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  1. #1

    Detroit Diesel fuel pump prime (6-71,8V71, etc), by Genesis

    I do not put anything into the intake other than AIR!

    If your fuel system will not re-prime without all this garbage going on fix the problem instead of applying band-aids. If the correct solution for your configuration is an electric priming/boost pump, install one. A Walbro is about $100 from Depco and that plus a check valve will do it.

    Unless someone has radically screwed with the original configuration on these boats there simply shouldn't be a problem. I've seen some Berties screwed up so bad that they would NOT reprime unless the tanks were full to the vents, but never on a Hatt. At least not yet.....

    The fuel pump on the engine is a gear pump and is positive-displacement. It WILL pull air through and re-prime UNLESS you have an air leak in the system somewhere or its relief valve is stuck or has something in it preventing it from closing correctly. If you have either problem find and FIX IT. If you need to pressurize the "suction side" with the engine off to find the leaks, do so. Those leaks are hurting your performance SEVERELY, because any air in the fuel compresses and causes improper injection.

    Detroits require proper fuel pressure in the rail to run right, and they, like every other diesel, require clean fuel with no entrained air bubbles.

    The filter most people forget is the one on the engine - on the PRESSURE side of the pump. That has to fill before the engine will start. Problem is that while the pump will do it, at cranking speeds it takes a LOOONG time. So drop that cartridge housing and fill it with Diesel Kleen (or fuel) - just make sure you don't cross-contaminate the output side, because that is your "last chance" filter before the gunk gets to your injectors and fouls them!

    From right to left....

    (not visible - Fuel shutoff), Fuel inlet, strainer (30mic relative, plus water-block), filter (10mic "stratapore", rated for the new common-rail engines like the QSM-11s - radically more than you need for Detroits but overkill is good), output shutoff (to isolate for filter changes), then suction for the Walbro, a check valve, pressure on the Walbro, vacuum gauge (not visible in the first picture but is in the second), then diverter valve back to the return line. Both the strainer and filter elements have water drains at the bottom (finger-operable; no wrench required)

    The check valve in the line to the engine's fuel pump has been REMOVED. It was a 1/4" bore thing - Detroit original - but that HAD to be restricting flow at higher output.

    The wiring to the pump is temporary (I had to run jumpers there to prime the system so I could start up after installing it, and of course to test for leaks); that will be replaced by a spring-wound timer in a utility box on the forward firewall.

    Now, the idea is this:

    1. The Walbro makes it trivial to reprime the engine. Turn it on, it sucks fuel through the filters. To prime you OPEN the diverter valve (to return) and energize the pump until it stops cavitating. Now close the diverter. The pump will go into "tick-over" mode as it reaches its rated pressure (4 psi). When you hit the starter you now are FORCING fuel into the pump, bypassing any air that's in there immediately. On a completely dry start (which the first one was after installing this) I got instant light-off. If the pump is left running there's no foul, since the check valve allows full flow through to bypass the pump if necessary.

    2. To polish the fuel tank, you open the bypass and energize the pump. The pump is continuous duty rated, so it can be run for a long time. It will turn over my tank (300 gals each side) in about an hour and a half; figuring you need 10 "turns" to insure you filter it all, overnight will just about do the job.

    3. To TRANSFER fuel, you set the suction and return selectors in the lazarette as desired, open the bypass, and energize the pump. Voila. Be careful - you can overfill a tank this way REAL easily! However, this allows you to easily transfer fuel if required as well.

    Oh, by the way, note the size difference between the two systems - the new one and the old dual-RACOR (redundant.) Which would YOU rather have in your engine room? Never mind that there's no more taking the RACORs apart and cleaning them, plus changing that damn bottom seal (the one you might not even know is there between the tube and baseplate - its a real pain to both get apart and reassemble.) With the priming pump I can replace a pair of clogged filters within minutes if necessary and re-prime the system, so there's no real need for the "instant swap" capability of the RACOR setup. Plus the vacuum gauge is "in your face" from the top of the engine room hatch.

    Sorry about the very visible mess - I took these this evening and I'm not quite done, as I have to put in the timer box and of course do the other side.

    Something to think about.... this wasn't terribly expensive to put together either....

    Genesis
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    note: the complete thread in which this post appears can be viewed as :

    http://www.samsmarine.com/forums/showthread.php?t=1440
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    Last edited by Pascal; 06-06-2005 at 08:56 AM.

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