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How it Works - Fuel Selector Valve (FSV) and IDI Fuel Math AKA "What fuel pump should I run?"

Discussion in '6.9L IH & 7.3L IDI Diesels' started by hacked89, Dec 27, 2020.



    Likes Received:
    Feb 5, 2010
    edmond, ks
    Yes that's my idea. I'm going to use a large rear tank to replace the leaky one in my Blue Truck. Since I bypassed the FSV about 5 years ago, I'm going to use a transfer pump to move the fuel from the rear tank into the front.
  2. CBRF3

    CBRF3 Full Access Member

    Likes Received:
    Mar 22, 2020
    Riverview Florida
    I prefer to use the rear tank and block all venting and fill port then use a large tank in the bed to gravity feed the rear tank I return the fuel to the in bed tank with a 1 way check valve and pull fuel from the under truck rear tank this also gives a positive pressure at the fuel pump / injection pump meaning if are ever priming system its much easier and if have any leaks in the return system it wont make for hard starts otherwise gravity would draw all fuel from the injectors breaking the seal allowing for fuel to draw backwards and be replaced with air in system.

    I want to point out with my setup I have 0 leaks and my return line setup doesn't leak I have 3 total 1 way check valves in my setup and I have just under 1psi positive pressure at my injection pump via just gravity with both tanks full and motor not running. I limped my truck home without any fuel pump at all before idled it and shifted gears and let it lug at a idle it made it 25 miles with 0 fuel pump only gravity with my setup. I had to remove the pumps and bypass them had a total electrical failure I zip tied kill solenoid open in injection pump and push started it and limped it home batteries were 100% dead 0 juice at all.

    I also remove the front tank / switching valve system to allow for dual exhaust for a naturally aspirated truck as we get a huge power gain by doing so and for a turbo truck having the front tank and switching valve removed makes things much easier for after market fuel filtration and bypass oil filtration setups and such to go where the front tank used to be simply put I find the front tank / switching valve a headache and a waste of space you could use for other more beneficial things. The switching valves are iffy and often cause problems add to it theyre a major restriction to fuel flow and our injection pumps do not like going negative in fuel feed pressure it kills the transfer pump side of our injection pumps quick costing us $500+ to repair / replace and likely a tow bill I am sorry 16-18 gallons of additional diesel and risks involved just don't add up to being worth it. I will continue with my gravity feed setup I built a custom aluminum 50 gallon tank with 1 entrance at top of tank and 1 exit at bottom aka sump pickup 1/2 inch bottom 3/4 top ( I have a 1/4 vent line from top of the rear under bed tank going to top of large in bed tank ) I transfer via gravity from my 80+ gallon tank in the bed to the 50 gallon tank under rear of truck.

    PLZ do not use the factory tanks with the gravity fill setup with large tank in bed the pressure is to high from gravity and the tank will likely crack / split had 2x 40 gallon bronco tanks did this to me so I built my own custom tank out of aluminum perk of being a welder / fabricator and fleet mechanic.

    there are a few options also to use both tanks together put a sump pickup on both tanks and tie them together this way and return to rear tank and draw from front tank and vent them together with a 1 way check valve only allowing positive venting from front tank to rear tank causing the front tank to draw vacuum and constantly cause a siphon from rear tank allowing gravity to keep both tanks always at same fuel lvl and remove front tank filler neck ( aka seal off front tank ) several older chevy trucks were like this and many trucks factory with saddle tanks are like this many older 2002 and older f650 and such are this way and alot of the older chevy 5500 and such also are. I simply do not feel comfortable relying on a electric fuel pump to transfer from 1 tank to another and would prefer gravity to do it for me can always rely on gravity it will be here as long as the world spins lol.
    Last edited: Feb 23, 2021
    The Gill Fish likes this.

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