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Old 01-14-2017, 04:01 AM
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e53 Fuel Supply Detailed (fuel pump / siphon pump diagnostic help)

after spending 20-30 hours searching every corner of the web to find out 'what the hell are all these parts' and why my x5 showed less than 1L of gas on the right tank and only went up to 1.1L after I made the recommended fix.

This is a detailed description, in most cases, the descriptions are verbatim or paraphrased from bmw technical manuals.
The right-hand side of the saddle-shaped fuel tank has a surge chamber with the electric fuel delivery pump.

The surge chamber guarantees correct fuel supply from the fuel delivery pump in all operating states of the vehicle.
the 'surge chamber' is a 'well' that contains about 5L of fuel. This well is clearly visible in the attached photo below.



The little 'foot' at the left of the photo (right side of tank) is literally where every drop of fuel your x5 consumes comes from.

Many bmw fuel tanks are 'saddle' type tanks but the x5 tank does not really qualify.

If you look closely at the photo, the x5 tank is closer to the shape of an old fashioned carpenter's folding ruler, that has been twisted 90 degrees about 40% up, or maybe better: a couple of 2x6 'lego' blocks attached at right angles on the corner, with a 1x2 on the bottom of the right side for the 'foot' or the 'well'.

This has a few implications:
  • there is not a well defined 'left' and 'right' side; the 'saddle' hump might be 2cm tall; you can see in some pictures, where the outlet of the siphon pumps are attached is slightly higher than the well (aka 'reserve tank').
  • once the fuel level gets lower than the flat area between the two sides, gas will no longer stay 'on the right side' other than exclusively in the reserve tank
  • this means, that since the right side sensor resides in the reserve tank, once your fuel level gets below about 33L, the right side sensor will read a relatively constant and very low amount (my wife's x5 reads 1.4L on a level ground, driving highway speeds)
  • the system is entirely dependent on the siphon 'jet pumps' (there are several other terms used interchangeably) at all times, since the car is always using 'the last 5L of gas).
Fuel is transferred from the left-hand side of the fuel tank to the surge chamber by the suction jet pump integrated in the tank expansion line. Both suction jet pumps are driven by the fuel return line. The pressure relief valve controls the pressure required for the suction jet pumps to operate.
Think of the pressure relief valve as a pressure regulator; if the return line is supplying too much pressure, or a jet gets plugged, the return fuel will vent into the surge chamber; sadly this won't get you more than 30-40 miles, but at least you will be able to consume the gas from the surge chamber and not have the system simply suck the reserve tank dry and put the gas on the left side of the tank.


The fuel is fed from the electric fuel delivery pump to the fuel rail via the fuel feed line and the fuel filter. Excess fuel is diverted back to the fuel tank by the pressure regulator fitted to the fuel rail and the fuel return line.
The fuel filter and pressure regulator are 'one in the same' on the x5. The fuel from the surge chamber (aka reserve tank) is pumped to the fuel filter/regulator and a constant stream of fuel comes back to the LEFT side of the tank.
The fuel level is measured by a lever-type sensor on each side of the fuel tank. The right-hand lever-type sensor is integrated into the fuel delivery unit. The left-hand lever-type sensor is located in the left-hand sensor unit. The actual fuel level in the tank is determined by linking the ohm values of the left and right-hand lever-type sensors
Since the right side sender float goes down into the reserve tank (which holds 5L) but it reads 1.4L when full on my wife's car when operating properly (meaning the surge tank is overflowing over 'the hump' back to the left side of the tank).

The low fuel light comes on when the calculated amount remaining is 8L or 10L of gas. There is some fuel (about a gallon) that remains when the computer measures 0.0L of gas. This will net you about 15-20 miles range past zero miles DTE.
Pressure regulator: 3.5 bar (S62: 5 bar)

Operating pressure of suction jet pumps: 1 - 1.3 bar
Ok that's a new tidbit of info; the suction pumps take 1 to 1.3bar of pressure to operate. I'm quite sure that info will help somebody out if they can only figure out how to connect a gauge; actually just a 't' adapter at the top of the left sender unit should work.

Here's a more photorealistic drawing of the fuel pump including the jet siphon pump. Note that the drawing indicates that the siphon pressure relief valve is actually connected to the electric pump but I'm not sure what variant of x5 does that. Not mine.



I've attached a diagram of what the tank more or less looks like from the back view; it's not really to scale, the 'well' on the right side is not nearly as big relative to the left body of the tank, but it's FAR closer to realty than any of the 'saddle type' tank diagrams i've almost universally seen.



(i'm not sure if the euro models or diesel especially may have a different tank, i can only go with the e53 gas like I have and that I exclusively see when searching for a part online).

I'm marked up a diagram of the fuel pumps including the jet suction pumps; i put red arrows to indicate the flow of fuel, the dashed red indicates the direction of fuel if the pressure gets to high in the return line and vents directly to the reserve tank.

When i repaired my wife's fuel supply system recently, I replaced the fuel pump first, afterwards I expected the fuel level in the right tank to rise since every single (of dozens) of articles online insisted that the jet pumps push the gas from the left side to the right side and that would make the left side drained and the right side hold the fuel.

