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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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz