Gas Sending Unit faults -Help!

[taosx5]

I have searched several forums and some have had similar but not exact issues. Here is mine. From what I have read I think I have a sending unit failure.

Wife drives an 03 E53 with 4.4 since new. Now has 150K miles. This car has been pretty good overall and we have had longer than any other we have owned of over 35 vehicles. She is good about filling up at 1/4 tank or as soon as fuel gauge light comes on. Light usually comes on with 50 miles left. (I drive into her to fill up at 1/4 tank and not let the light come on before she fills up but sometimes that is her queue).

So long story short, I get a call that my wife's car broke down about 15 miles from home. She described it went into limp mode and just quit running. I go there with my scan tool and have to jump it to get it started but it won's stay running. Didn't think about it but gas gauge is on 1/4 tank. No error codes what so ever.

We have road side assistance so I have it towed. Car is put on a slide tow truck and dropped off at an independent shop (I say this because car is winched at steep angle meaning fuel in tank is moving around quite a bit). I am thinking alternator went out and is not charging battery and it got so low that it created some electronic issues.

Fast forward to shop test. It is They turn on key and gas gauge is left of E (but no light for low fuel) indicating to the trained eye it is OUT OF GAS. They put 5 gallons gas in and it starts and runs fine, no errors on scan and no other issues. We take car and I am making fun of my wife for running out of gas.

First thing she does is fill the tank. It was on 1/4 tank when she picked up but took 23.3 gallons to fill. Then the gas gauge registers 3/4 tank. We take it on a road trip and miles left shows around 300. We drive about 140 miles and the trip meter range goes back and forth from 220 to 330 as we climb a mtn pass. It changes in 10 mile increments (never done that before). Goes from 300 down to 220 miles at top of pass then back to 300 +/- after we get back to normal elevation. We get to our destination 3 hours later and we have 300+ mile range (this car usually gets about 450 miles per tank). The gauge reads about 2/3 full. We hang out for about 6 hours, temp climbs from about 60 degrees to 79 degrees, we get in car and start it up. Gas gauge then reads 1/3 tank and range is around 210 miles. WITA?

I decide to fill tank and it takes 11.5 gallons to fill and I filled it to the rim. Range goes from 220 to 320 and gauge goes from 1/3 to 3/4 full. We drive back over the same pass we just came and range goes from 320 to 220 then after 140 miles we get home and range is showing just under 300 and gauge is at 2/3.

I have to say since this car was new the computer, gas gauge, and mpg have been super accurate. This issue went from working perfectly to whacko in one fell swoop. I have read many threads and suspect it is a sending unit on one side of the tank (understand there are two).

If I have to take the tank out does it make sense to put in a new fuel pump (over 150K miles)? Do I replace both sending units? Do I try to clean the contacts and see if it clears up? HELP Please!

Is there a good DIY on taking the tank out and checking contacts and possibly cleaning the tank and reinstalling? I do some of my own DIY but is this above average mechanical experience? Thanks in advance for any input.

[upallnight]

Most likely your transfer pump is history, and the gas on the left side of the tank is not being transfer to the right side. The gauge is on the left side, but the fuel pump is on the right side. If the transfer pump doesn't transfer the fuel, the right side of the tank will be pump dry by the fuel pump.

[srmmmm]

In my similar situation, both the left and right side sending units had gone bad. My fuel gauge dropped so fast, you could almost see it move going down the highway. The transfer siphon worked fine. Since the fuel pump is part of the right side assembly, it was replaced as part of the service. The job is done by accessing the top of the fuel tank from underneath the rear seat cushion. My replacement was done at 245,000 miles.

2002 X5 3.0 282,200 miles
2004 325i 120,900 miles

[upallnight]

His no start situation and later able to restart without any repairs done to the X points more toward a transfer pump issue.

[mferek]

On one of my previous cars, the electrical contacts on the sending unit got dirty and caused the gas gauge to malfunction. Just cleaned it off with a Q-tip and it worked fine after that. I haven't tried to access the sending unit(s) in the x5 but they're usually not too hard to get to. Worth a shot IMO

[taosx5]

Originally Posted by upallnight

Most likely your transfer pump is history, and the gas on the left side of the tank is not being transfer to the right side. The gauge is on the left side, but the fuel pump is on the right side. If the transfer pump doesn't transfer the fuel, the right side of the tank will be pump dry by the fuel pump.

Thanks for your input Srmmm, and mferek. Upallight thanks for the insight. I knew there were two sides to the tank but just assumed that the bottom of tank is flat and any fuel in the tank would get to the fuel pump. I had no idea that there was a transfer pump.

Upallnight, I am big on preventive maintenance. With 150K miles I suspect that if I were replacing this transfer pump wouldn't it make sense to replace the fuel pump as well. Are there two sending units that tells the gauge how much fuel is in the tank? Does the fact that the low fuel light did not come on give any credence that the transfer pump is history? Do you have part #s and is this a job for a moderate do it yourselfer?

[cn90]

Agree on siphon pump being the culprit.
Common issue in E39 and E53 etc.

If you don't want to fix it (the siphon pump), then keep gas ABOVE 1/4 at all time.

[taosx5]

Originally Posted by cn90

Agree on siphon pump being the culprit.
Common issue in E39 and E53 etc.

If you don't want to fix it (the siphon pump), then keep gas ABOVE 1/4 at all time.

CN90, that would be great advise but it won't work in our case. This just started literally on one trip near town. The gauge right now will not register more than 3/4 after a fill up to the gills. From there it fluctuates from 2/3 to 1/4 just by turning the ignition off and starting it again. It does not go below 1/4 so it could run out at any time which it did. The only thing we can do now is to fill by accumulated miles. Normally the car averages 18 mpg both around town and highway.

With this average, 1/4 tank of 23.7 gallons (full tank) is about 5.9 gallons. 23.7 - 5.9 = 17.7 gallons X 18 mpg gives a 320 mile range. We could probably get along by filling up once it hits 320 miles but on road trip we can get as much as 24 mpg which is about a 550 range give or take a gallon. Conversely in stop and go snow and mountain driving with 10% ethanol in the fuel which is what we mostly are in during winter months it can get less than 16 mpg.

The bright side is when my wife broke down, I was seeing $$ signs to fix. But now it is just an inconvenience that we will have to deal with until I can figure out what is going on. Any input is always welcome and Thanks to all of you who have volunteered info.

[cn90]

The E39 crowd uses Techron and it usually fixes the sticky sending unit.
Put a bottle of Techron in the gas tank here and there and report back later.

[taosx5]

Originally Posted by cn90

The E39 crowd uses Techron and it usually fixes the sticky sending unit.
Put a bottle of Techron in the gas tank here and there and report back later.

CN90, the more I read and with the symptoms I am having I am kinda doubting the transfer (siphon) pump. Here is why! The fuel gauge errors are consistent with wrong readings regardless if the tank is empty or anywhere in between and at full. I am gonna go pick up a bottle of Techron as you advise and see if that helps.

Question for those that have cleaned contacts on the sending unit. I have taken the seat off and see the inspection (or replacement) holes for both the fuel pump and what I am guessing is the transfer (siphon) pump.

Can anyone point me to a thread that shows how that is done?
Big thanks in advance.