This simply didn't happen and it's obvious why; the top of the reserve tank is flush with the 'plateau' between the left and right chamber so the 'right side' holds NO FUEL once the level of the gas gets below the plateau between the two sides.

That being said, i noticed that when my 'left side' got below about 27L of gas, the right side even with the new pump would't read even 1L of gas.. it stuck at 0.9L for 20 miles or so (where i maybe had 150miles to empty).

it was 0F outside when i needed to work on the truck so i 'bet on' it would not be fuel filter/regulator and would be something to do with the suction pump system.

I was correct; I found that the o-ring that seals the connection from the sender unit to the inlet of the suction jet pumps was totally dislodged from the groove; gas was obviously rushing around the seal and weakening the pressure.





From this knowledge I determined that the reason i could use some (maybe half) of the fuel from the left side was that the weight of the fuel aided the pressure at the inlet of the jet pump 'foot'; once the depth of the fuel was too shallow, it couldn't suck the fuel up and over, and i would run out of gas with maybe 15-20L of gas remaining in the tank.

I fixed the o-ring and started the car; the 0.9L of gas reading of the surge tank became 1.1L. At this time, i was still under the impression the sides were split and the value should grow on the right side, so i lifted the sender unit up on the left side while the car was running and imagine my surprise when gas was spraying out like a shower from the 'unlisted seal' that i've marked up on the diagram; where the hoses to the jet pump feet are attached; there was no o-ring or any type of seal and fuel was spraying out like a solid morning pee..



The left side of this pic (of the o-ring properly in place) also shows the mangled clip that holds the hoses to the jet pump feet

it took me a couple hours to re-engineer the part to get some o-rings in there and tie the thing back together because the 'only attach once' clips were not planning on holding very long.

Once i got it back together with o-ring seal at the 'unlisted' joint, started up and checked test 6; the value jumped from 1.1 to 1.4L. Time for a test drive.

Drove for about an hour and got the level down to below an eighth of a tank; got DTE to about half of the previous record.

I haven't had a chance to calibrate the fuel gauge by which I mean, put a portable tank in the boot and go for a drive down to single digits DTE before putting in some gas and comparing to mfg. spec.

I've read hundreds and hundreds of posts of bmw owners with siphon related symptoms and with mine having a leak at a factory seam that has no seal, i'm sensing 'design defect'; people end up replacing a $100 part which is perfectly functional other than there is no 50 o-ring.

It appears the seal is supposed to be a "flare" like used in natural gas connections however it's made from plastic and no way to tighten.

It was tragically difficult to get o-rings into that seal, if i ever have to do again, i will use a piece of rubber tube around the OUTSIDE of the seal and a hose-clamp.

I have extreme confidence that people are replacing the whole sender unit or jet pump assembly when they probably just need a seal.

Some possible failure points if your jet pumps aren't working:

clogged venturi valve; some people have used fuel system cleaner to get them cleaned out, some people have used 'a pipe cleaner'; if one is working and one isn't you could make some determinations by driving to a 'nose up' or 'nose down' attitude and see what happens with fuel test 6.

disconnected line to one of the jet pumps.

pressure relief valve (this is on the right side of tank)

weak main pump

pressure regulator (which is in the fuel filter)

A couple pretty easy checks you can 'poor man mechanic' try (no special tools needed).

Start with about 1/4 tank of gas then:

pull the right side pump out an inch or so, use mirror if needed and of course a flashlight and observe the jet pump outlets and the pressure relief valve.

gas should be pumping like crazy out of the two outlet hoses and NOT a drop from the pressure relief valve.

pull the left side sender out an inch or so and observe the seals and connection at the interface from the hoses to the sender unit (that's where my failures were)

put a 't' connector in and get some pressure readings. 1 to 1.3bar on the return line (l'll leave it to you to do the math if your gauge is psi).

I've put this other places but deserves mention logically here as well.

There are some myths about gas and running low on the gauge, they are all false, but the big ones are here:
Don't run below a quarter tank, the pump is cooled by being submerged and won't be cooled properly.

absolutely false; on the bmw x5, since the pump is in the surge chamber (aka reserve tank), it will be submerged until the car gets to about 2 or 3L of gas, in addition, any time you get below about a half a tank, the car is always using 'the last 5L of gas' it literally means once you get below half a tank, the car is operating at 1/12 of a tank always (until the low fuel light comes on). (At which point it will happily operate down to less than 1/30 of a tank)
Don't run below a quarter tank; it will increase the relative amount of sediment that also can cause problems for the fuel system.
absolutely false. When i opened up the tank of my wife's car with 131313 miles (should I be surprised it had bad luck)? There was more dirt in the tank from me opening the tank than the 400 fill-ups that preceded my work. (quite literally; i knocked a crumb of dirt in there and it was the only thing i saw in the crystal clear gas).

