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  #1  
Old 11-19-2006, 01:58 PM
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OM606 Fuel Flow Schematic...

I am trying to figure out the fuel flow through the series of plastic lines under the hood. It seems that all of the fuel enters the system through the pre-heater via a black hose.

From there a hose feeds it into the center (top) of the pre-filter and then into the main filter where one hose goes directly to the shutoff and a second hose goes to the mechanical lift pump and then into the shutoff. The third hose on the shutoff seems to be a return hose back to the pre-filter housing but on the filtered side of the pre-filter. I know the shutoff feeds fuel directly into the hole in the side of the IP so all of the hoses attached to it either must bring fuel into it or send overflow back.

So I am thinking there is actually a parallel path for the fuel, one which is gravity fed into the shutoff and one which is pumped from the pump into the shutoff. Any overage is returned to the prefilter via that third hose.

This explains why some cars will run until the tank gets too low or if they are parked uphill at which point the lift pump would need to draw fuel from the tank but if the pump is faulty the car will not run. It also explains how you can have a fuel leak when the engine isn't running but gravity is feeding fuel into the system via the hose which does not go through the lift pump.

Does this make sense and am I right? Are there two "parallel" hoses feeding fuel into the shutoff, one pumped from the main filter via the lift pump and one gravity fed from the main filter?

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2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz
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Old 11-19-2006, 02:25 PM
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Location: Walnut Creek, CA & 1,150 miles S of Key West
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I remember the fellow in the below post seeming to have a good understanding of the fuel path. Scroll down toward the bottom. Was concerning a 96 but should be appreciably the same as the 98/99.

1996 Mercedes E300 ifi Electrohydraulic shut-off
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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)
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Old 11-19-2006, 02:49 PM
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Location: North Shore, MA
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I hope this image is not copyrighted since I found it online somewhere - but here it is

The suction side of the fuel goes from the pre-filter through the shut-off valve to the lift pump. This make sense since the system maintains the partial vacuum on this side when the shut-off solenoid is closed. After the lift pump, the fuel is filtered and passed to the other side of the shut-off valve where it enters the IP. The last line from the spin-on manifold is the overflow back to the tank.

I'm glad you brought this up since I now am understanding how my fuel leak is even possible. When the car is off, the fuel uptake "bolt" in the top of the spin-on filter is acting like a siphon.
Attached Thumbnails
OM606 Fuel Flow Schematic...-fuel-lines.png  
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Scott C.

2006 E320 CDI (120k miles)
FOR SALE: 1998 E300 Turbo Diesel - Black w/Tan Leather - Euro delivery (236k miles)
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Old 11-19-2006, 03:01 PM
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OK, so I was wrong then apparently all of the fuel is pumped via the mechanical pump, it only passes through the shutoff on its way back to the main filter and after filtering comes back into the shutoff to be fed into the IP.

I still wonder if there is enough gravity feed pressure to allow the car to run even if the lift pump was not working?
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Old 11-19-2006, 03:22 PM
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Quote:
I still wonder if there is enough gravity feed pressure to allow the car to run even if the lift pump was not working?
Well, speaking from my first fuel filter change experience, I think the answer is yes. I missed the pre-filter O-ring and the lift pump was sucking in air so the only way for me to get the system to run was to fill the spin on filter right to the top with fuel. It gave me about 5 minutes of idling time - just enough to diagnose the stream bubbles entering the system at this point. The lift pump may have been filling the spin-on filter with air and this pressure was enough to keep the system briefly running, but I'd bet that even if the lift pump was disconnected the engine would have started.

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2006 E320 CDI (120k miles)
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