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im in a similar bind. i've done HOURS of reading over hundreds of threads on several forums and this has been the closest thread i've found to a solution. forgive me in advance as i am not doing a 4bt but instead a 6bt swap into a 1994 f250 460 c6 truck however our issues are the same. i want to keep both tanks but haven't figured out how to utilize them along with the stock fuel lines to supply fuel to the lift pump on the 12 valve engine.
 

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Basically, you need 2 things. One, change out the in tank parts for the Ford diesel pickup and returns. Those are being reproduced now as the OEM stuff is long gone. I believe the fuel tanks for gas and diesel were the same, just the pickup section that is different. The pickup units plus the filter screens will cost you around $250-300. Not 100% sure the dash gauge will function normally, but I believe it will. Second, you need the diesel tank selector valve. It is a 6 port switch valve to accommodate the pickup and return. These are also available aftermarket for under $100. Oh, you may also want to enlarge the fuel inlet for the tanks. Some diesel pump nozzles are much larger than the gas ones.
 

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I’m still trying to wrap my head around this solution. I can’t quite understand it or visualize it.

Having done it both ways, the simplest solution is to leave the pumps in the tank and plumb them right into the lift pump. However, the super secret trick is to install a "T" before the pump with one leg pointed up. make an orifice with about a 1/16" hole to fit in the T and hook that into the return line. Jump the FP relay so it turns on with the key in start and run. The orifice ensures there's no pressure and air goes back to the tank.

After screwing around with selector valves and re-plumbing the entire system a few times I did it that way with great results. Several have been on the road that way for more than a decade without problems. I would guess the famously unreliable Ford in tank pumps have an easier life pumping diesel instead of gasoline and at no pressure.

If you don't like it you can always change it down the road, but it takes a pretty big undertaking and turns it into a 20 minute job.

There's also the bonus of bleeding the fuel system is a breeze. One feature I would add (but never did) would be to retain the fuel pressure relay and control it with a 5 PSI oil pressure switch. That way fuel pumps only run when needed.
 

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Basically, you can't use the in tank pumps for diesel operation. Also, the in tank filter screens will eventually clog with diesel and leave you sitting. Options are either modify the current in tank parts and remove the pumps and filter sock and be sure you have a return line. You will have to have the diesel selector valve as the gas units were different. Second option is replace all the in tank parts with OEM style diesel parts. In a diesel swap, a few hundred $$$ is nothing. Another possible option is check a few salvage yards and see if you can find a diesel truck of your vintage. 1994-1997 used the same parts. I'm also pretty sure you'll get into some electrical issues. Wiring harness for the intake pumps and diesels are different. No such thing as an easy fix.
 

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Yes, 3/8" and 5/16" are adequate for most engines in the sane HP range. For an all out high performance engine 1/2" and 3/8" may be required for adequate fueling. Have read of a few who used 5/8" feed line but that was on 6bts over 1000 HP. Most of us aren't driving one of those.
 

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I kept nearly all of the original lines, took the front pump out in favor of a draw tube, and use the pump in the rear tank to transfer to the front. I have had no problems. I also enlarged the filler necks and added an external vent from the rollover valve to the top of the filler neck which helps with fill-ups.
 
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