- Thanks to the O.P. for the detailed explanation.

- I know I don't have siphon pump issue b/c I ran it all the way until the YELLOW light came on, then stop at gas station to fill up.

- I did some experiment today, filling to about 35-40%. Then drove a few miles, then some more photos.

- Then I simulate "off-road" condition by driving EACH side on a concrete curb at a shopping mall, just to tilt the car to the right and left.

- Re "How to test siphon pump": the design is such that the Fuel Pump sits in that well on the RIGHT side of the tank that holds some 5L, and this is all the Fuel Pump "sees". As mentioned previously, when fuel is below approx. 32L, most of the fuel sits on the LEFT tank.

- So if your gas tank gauge is below 35%-40% AND the siphon pump fails, you only have minutes before the car stalls when the Fuel Pump consumes all 5L in that well! In other words, there is "no good way" to test the siphon pump as running out of gas is detrimental to the Fuel Pump. I drove around with Test #6 on, the well shows anywhere between 2.6L ---> 3.9L.

- If you want to test the Siphon Pump, then drive around with:
1. Fuel less than 35%...
2. Test #6 on and...
3. Keep looking at LEFT tank, it should go down slowly as fuel is being pumped from L ---> R. So, if the RIGHT side shows <1L, then the car will stall soon, potentially damaging the Fuel Pump.

- In other words, once fuel is below 35%-40% AND the Siphon Pump fails, it is a matter of minutes before the car stalls, simply b/c there is only some 5L to be consumed!

- Back to what I said in another post, on looooong highway drive, it is best to keep fuel above 40% at all time, just to avoid that 0.01% chance your Siphon Pump fails. Or open the left tank and fix any potential siphon pump way ahead of time.

- Photo...


---

The last 5L will get you 15-20 miles it's not mere minutes.

Watching the test six once the tanks split left/right (about 30L total) the right tank will stay the same until you run out of gas. You can tell of there is a soft failure without running out of gas very easily:

The right tank number will stay the same on level ground. In my wife's case 1.4L (there is an additional 3.6 L not displayed).

You can drive for quite a long way while that number slowly drops, for example once the number drops to 1L there is still some gas coming from the left tank just not more than is being consumed. I've driven 40+ miles watching the level in the right go from 1.4 to 0.9L.

So: drive down to 1/3 of a tank and watch the right tank value. If the number drops, you have either a leak in the siphon jet or a weak electric fuel pump.

The flawed design of the siphon jet guarantees a failure at about 3-4000 hours. I recommend fixing the o-ring preventatively at 3000 hours. (100000 miles if you average 33mph.)


Sent from my iPhone using Tapatalk Pro

- On the issue of preventive maintenance...

- If I open the LEFT tank to check the Siphon Pump, and let's say it is good with no issue, what can I do to prevent O-ring issue? Should I apply a hose clamp around the connection? Should I secure the Siphon Pump better?

Zip tie will prevent the problem. See where I put in the thread on the diy fix for siphon jet


Sent from my iPhone using Tapatalk Pro

I don't think that vent is attached to vacuum. You can disconnect the hose to see.

When you drive a while do you get a vacuum on the fuel cap? I think the problem would be in the tank venting system that has a valve to close in the case of tip over. Maybe that valve is stuck shut.

As fuel is consumed, ambient air must be allowed in or you will create vacuum: drink some soda out of a bottle with your lips sealed on the bottle and the bottle will collapse. It is what I suspect is happening with your tank.

Look into the carbon canister venting system. If the tank is dealing with vacuum that's the main suspect.

I will look into the air intake valve, I'll have to find it first... any ideas where this is?



The fuel tank vent valve is my main culprit for now since its linked to the manifold but having no way of finding out when it opens, I can't be sure.

This is definitely the most beneficial thread I've ever seen on the www; makes my head hurt.


Knowing when the siphon pump is supposed to kick in (@32L?) is priceless.



I want to replace the entire siphon pump on my X5 3.0i; not comfortable with the task of separating the components without busting them. I'm wondering if any of today's siphon pumps being offered have been upgraded with the better O Rings, etc.


I read down to page 5 or 6 and haven't seen a part number for a siphon pump; any help would add to the pile of appreciation I have for this community.


Onto page 7



Thanks again for all the info. Salute!


G

The siphon pump assembly is not sold as a complete part - you will have to buy two major parts (plus the o-ring) and assemble them yourself...

BMW Part #16111184427

https://c1552172.ssl.cf0.rackcdn.com/370687_x800.webp

BMW Part #16116762044

https://c1552172.ssl.cf0.rackcdn.com/173720_x800.webp

Might as well simply replace the o-ring...

BMW Part #16116761645 (o-ring)

I did the teflon seal mod a few weeks back, it was a pain to separate but being gentle went well and I have no issues to report.



I haven't had a chance to test it to 0 because my GF is using the car so she's been ordered not to play with the reserve but I will soon.

thankyouthankyouthankyouthankyou!

I remembered 'i saw this somewhere' but it took me a couple hours to figure out where! I 'knew' i saw it on my detailed fuel supply thread, so i just had to read post-by-post until i found it!

there is literally NOTHING on the internet that returns anything helpful when you search anything related to 'reset fuel gauge bmw' literally nothing useful.

I remembered that 'somebody posted how to reset' after 'replacing a fuel sender' and somewhere, and i sure was hoping it was on this thread!

Story to follow.