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Author Topic: Fuel line routing problem? (diagrams)  (Read 693 times)
Jnbroadbent
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« on: May 18, 2013, 10:11:16 AM »

Rebuilt air purge valve, went to start it up, cranked right up then sputtered and died. Realized I left the fuel shut off valve off. arg! So it's a good time to figure out how to prime. As I'm following lines and realized it's not hooked up as it should per my DD 71 manual.



Lines are sloppy long, obvious someone re routed everything. It ran great, never any hesitation or symptoms suggesting fuel delivery issues. Any reason why I should keep the lines as they are? I'm going to head up to Napa and get some fittings and fuel hose.

On the secondary filter housing, there appears to be a solenoid (I think) on it with a rubber boot with exhaust ports. It's hooked up to the hot of the starter. Not sure what this is.
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Jon
1980 Mc9 w/ veg oil
8v71
Jacksonville Fl
Lin
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« Reply #1 on: May 18, 2013, 11:36:50 AM »

Since your current setup would have been easier to do from the factory, I would bet that there are good reasons they did not do it that way.  I can only guess at what they are though.  I will watch to see what those in the know have to say.
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bevans6
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« Reply #2 on: May 18, 2013, 12:19:28 PM »

It's a question of making the fuel pump suck harder to get fuel.  The normal way to route the fuel is primary - pump - secondary, and the reason is that the fuel pump pulls fuel out of the first filter, which has coarser filter element and is less restrictive, then pushes pressurized fuel through the second filter, which has quite fine element and is a lot more restrictive.  The first filter is kind of a trash, algae and water trap filter.  The second filter is the real fuel filter.  The sensor mounted on the secondary filter head is a thing (pressure sensor switch? I don't know what it's called either at this senior moment in time) that doesn't let the starter motor crank if there is fuel pressure at the secondary filter.

Edit:  My manual calls the first primary filter a "fuel strainer", not a fuel filter.  The switch is called a fuel pressure switch, and it provides ground to the starter solenoid, opens between 8 and 10 psi.

On my MCI there are additional fuel lines on the heads - pressurized fuel goes into both heads as you show, and the return from the left (driver's side) head goes into an "in" port on the right hand head.  Return from the right hand head comes off the back of the head, goes through a restrictor and back to the fuel tank.  So the right hand head has two hoses connected to "in" ports which confused the heck out of me for about a day when I was first doing the fuel line plumbing when I did my re-power.

My priming pump is just mounted in-line with the main fuel line prior to the first fuel filter.  The main pump pulls fuel through it just fine.  I turn it on, let it run till it's sound changes so I know the lines and filters are full (maybe 10 seconds) then I turn it off.  If I leave it on the engine runs fine, the the pressure trips that pressure switch so my engine won't crank.  Caused major panic attack the first time I used it when I left the pump on and could only fire the engine once!

Brian
« Last Edit: May 18, 2013, 12:32:06 PM by bevans6 » Logged

1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
luvrbus
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« Reply #3 on: May 18, 2013, 01:09:03 PM »

French fry oil burners do it that way why I have no idea if you find the reasoning tell us, several ways you can hook up the fuel filtering system on a DD and they all work
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Jnbroadbent
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« Reply #4 on: May 18, 2013, 03:32:42 PM »

It's a question of making the fuel pump suck harder to get fuel.  The normal way to route the fuel is primary - pump - secondary, and the reason is that the fuel pump pulls fuel out of the first filter, which has coarser filter element and is less restrictive, then pushes pressurized fuel through the second filter, which has quite fine element and is a lot more restrictive.  The first filter is kind of a trash, algae and water trap filter.  The second filter is the real fuel filter.  The sensor mounted on the secondary filter head is a thing (pressure sensor switch? I don't know what it's called either at this senior moment in time) that doesn't let the starter motor crank if there is fuel pressure at the secondary filter.

Edit:  My manual calls the first primary filter a "fuel strainer", not a fuel filter.  The switch is called a fuel pressure switch, and it provides ground to the starter solenoid, opens between 8 and 10 psi.

On my MCI there are additional fuel lines on the heads - pressurized fuel goes into both heads as you show, and the return from the left (driver's side) head goes into an "in" port on the right hand head.  Return from the right hand head comes off the back of the head, goes through a restrictor and back to the fuel tank.  So the right hand head has two hoses connected to "in" ports which confused the heck out of me for about a day when I was first doing the fuel line plumbing when I did my re-power.

My priming pump is just mounted in-line with the main fuel line prior to the first fuel filter.  The main pump pulls fuel through it just fine.  I turn it on, let it run till it's sound changes so I know the lines and filters are full (maybe 10 seconds) then I turn it off.  If I leave it on the engine runs fine, the the pressure trips that pressure switch so my engine won't crank.  Caused major panic attack the first time I used it when I left the pump on and could only fire the engine once!

Brian

Thanks for the info. I suppose it's working pretty well so maybe i'll keep this setup untill I have a reason to switch over. Though the stress on the pump could be enough reason.

Built a little T-valve with a butterfly and some fittings, did the garden sprayer thing, 1.5 gals, squirt with some starting fluid and it cranked right up. Though it won't air up over 80 (takes 2-3 mins) then just sits. Hmm. Time to figure out the next issue!  Grin
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Jon
1980 Mc9 w/ veg oil
8v71
Jacksonville Fl
bevans6
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« Reply #5 on: May 19, 2013, 03:43:17 AM »

If it's a veggie oil thing, then it's probably down to wanting extra filtering for the oil before it gets to the pump, or maybe different viscosity of fluid.  A given filter element will have slightly different efficiency and filtering characteristics under higher pressure than under low pressure differential on the suction side.  But as you say if it works that's the seal of approval!

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
Spicer 8844 4 speed Zen meditation device
Vintage race cars -
1978 Lola T440 Formula Ford
1972 NTM MK-4 B/SR
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