I have a 9400 which the owner and I have tried all ideas we can to fix a fuel priming loss problem. We have replaced the filter canister hand primer, filter and head o ring, return hose from the engine head down to the filter, and lastly, I installed an inline check valve right at the tank outlet which I thought would finally be the fixall. He called this afternoon and it started then died and won't restart without a can of ether, two people, and an half hour of hand priming. Anyone have any experience with this on a scraper or farm duty tractor? It is the electronic controlled in head system (only has a charge pump on the backend outside).
A thought... there may be a seal somewhere that can allow air in, the return will act as a siphon if the tank is lower than the pump? You might try putting the check valve in the return wherever it comes off the engine, a couple psi back pressure may help keep the system from draining, if in fact that's what the problem is.
Replace the fuel pump with a new one...
Have you changed fuel filters? I've had water separators suck air and since its on the suction side you may not have a visible external leak. I chased one on a 3208 cat for several hours. Finally noticed bubbles in the glass separator. One had o-rings on the filters get tweaked just enough it sucks air and the second you shut it down you lose your prime. Maybe take the fuel supply line off at the tank and pressure it up to 20-25 psi and see if you have a leak on the suction side. May also have a line collapsing or a little flap of rubber inside a line that you can't see. My bet is supply/suction side. Heck there could be a rag or something in the tank, pickup sucks it up creates a vacuum and the second you shut it off it drops and pulls the supply line dry. It will be one of those things, when found, you'll go *** how did that happen?!? Or how did I miss that?!? Does the filter head have a check valve in it? Some Cat's do and they go bad on occasion. Be sure to let us know what you find and what fixed it, we will all learn! Junkyard