Sorry guys, I thought these two forums have different people on them, so posted in both places to get more ideas. Didn't try to do anything stealthy.

I will now only carry on the discussion here. Once I have this all figured out, will post the same findings there.

Thanks CN90 and 80stech, I read up on the o-ring thing it all fit problem. Put some gas in and it started up right away. Now onto open up the sender side and checking out the o-ring.

The key thing is finding the O-ring a tiny tiny bit bigger than the factory O-ring.

All info is in this DIY I wrote back in 2018...

https://xoutpost.com/bmw-sav-forums/...flon-wrap.html

Also,

If you don't have time/desire to fix this issue, then keep fuel level > 3/8 at all time.

It took some doing to open up the sender side. The plastic part is semi-transparent especially if I shine light onto it, so I can clearly see the o-ring through the plastic. It looked perfectly seated in its grove, nothing like what was shown in the pictures posted on-line. I was very baffled and decided to take the thing apart anyways. When I pulled the pieces apart, the o-ring just popped out of the grove! Turned out, it was broken. So even still seated nicely, it was no longer air tight.

Onto to ordering replacement o-ring and putting everything back.

Thanks to all the replies to my posted and guided me to this fix!

Yes, I read through your post, that was what gave guts to try pull the sender side apart! Thanks for that!

Where did you find the larger o-ring? Do you remember the size? I can't measure from the broken one...

Did you check with the pump running if the O-ring was leaking ? You should be able to get away with the same size and a few wraps of teflon tape as well. I think the o-ring is usually pushed out if it has failed. Sometimes the lower part which is a plastic to plastic fitting can leak too which you could see with the pump running.

I found this on Amazon, figured one of them should fit :-)
I am thinking either the 22 or 23.5mm ones.
The o-ring grove measured 23.38mm, a very strange size.

Ah, didn't check with pump running, but now I already took it apart :-(
But does the pump send fuel back to the sender side?

Get O-ring size 22 x 2.5 mm (I.D. x thickness).
You can find it at "O-ring and more" website, Henrico, Virginia.

PS: iirc, factory is 22 x 2 mm. The difference of 0.5 mm in thickness makes a difference.

A note for everyone...

After you fix this O-ring issue, to test the siphon pump system, once the fuel level is approaching 1/4, carry a gas can (the stuff you use to fill the lawnmower) in the trunk with 2-3 gallons, then keep driving so it drops to 1/8. This is how you safely test the system w/o being stranded.

On long road trips with gas stations far apart, keep fuel > 1/4 at all time if you can. It is really not fun being stranded in the middle of nowhere. In the city, you can easily get help. In the middle of the corn field, only the cows have sympathy for you.