On the left side of the tank there is a 'dent' to align the sender unit and because gas stays there permanently there was a little dirt in there; maybe about 0.2g worth, the rest of the tank stays perfectly clean because it's constantly swirled to shake up any dirt that could settle, the jet pumps move the fuel to the surge tank and unless the dirt is too big to fit through the pre-screen filter at the foot of the pump that dirt will end up in the fuel filter; which is ginormous; literally 50-100x the size of the 'old school' inline fuel filters of yore.
Don't use less than 1/4 tank when it's cold; the water in the gas will find a low spot and freeze.
You would have to have 'powerball' level of bad luck to get crappy enough gas for this to happen. I run my car down to single digit DTE about 3-4 times per year and it does not matter if the temp outside is below zero, the gas delivery is flawless; I live in the midwest so have to use 'crappy gas' with ethanol which dissolves any water that could be in the tank and burns it up.

I really hope that many people can learn what they need to actually solve this problem without the 20-30 hours of research and 10 or so hours in the garage, armed with the knowledge here that is literally a work week worth of research condensed into half an hour, you should be able to diagnose the problem accurately and more than likely fix your problem without even having to replace parts.

Learn how to use the hidden test menu, it's the only way to diagnose fuel supply system problems, but know what 'working' looks like; get a control reading when your x5 is operating properly.

Be aware that it's likely each x5 may read slightly differently but that your car should be basically the same always.

Once the tank is below about half, at this point, all the fuel being consumed is coming from the 8L reserve tank, it's absolutely vital for the siphon jet pumps to be operating or you will run out of gas with 70 miles on the distant to empty.

please report back if this thread helped you solve your 'sucking problem'.

-awr
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Old 01-14-2017, 06:04 PM
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Great post.

To answer your question about when does the low fuel warning light comes on...it depends on which engine you have.

The data sent to the IKE (instrument cluster) by the fuel level sensors...determines when the low fuel level light comes on. TEST 6 can be used to verify your particular BMW...when the low fuel level light/warning comes on...then open TEST 6 and look at the reading of the right fuel level sensor...it should be either 8 liters (I6 engine) or 10 liters (V8 engines). This is your "reserve" amount which is mentioned in the owners manual:





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Old 01-14-2017, 06:16 PM
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e53 Fuel Supply Detailed (fuel pump / siphon pump diagnostic help)

I'm just curious if the test six is compensating for the amount of fuel in the surge tank. When the float is at the top and there is 8L of gas the test six registers 1.4 in my wife's x5. Mine is not running to do a comparison. I would love to see some test six feedback from others with less than 1/2 tank. I am planning to open the tank and take a look when the low fuel light comes on and then maybe also when the the DTE is "on fumes".


Update: After getting a chance to run the car down to the reserve tank I was able to establish a few things:

➀ the reserve tank holds 5L of gas
➁ only the baseline right tank is included in computations
➂ the light comes on at a calculated 8/10L
➃ that leaves up to a gallon of gas once the calculated amount and DTE are both zero
➄ that means you will have 15-20 miles left when DTE and test six both are 0.0

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Old 01-14-2017, 06:57 PM
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Great post! I read it all, comprehended about half, and will re-read to allow for full comprehension. Definitely the most comprehensive post I've read on the fuel system and confirms my personal beliefs on it being perfectly okay to run the tank down. Thanks for taking the time and effort to gather that information.
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Old 01-16-2017, 07:59 PM
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e53 Fuel Supply Detailed (fuel pump / siphon pump diagnostic help)

BMW does recommend against actually running out, but you clearly have 11-13L of gas to consume once the light comes on before you are low enough to be a problem.

It is important to run down close to empty occasionally if for no other reason to test the siphon pump. My wife insisted on never getting below 1/4 tank and that masked the defective siphon pump leaving me stranded at 6F for 3 hours waiting for a tow. I recommend having a gas can with and running down to single digits DTE at least annually.

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Old 01-18-2017, 10:48 PM
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Quote:
BMW does recommend against actually running out, but you clearly have 4-6L of gas to consume once the light comes on before you are low enough to be a problem.
Oh, of course. I run from full to ~20mi DTE. Never run a tank to dry and run out of gas. Just meant that I never worried about my practices and now I have even less worry.
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Old 01-19-2017, 12:27 AM
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You definitely don't. I feel quite vindicated in my practice of wait for the low fuel light and then look for the next convenient station within about 1gal away. The surge tank system in the x5 is nearly flawless. I've had cars with 1/4 tank not start from being on a sloped driveway, that is impossible with x5! You can use 95% of your fuel tank at any pitch or roll attitude!


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Old 01-21-2017, 04:33 PM
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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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Old 01-23-2017, 08:16 AM
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Old 01-23-2017, 08:31 AM
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Thanks! (And you're welcome).

It took me so many hours to weed through literally hundreds of Internet postings almost all of which were inaccurate, then finally trial and error on my wife's car to arrive at some hard facts.

My original essay may have errors if anybody finds one in there let me know I can update it so that future readers will have the most accurate information.

I went through so much grief trying to find actual hard facts that I wanted to save others from the same headache and give them the most accurate and detailed information so they can diagnose and fix their problems more quickly.
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