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Old 02-15-2012, 05:17 PM   #74
Registered User
Join Date: Mar 2009
Location: Indiana
Posts: 21
My Ride: 2003 330CIC
As far as I know, the "Sucking Jet Pump" (also called siphon pump) part #3 in second chart link below is the part that sends fuel from the left side (driver) to the right side (passenger). The right side unit part # 1 is the main fuel pump (called Delivery unit) that feeds fuel to the engine. The left unit is just a fuel level sensor (also called fuel gauge sending unit, part #2).

Note added: The siphon pump (part #3) is actually not an electrical pump. It has two hoses, one connects to an adapter on the left level sensor, and the other runs all the way to the right main pump and goes through the tray under it. So the siphon (pumping) action from the left tank to the right is related to all three parts. I believe the main suction action is created from the main pump (at least in my case), but also requires that the siphon pump is not clogged (rarely happens), and that the siphon pump is still connected to the adapter on the left level sensor (has something to do with flow). Any of those fail and you wont have fuel flowing from the left side to the right.

Problems with the main pump is the easiest to detect since you would have some drivability issues with loss of power or stuttering with a full tank. If you are just running out of gas and you know there is gas in the left tank, but not in right tank, then most probably you either have a clogged siphon pump on the left side, or the siphon pump have come loose from the weakened adapter on left level sensor, which requires replacing the left level sensor unit.

Check these charts out for 330CI Convertible (Note the arrow in the diagram showing the front of the car, so left and right are reversed here):

Last edited by roumani; 03-02-2012 at 10:57 AM. Reason: To clarify
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