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Old 01-21-2017, 04:33 PM
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andrewwynn andrewwynn is offline
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Join Date: Jun 2015
Location: Racine, WI
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So: pictures!

I made a computer model to diagram a simplified example of the e53 fuel tank.

Here is a photo of a real tank:



And here is a simplified computer model that is "section cut" from top to bottom showing where the "shore line" is as the level of fuel drops.






As you can see, once the fuel level gets below about 40-50%, the little "well" on the left side (right side of car), is separated from the main tank and at that point is entirely dependent on the siphon pump to use *any* of the fuel on the left side (right in model).

Depending on the mode of failure you can have fuel starvation at any level of total gas once the level is below the hill.

In my case the failure was two fold: weak pump and also leaking o-ring on the siphon pump.

If the pressure is weak from the fuel pump the siphon just won't be full power. What happens then is the depth of the gas on the left side helps out the weak pump so you will get some of the fuel from the left side. This is why I advise running your tank down to single digits distance to empty DTE at least a couple times a year.

The main pump will usually get weak as the brushes and commutator wear LOOOONG before the pump fails completely. The idea is this: while you are "close to home" and could have somebody bring you a can of gas, run the tank down to bottom, if the fuel pump is starting to soft fail, you may run out of gas at 10-20-50 miles from empty. The main pump would likely operate for 10s of 1000s of miles at this point but you can then do an easy DIY replace of the electric pump for $120 in my case, run the down to single digit test again to confirm fix is good and avoid being stranded with 100miles to go on a freeway halfway across the country on a vacation.
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E53 / M54 2001 3.0i gas LHD (pair of them: his & her)
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