[Synchronixity]

Hi, I just bought this car and the gas gauge is reading weird. Before I tear it apart I was looking for any insight.

When the tank is full, it will always read half a tank.

It will then decrease with driving as normal, but does not seem to correlate to being "half scale" (divided by 2) of what is supposed to be there as if a float arm is bent incorrectly.

Additionally when you turn it off and on the reading will go up to a higher reading.

At this point I am usually starting each drive with no gas warning, to triggering the gas light every drive shortly into it, despite not filling the tank at any point between this. Up, down, up, down.

The previous owner mentioned he replaced the fuel pump and then this issue started.

So is it possible he installed the wrong sending unit, or am I looking at more of some type of sensor?

I am a long time mechanic but a BMW newbie so all the sensors and things are new to me.

Thanks
Synchronixity

[Synchronixity]

I have also noticed a slight smell of gas outside of the car sometimes, if it's related. I have yet to smell it inside of the cabin, and no leakage is apparent.

[gbalthrop]

Originally Posted by Synchronixity

Hi, I just bought this car and the gas gauge is reading weird... When the tank is full, it will always read half a tank... Additionally when you turn it off and on the reading will go up to a higher reading...The previous owner mentioned he replaced the fuel pump and then this issue started...

There are TWO (2) Fuel Level Sensors, one on EITHER side of "Saddle-shaped tank". The two sensors are wired to the JBE Module. The Gauge shows the TOTAL of the values from EACH of the 2 sensors. Proper Scan Tool, or Diagnostic Software such as INPA or ISTA will show you, ON Screen, the Value reported by EACH of the two Level Sensors. So you can determine Which Sensor has a bogus value reported, WITHOUT dismantling anything.

Actually, "Hidden Menu 6.00" will display that information on your Instrument Cluster:
https://www.carsaddiction.com/articl...er-hidden-menu

You need a Tool that can "Connect" to the JBE Module to see each Sensor Value, as received by the JBE Module. The fuel pump is on RIGHT side of tank (US Passenger side), and "Jet Pump" pumps fuel from Vehicle LEFT /US driver side of tank to Right Side where Pump is located. So when Fuel Gauge reads 1/4 Tank, Left Side is virtually empty & Right Side is ~ 50% Full.

Prior Owner or Mek-Nik could have messed up wiring or plumbing, but diagnose FIRST & let us know what you find, BEFORE opening tank or ordering parts.
George

[Synchronixity]

Thanks George I'll check

[Synchronixity]

L 09.7
R 00.0
S 9

So left has gas, right has none, and the sum is all from the left.

So I suppose the right sensor is bad.

I hear this is the one where you have to replace the whole tank or at least pull it, whereas the left side is the easy one?

Bummer.

[Synchronixity]

I now see it is the left side that is unserviceable. Lucky me.

Pulled the seat and the cover off and now digging into it. Wiring looks good.

The smell of fuel really knocked me in the face when I popped the rubber grommet out. Previously I was only smelling it outside of the car. Not sure if that is indicative of a fuel tank leak, or some kind of vent issue. Obviously I don't think the tank would have leaked as it is not exposed to the elements. We will see.

[gbalthrop]

Originally Posted by Synchronixity

... Pulled the seat and the cover off and now digging into it. Wiring looks good. The smell of fuel really knocked me in the face when I popped the rubber grommet out...

VENTILATE! NO SPARKS!!!

Fuel Vapor (Gas Gas ;-) is EXTREMELY EXPLOSIVE!

More than ONE person has been badly burned/ injured by fuel vapor explosion when spark from electrical tool ignited vapor in enclosed area.

If there is NO gas in RIGHT tank, then the issue may NOT be the Right Fuel Level sensor, but rather the "Jet Pump". If there is measurable fuel in Right Tank, but Right Sensor reads "0", then there MAY be a wiring issue.

AFAIK, unless you have N51 SULEV, BOTH Sensors are replaceable. ALSO, the plumbing in the tank may have been dislodged by someone (in a hurry ;-) working there BEFORE. When you post back, please provide Last-7 Characters of VIN so we can provide ISTA Procedure for whatever you need.
George

[Synchronixity]

Hi again,

So a weird turn of events... The fuel float is sinking causing the issue. It just doesn't float! It reads fine when moved by hand and is not sticking anywhere. It is also not getting caught.

I even submerged it, picked it up, and let go for a final test, and down it went.

The plastic float part just does not float. Some say this could be caused by a leak from a small hole eroded into the plastic, however previous owner states it did this since he replaced the whole assembly from day 1 and never bothered to fix it, and visually it looks fine. So I'm thinking it's just too heavy and it sure feels like it!

Perhaps bad quality control? LOL! Bottom barrel parts... Which is probably a sign to replace the entire assembly as I can't imagine the pump is much good either...

Strange, strange day...

I ended up disconnecting the entire rod and moving the plastic piece on the assembly to full, so at least now the annoying gas warnings are gone until I can get it fixed. It worked fine and is now simply "glass half full" rather than "glass half empty". Making my empty at half, and my full at full.

I'll see if I can pull a float out of my parts bin.

Two things:

A) For any future person reading this, the inside of the metal ring that holds the fuel pump in is oddly sharp, and I gave myself a nasty 1" gash on the bottom of my thumb that thankfully, barely, closed by itself. I guess now my fingerprint will always be a reminder of the day! No bowling or typing for a while...

B) I think I am missing the inner ring under this ring, as I had to try several times to seal the ring with a combination of perfect angle and a lot of tightness, otherwise as soon as the assembly filled up with gas it was pissing all down the side of the tank and all over the ground. Unless it is really that finicky, I think I am missing a part here. The rubber ring is in place. I assume that is also the smell outside of the car (and now for a while inside the car, and my clothes, and my skin...)

All in all, a new float should fix it (like I said might just get an entire quality assembly if a sinking float is any indication of quality...), and a new ring to stop the smell.