Yeah, I get that the tank is split. It just seemed logical that when refueling, the fuel would flow into the passenger's side of the tank, then 'flow' over to the drivers side, since the fuel filler is located on the passenger side. But I guess this is not the case..

Nope, no M&M's found..

This is a myth. Especially with the x5. The fuel pump will be submerged completely down to about 2L (1/2 gallon of gas).

The x5 fuel tank when operating properly ONLY EVER has 8L or 10L (6 v 8 cylinder) gas on the right side of the tank once you have less than about 2/3 tank of gas. (E.g. there is no difference from the fuel pumps perspective from 2L and 98L of gas in the tank)

That said, the system that pulls the fuel from the left side of the tank and keeps the reserve tank (8L and only 8L) full has a few failure points.

➀ fuel pump weak doesn't supply enough pressure to filter/regulator (45% chance)
➁ jet siphon pump defective (45% chance)
➂ fuel filter/regulator problem (5% chance)
➃ some other extremely rare situation

Put in some gas at least 4-5gallon (15-20L) and enable test mode 6 on the dash. You should see over 1L when driving level. If the jet pump isn't working full power it needs some depth of fuel on the left side to help and your won't get full range: in my example i would run out of gas with 70miles to empty.

I replaced the fuel pump first and got little improvement in the reserve tank quantity.

I took out the jet pump and found an misplaced o-ring :

https://uploads.tapatalk-cdn.com/201...81e2dd6f4d.jpg

https://uploads.tapatalk-cdn.com/201...a7e143fa7c.jpg

I replaced the o-ring and it helped a little but the second joint in pump was leaking like a sieve. There was no o-ring : I added an oring and and it solved the problem

Myth #2: there will be no "crud in the tank". Examining in great detail my wife's fuel tank with her x5 Just passing 131313 miles (no wonder the bad luck with fuel pump), there was not 0.1 g of foreign material in the tank save one spot that does not get consumed. (There is a little pocket to guide/hold the bottom of the fuel pump/sender units) on one side there was what could have amounted to 0.2g of "dirt" that accumulated from the roughly 7,300 gallons (350 full tanks worth). It's an absolute myth that any amount of sludge or gunk is interfering with fuel delivery.

(And repeating from above also absolute myth that the car cares in the least that you refill before it is empty). The x5 fuel tank is a brilliant design that keeps the fuel pump completely submerged in any pitch/roll condition down to about 3-4L of gas. This is so you don't run out of gas when going up or down a hill. The gas can't "pool away from the pump" because the pump is at the bottom of an 8L well that is maybe 30x15cm. The car would probably tip over before the pump is starved if you have more than 4L of gas.

Once below about 2/3 of a tank, only the reserve tank (8L) will have any fuel on the right hand side and ALL the rest will be on the left.

There is some clever arrangement of tubes that it seems "senses" if the reserve tank is full and stops the siphon so it doesn't bother overflowing the right side tank back over to the left needlessly. There is a dip tube for the siphon at the front and back of the left tank so in spite of going up or down a hill (or brake or acceleration), the siphon will drain almost every drop from the left side of the tank.

The sedans may work differently but this is how the x5 works. Fire up the hidden test menu test six and see yourself.



Sent from my iPhone using Tapatalk

If it's a myth how do you explain the OP finding a "lot of crud" at the bottom of the sump?

The pump is not submerged if the siphon pump is not working, that's why people can't start their truck even though the fuel gauge may indicate a 1/4 of a tank of gas remaining because there is no more gas at the bottom of the sump. Most mechanics will tell you that running an in tank pump without gasoline will eventually lead to early failure of the pump.

You can believe whatever you want to. I'm just posting what I believe and what the original poster posted. After all, I did post the solution to his problem, I didn't see you post any solution to his problem.

^Agreed about running the tank to empty not being an issue unless the pump is near it's EOL anyway. Over about 15 Bimmers, I've had 1 pump failure and I run from full to empty on almost every tank (at least 95% of tanks).

Glad the pump got you back in business. Enjoy her until the next thing goes south. :) Hopefully that failure will be an easy fix.

I don't know if the pump heats up or not, I can see some logic either way. Somebody should video a pump running on a bench and see how much the temp changes running it dry. Until then, I will be running my tank down to 14 or less miles that I have been doing since I bought my X5 new-- just so I can get gas at the station near me that has amazing Texas tacos plus it's fun and exciting to watch the distance to empty drop.

The "a lot of crud" that OP mentioned was under his seat-- Cheetos, MacDonald french fries and something he had no idea what it was and didn't want to smell it. As to stuff in the bottom of the tank--that depends on what went into the tank. The sentiment you found in your wife's X5 tank is hardly enough to substantiate a myth or my sighting of the Loch Ness Monster would have ended the debate.

I'm actually not sure if the distance to empty includes the reserve but believe it does. It will cause no damage whatsoever to run the pump down to 10-20 miles to empty.

A study showed an interesting thing for those of us forced to use shitty gas (diluted with ethanol): it fantastically reduces wear in the fuel pump. E10 maybe 20-40% less wear and E20 less than half the wear of pure gas.

There can be a lot of crud on top of the tank: mice can make a home there: I think really much more of a problem with sedan vs SUV.

My favorite DTE thing is when after driving city a lot then switching to highway miles: I'll drive 60-70 miles with the DTE NOT DROPPING even rising.

I'm going to open up wife's tank to inspect exactly how much fuel when the low fuel light comes on.

I think it's when the two senders are both zero but the right sender reads about 1.5L while there is still fuel on left side so maybe the top 1.5L of the reserve tank isn't counted in the 8L.


Sent from my iPhone using Tapatalk

Ps: I kept the pump I removed from Lumba2. I can do exactly as you described. I'll pump water in a circle back into the bucket for a while to see if it can even generate enough heat to warm standing water then I'll run it for an while dry to see how much it heats. The reality is that you'll only have about 2L or ten miles of driving from the time the top of pump is dry until you are calling for road service. 20 minutes max. Not going to hurt the pump is my estimation.


Sent from my iPhone using Tapatalk

Here's the original poster post on the "crud".

Never saw an episode of Myth Buster that said that running a tank to empty will not cause a problem with an in tank fuel pump, but Consumer report did issue a statement on this practice.

Here's the link, I will tell you what they reported, but then you won't look at the link.

Running on empty: Low gas in the tank can be costly