fuel tank sucking jet pump

[Rev Engineer]

Apparently I have a problem with the fuel transfer on my newly acquired 2001 X5. The previous owner said that the gauge was 1/4 tank off. The instrument cluster test shows the left tank having 30-40 liters and the right side only has a few. So, this means that the fuel isn't transferring. The previous owner said he had replaced the fuel filter and I can see non-bmw style hose clamps on the filter. From here it seems like it still be several things: fuel pump is wearing out, fuel pressure regulator is bad, the siphon jet needs to be replaced, or something isn't connected right inside the tanks. Can anyone help how to diagnose this a little further before throwing some parts at it? I took a peek inside the fuel tank on both side and didn't see anything unusual, I also confirmed that the floats work via the instrument cluster test. Perhaps I should pull out the jet pump and check it for a blockage? Can I learn anything by checking fuel pressure or doing a fuel delivery rate test? Anyone know the specification? Or, any other ideas on how to diagnose this problem?

[Qsilver7]

Can you clarify the issue a bit more? What happens if you fill up the tank? Does it consume the amount of gallons that you refueld with...or does it only consume some of it...leaving you with a partially unused tank of gas...which would indicate that the jet is not sucking from the left side (as you presumed). But if the total amount of fuel you put in is being consumed...but the fuel gauge isn't correct...then the issue may just be a faulty fuel level sensor.

The left/right fuel level sensors inside the tank read from left to right. The left side of the tank is sucked over to the right side of the tank (the left/right perspective is from the driver's position behind the steering wheel).

If you have a full tank of gas...and you run OBC TEST 6...what does it reveal? Does the left side indicate "full" then the right side starts decreasing until the tank is empty?

If so, then the sucking jet is doing its' thing and the issue is probably the left fuel level sensor. Even though the fuel tank is bifurcated (2 sections) and there's a sucking jet that pulls the fuel over from the left side...that doesn't really occur until the fuel drops below the "hump" that separates the left right side. Even though the left fuel level sensor is supposed to go from full to zero...then the right sensor goes from full to zero...the fuel in the tank empties from top to bottom (until the fuel is actually separated by the hump in the middle and is sucked over to the fuel pickup on the right side.

What does the 2nd screen of TEST 6 reveal? Does it say Phase 1, 2 or 3?

(left side goes down to zero 1st..then right side...image below shows that fuel gauge showing approx 1/2 tank of gas is correct in that the left sensor is showing 2.8 liters...while the right sensor is showing 35.2 liters...while the 3rd nbr represents the approx total amount of fuel left in the tank 38.1 liters)


(perfectly working fuel level sensors will say PHASE 1 in the 2nd screen of TEST 6)



See explanation of TEST 6 and what the 2 screens information means:

[Sarek]

On the e53 the right side (pump side) of the tank is much smaller than the left side. Keep that in mind when diagnosing. I usually consider replacing as much as possible. sensors, pump, filter and sucking jet pump if I am cracking the egg. All of these have issues. Just do it once. Just my 2 cents

[Rev Engineer]

Thank you for chiming in! Sorry if I wasn't too clear in my first post. Before I suspected a problem with the jet pump, I was suspicious of the senders based on the PO's comment that the gauge was off. So, I pulled both senders partially out of the tank and moved them through the full range of motion, while watching the cluster test and measuring the voltage. They worked as expected. The next time I filled up, the gauge did not go to full. After just confirming the senders were ok I was baffled. So, I open the left side tank again and found the float stuck as the gauge went to full when I re-seated the driver's side sender. Thinking I just solved the problem, I happily went through most of the next tank when I checked the cluster test again and found the right side showing near-empty (a few liters) and the left side around 37 liters. Doesn't this clearly indicate that the sucking pump isn't working as the right side should be mostly full while the left side gradually goes down (when the tank is say around 1/2)? I'll need to do some more checking with the cluster test and check the phase you mentioned. I haven't tempted fate by running out of fuel yet, but with the cluster test only showing a couple liters in the right side, I suspect it is going to run out. Further, maybe the pump is wearing out quicker as it spends most of it's time running in a near-empty tank.

I just got the vehicle and have only put a few hundred miles on it so far. I'm trying to be cautious about throwing money at it as there are no shortage of X5 issues that can empty the pocket book. I'm happy tooling around the X5 and learning more about it, but I'm being frugal and only fixing what needs it. It's not my primary vehicle so I don't need it to be ultra reliable for the long-haul.

Cheers!

[Sarek]

As I stated before, the tank in an e53 is a very strange shape. The pump side is a lot smaller than the passenger side. The sucking jet is constantly pumping fuel from the left to right side using return fuel pressure to cause a venturi style transfer. In a normal type tank like in an e46, the tank has a hump in the center that splits the tank amost equally in half. Your theory of transfer at about half a tank prove accurate. On the e53 the tank shape is not equal on both sides. The pump side of the tank is very small and the other side is huge, but has less of a hump in it. Transfer tends to get critical at around a quarter tank. The left side will pretty much always have some fuel in it due to the erratic shape of the tank. As the tank gets around a 1/4 tank you should see the pump side of the tank level stay steady and the left side drop until it is empty. The pump side at that point will be barely holding any fuel. I think it was like 5 or 6 liters. . Still think u should consider replacing all of that plastic stuff in there. Peace of mind.

[Rev Engineer]

UPDATE

Thank you for the thorough explanation on the e53 tank! I was wrongly thinking it should behave like an e39/e46. With a low tank, I ran the output of the pump into a fuel container and proved the jet pump is working just fine. Turns out my problem is related to the left level sender. It seems to get stuck at around the 37L reading on the left tank and then the reading doesn't budge until refueling (using the cluster test). I've checked the sensor and float a couple times and can't see how it could get stuck on anything, the jet pump tubes are not that close to it. Electrical contacts look fine and they would result in reading empty if the connection broke (with the cluster test). The last time I put the left sender back in, I noted the bump on the bottom of the tank where it is supposed to center on. On my e53, it's actually towards the back and I put the pump back in a slight angle towards the back to hit it. The sender reads about 10L higher with it mounted this way versus putting it back straight down. I just ran my last tank of gas down to the reserve light without the gauge getting stuck or running out of fuel, so I'm hoping it's finally fixed.

[cn90]

The E39 people have good luck with a bottle of Techron here and there to help free the gunk buildup on the level sensor.

If you have issues with siphon pump, a work-around solution is to keep fuel above 1/4 at all time.

[tmv]

Cam.

[Rev Engineer]

I'll try that if the problem comes back. I've gone through a couple more tanks and the level sensors have been working fine. The main issue seems to have been the installation of the left side level sensor. It needed to install it at a slight angle, with the bottom centering feature tilting towards the back of the vehicle. Maybe something happened to this X5 in it's 120k mile life that messed up the left side tank. From what I've read, the level sensor is supposed to get installed straight-down and not at an angle. If I do install it straight down, it reads incorrectly when the tank gets low and seems to get stuck.

[tmv]

You can try it as a fuel system cleaner. No need to wait for the problem to come back.
BTW, they are BOGO at Advanced Auto and AutoZone now. I've just got 4 bottles.