there and then run it down to the pumps inlet. That will trap fuel to your pump.

If that’s not your problem I would look at your vents on the tank. If it’s not open to vent when truck heats up the diesel is under pressure from thermal expansion. As things cool off tank pressure can contract drawing fuel back into the tank and FASS pumps like fuel head pressure to prime. Also make certain no bumps show up over there. I don’t think a draw tube into bottom of tank is going to be a random event.

Hope this helps

 

[Eamon]

I have a FASS 125GPH and run a 5/8 draw tube and 1/2” up to filter housing as well as return to tank. The 5/8 supply hose comes over top of tank and drops into the pump inlet.

By using the draw tube supply once fuel is primed to the pump every time trucks shut off that 5/8 supply hose is full of fuel above the FASS pump. So it always has a load of fuel at the pump for each consecutive startup.

Now if you are running a sump in your truck you want to run your supply hose up to the bottom of the bed and strap hose there and then run it down to the pumps inlet. That will trap fuel to your pump.

If that’s not your problem I would look at your vents on the tank. If it’s not open to vent when truck heats up the diesel is under pressure from thermal expansion. As things cool off tank pressure can contract drawing fuel back into the tank and FASS pumps like fuel head pressure to prime. Also make certain no bumps show up over there. I don’t think a draw tube into bottom of tank is going to be a random event.

Hope this helps

It definitely gives me an idea of where to start. Thank you!