Another E36 fuel guage issue

Ok, the car's done this before. I mean the guage stayed on empty after filling the tank. A week ago, I switched on the ignition half way through
filling up and the guage moved to half. Switched off the ignition again but after filling the tank completely, it was still showing only half full.
Drove about 250km and the needle showed almost empty. (Now how can the needle move regularly towards empty while it is still over half full?) So I filled up, but only about 30 liters went in and the needle is still stuck on empty. The fuel low LED has now been on for the last 200km and the needle is still on empty but fuel consumption computer shows 8 liters per 100km which is excellent. If I could just get the needle to show what I've got in the tank. Can anyone explain how this works?
PS: My brother drives a C180 merc and says that his car sometimes still shows empty just after filling the tank too. Does the C class merc have the same crap system for measuring fuel?
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You have dirty contacts on one or both of the sender units. The contacts are wiper-like thingies that work much the same as a rheostat -- potentiometer. They could work exactly the same way, but saying "much the same" allows for error.
In any case, my E36 did precisely the same thing as yours is doing. I removed the left-side fuel sender unit -- mostly because I felt it would be easier -- and cleaned the contact wiper with a combination of contact cleaner and a business card to mimic very fine sand paper. I cleaned a significant amount of black crap from the contact patch and wiper are, and the sender unit worked properly after that.
The left side and right side sender units work the same way, but the right side unit also contains the fuel pump. There are two sender units because there are two discreet gas tanks in the car. There is a pump system that tries to keep the level in each bladder equal, but there are two senders none the less. (I like to think that if the levels could be kept equal, then only one sender would be required.)
To gain access to the senders, remove the rear seat bottom, and carefull pull back the tar paper material to expose the round cover. Remove the cover to expose the gas tank. Take off the hoses and wires that are connected, and remove the ring that holds the sender unit in place. The sender can be lifted out, you will need to jockey it around to get it out. It (they) only fit in one way, so you can not put them back in wrong. Take care to not damage the gasket/o-rings.
I said you can't put them in wrong, that is a bit of an error. You can put them in crooked, and this affects the sealing when the outer ring is replaced. You are required to put them in the proper orientation, and this can not be screwed up.

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How much fuel can be in the tank when I attempt this, or are the senders right at the top?
Thanks again for your very detailed description of how to tackle this job Jeff, I'm sure I'll be able to fix this myself.
Kind regards MW
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Well, you open the tank from the top, so technically the tank can be full. But as a practical matter, I'd suggest you plan your trips so you get home with a nearly empty tank to work with.
I keep track of my odometer, so I know that if the guage shows E and the light comes on, and the odometer says 350 miles, then all is well with the universe, and the tank is in fact empty. (I don't recall at this moment the actual range, but the point is that the odometer plays a role in the checks-and-balances of the indicators.) When the indicators are lying, as yours are, then I use the odometer for my main data set.
Fill the tank, reset the odometer, then refill again when the odo reports 300 miles or so. Try to get back to your garage when the odo reports 250+ miles. This should leave it sufficiently empty to do your work.
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