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Old 10-29-2018, 08:52 AM
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andrewwynn andrewwynn is offline
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DIY: E53 X5 Siphon Pump with Improved O-ring and Teflon Wrap

The zip tie, not the o-ring is the actual fix. The failure happens because the o-ring is not held square not because the o-ring is too small or somehow wears out.

There is nothing in the original design to hold the o-ring square. It's far more important to solve the twist of the seating surface than use a thicker o-ring. Time and pressure will win just like geology.


This photo shows the problem. The blue lines are parallel and where the bottom part should be. The red line shows where it is. The seal fails because nothing in the design holds the right side of the o-ring seal up. It's held only on one side it's like wearing one suspender of course your pants will fall down on one side.

EDIT: this photo just is a representative of HOW it fails I think that this picture was taken during disassembly on a unit that I believe hasn't actually failed yet (hence the indifference on the importance of the zip tie). It's a better representative photo than any I have as I already had mine either apart or from the wrong side to show the failure mode.

The zip tie forces the left side down forcing the right side up. It has to be a wide enough zip tie to touch the bottom part when assembled.

The other solution I recommend is to drill a 7/64 hole outside of the high pressure zone of the o-ring and install a 3mm stainless screw that's even better than the zip tie but the zip tie is instant.

Also after about 3-4000 hours on my car the second seal was leaking significantly but just drip drip drip not spraying like the open o-ring. There is no actual seal. The factory design has a plastic to plastic flare that will absolutely fail. In my case the female side was 0.2 mm larger diameter and has no choice but to leak.

The teflon wrap is a quick fix the better fix is to add an o-ring groove and add an o-ring, that said the teflon will eliminate the gap and reset the clock for a long time.

Nice complete step by step I only took the critical steps photos it'll be a lot easier for somebody to follow this new recipe.

The only other tips I could think of off hand are these:

Run the car with the electric pump unplugged to drain the fuel lines

A bent metal bar and a big vice grip will substitute for the proper tool to open the monster nut, other options I've used when I've misplaced my tool I made: lock a small vice grips on opposite sides and use a long rod between the two: it's
Better to use even pressure than try to use a hammer and chisel method that only puts pressure on one side

I wet the seal with gas v oil I figure that way it will evaporate and be rubber to plastic seal no oil left over

Not specifically mentioned above but the photo shows: the seal around the sender unit will typically come out with the unit leaving most to believe it should be on the unit when installing but that is impossible. You must remove the seal and install into the tank first (shown in the photo but I didn't see the description). Wet the ID of the big seal (I'm wearing surgical gloves when I do this so I just dip my fingers in the gas tank and wet the seal with gas)

Have a fresh roll of paper towels handy to soak up any fuel and "dispose of properly" (by which I mean burn them in a safe container): liquid fuel will safely burn; evaporated gas that reaches the proper stoichiometric* ratio will explode! Eg much safer to burn a soaked rag than risk it evaporating into an explosive situation.

I did the first two fuel pump repairs with the seat in the car but with impact tools it's so easy to remove the 16mm bolt and there is twice the elbow room with the seat out I now remove the seat bottom from the car when doing this job it's worth it for the extra room but totally not necessary. I've just had the seat fall on me enough times I've retired the leave the seat in method.

*Thank you MythBusters for teaching me that word.
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2011 E70 • N55 (me)
2012 E70 • N63 (wife)

Last edited by andrewwynn; 10-29-2018 at 10:06 PM.
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