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-   -   Reserve fuel problem (https://xoutpost.com/bmw-sav-forums/x5-e53-forum/109803-reserve-fuel-problem.html)

talljames 02-13-2019 07:46 AM

Reserve fuel problem
So, my car stopped when the fuel meter read 1/4 of a tank left. Couldn’t start it and had to get it towed to the mechanic. He discovered that it was a problem with the reserve tank (overflow tank). Purchased another sevond hand sensor for the non-main side off ebay. Had it put in and the same problem.

I was thinking do there need to be any codes cleared? Is it a problem elsewhere that looks like a sensor issue? Anyone have any experience that could help? 2005 e53 4.4i sports.


andrewwynn 02-13-2019 10:22 AM

Your problem is the siphon jet is not doing it's job. In the VAST majority of this case, the o-ring to the siphon jet I have a very detailed thread on how to fix that problem for free.

You can confirm the siphon jet is not working by unlocking the hidden dash (google how) and display test 6; it will show the fuel level in liters on each side of the tank. The right side should show about 1.5L of fuel always once the total is less than 27L. if the right side ever drops below 1.0L, your siphon jet is not doing its job. (almost always a leaky o-ring but also end-of-life electric pump won't make enough pressure to supply the engine AND the siphon jet; it's very rare for that to be the case; almost everybody that replaces the electric motor when the siphon jet is not working will fix/replace the siphon jet after because it wasn't the motor pump.

search for my thread about siphon jet and the detailed fuel system and read that one first to learn how the fuel system works on the X5.

sgrice 02-13-2019 10:24 AM

So to confirm - the car runs fine if you fill up the fuel tank, but runs out of gas while showing approx 1/4 tank?

Assuming the above is true, do a google search for posts by andrew wynn on the anatomy of the e53 gas tank system, and the usual trouble spots. I don't remember off the top of my head, but there are frequent failure points that disrupt the capability of siphoning fuel from one side of the tank to the other. I seem to recall replacement of an o-ring usually solving the problem?

Good luck.

andrewwynn 02-13-2019 10:26 AM

PS; there is no 'reserve fuel tank' in reality. The x5 is simply programed to turn the light on when your total fuel amount gets to 8L if a 6 cylinder or 10L when it's an 8 cylinder.

There is some 'emergency gas' in the tank as there is approximately 1 gallon of fuel that is not counted by the gauge. (i drove my wife's car down to 0.0 miles DTE and siphoned out and weighted the gas it was 3.6L of gas). Since her car reports exactly 1.4L of gas when the right tank is at 'flood stage' it means the 'surge tank' on the X5 is almost exactly 5.0L of fuel.

Read all the details on my detailed fuel system thread and how to fix the o-ring on the siphon jet on the other thread.

it's a 100% certain test to run 'test 6' and start with about 30L total gas; when you get down below about 27L of gas the right side should stay the same (level ground steady speed); most X5 will report about 1.4L of gas so you will see a number like 256014 (25.6L left 0.14 L right), but when the level in the left side drops and there is less weight of fuel on the left to help out the siphon the number on the right will drop if the siphon jet is not working properly. eg. if you see 190010 your siphon jet is not working and most likely due to a design defect in the o-ring of the siphon jet, it's an easy fix just need a cable zip-tie (and a fresh o-ring )

cn90 02-13-2019 01:02 PM

If you do Test 6 as "Andrewwynn" said above, bring a gas can (red gasoline container) with about 2 gallons of fuel. This way you can fill with gasoline to get to a gas station.

cn90 02-13-2019 01:28 PM

Ebay has the improved O-ring.

andrewwynn 02-13-2019 02:38 PM

I think the repair thread has the size listed

Sent from my iPhone using Tapatalk Pro

andrewwynn 02-13-2019 04:15 PM


Originally Posted by cn90 (Post 1155081)
If you do Test 6 as "Andrewwynn" said above, bring a gas can (red gasoline container) with about 2 gallons of fuel. This way you can fill with gasoline to get to a gas station.

You actually don't even need the gas can as you have about a gallon of fuel once the right side reads 0L. Once you see right side drop below 1L get some gas, and an o-ring for the siphon jet.

Sent from my iPhone using Tapatalk Pro

talljames 02-13-2019 07:22 PM

1 Attachment(s)
Thanks guys. Sorry for the delay in reply. I have a picture using test 6 below.

Attachment 75401

Also, when I took my car to another mechanic to fix a squeaky rear suspension it konked out just as the guy was going to move it. Sure enough on the 1/4 fuel mark and low fuel light on. Put some fuel in and started again.

hopefully the above picture is posted, otherwise will post again.

I will monitor test 6 as I drive.

andrewwynn 02-15-2019 05:40 AM

You exhibit the symptoms of a blown siphon jet o-ring.

The more fuel light should not be on at 1/4 tank so that's an oddity. Drive with test six up until the right side reaches the plateau stage (where all the excess fuel spills our of the right side to the left side). It should stabilize at about 1.4L on the right side.. typically when the siphon jet fails that number will slowly drop once the left side gets below 25L or so.

When the right gets to 000 you'd better refuel immediately. You should be able to drive 10-15 miles once the right tank reads 000 but that's assuming the circulating fuel from the FPR gets back into the right tank.

If the right tank number drops almost certainly youre o-ring has failed. There is a small chance of another component going like the fuel pump has gotten too weak or the pressure relief valve on the siphon pump but I've not seen a case of fuel starve at 1/4 that wasn't a bad o-ring. It's a design flaw. The o-ring typically will make it between 80 and 130,000 miles. Wife and mine both failed between 125 and 135,000. Exactly the same failure for exactly exactly the same reason (flawed design)

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