E30 fuel gauge jumps when less than 20 L in the tank

[E30REDD]

First of all the car is from the beginning a 316i but the engine has been replaced with a M42.

The fuel gauge works fine when it's between 60-20. But when it goes down to 20 it starts acting weird. Sometimes the low fuel indicator lamp lights up and sometimes it doesn't. It can also be jumping (slowly) between 10-20 litres.

Could we exclude contact errors since the gauge is working perfectly between 60-20 liters?

My guess is that there has something to do with the fuel sensors in the gas tank. Does even 316i and 318is have the same fuel system?

[Brianmoooore]

If you're certain that it only happens when there's less than 20 litres or so on board, then it is a level sensor issue.
If the car is a 316i saloon, it will have a 55 litre tank, and therefore only one level sensor, which is part of the fuel pump assembly.

[E30REDD]

Does this picture say something?

[Brianmoooore]

Sender is under the larger black cover. Held in by four small nuts and washers.

[E30REDD]

Sender is under the larger black cover. Held in by four small nuts and washers.

Yea but since there are two covers (one sender per cover?), does it mean I have a 318is fuel tank?

[FinalD]

That's just the standard floor/under seat area is it not? Check under the cover on the right (the small round one) for the sensor.

[Brianmoooore]

You can look under the round one as well if you like, but all you should see is the top of the fuel tank. iS fuel tank is the same as any other small engined saloon - 55 litre.

[E30REDD]

You can look under the round one as well if you like, but all you should see is the top of the fuel tank. iS fuel tank is the same as any other small engined saloon - 55 litre.

It seems like I have two sensors after all.

[Brianmoooore]

First of all the car is from the beginning a 316i but the engine has been replaced with a M42.

A 316i what?

[E30REDD]

First of all the car is from the beginning a 316i but the engine has been replaced with a M42.

A 316i what?

316i -90.

[Brianmoooore]

First of all the car is from the beginning a 316i but the engine has been replaced with a M42.

A 316i what?

316i -90.

Body type!

[E30REDD]

A 316i what?

316i -90.

Body type!

Coupe!

[Brianmoooore]

In that case it's had a tank change, or was a special order.
If you have two senders, you need to short each one out at its plug in turn, to find out which is causing the problem. You will get an incorrect, but stable reading when the faulty one is shorted out.

[E30REDD]

Should I just disconnect one of the senders and watch how the fuel gauge behave?

[Brianmoooore]

You have to disconnect ONE of the senders, then use a piece of wire to short together the two buckets in the plug.
After you're satisfied with what happens, replace the plug on the sender, and do the same with the other one.
It's the three pin plug on the fuel pump side, and the wires are brown and brown/green.

[E30REDD]

Okay, I guess this is something I should do when I'm closing down to 20 L in the tank. I just filled it all up so it will take a while until I get this fixed, but once again, thank you!

Btw, on the first picture, is the blue and red connectors for the fuel pump?

[Brianmoooore]

First time I've seen that pic. (couldn't get it to open yesterday), but it fills in a few gaps in the story!
Blue and red connectors are someone's half arsed bodge up of a fuel pump connector! I strongly suggest you get hold of the proper plug with a few inches of wire and solder it to the original wires (under the peeling PVC tape), before corrosion begins to set in and fuel pump troubles develop.
The fuel pump you have IS the correct one for a 55 litre fuel tank, so whether this is the cars original before the wrong tank was fitted, or an incorrect replacement is anybody's guess.
The little plastic cap over the 'spare' pipe on the pump isn't really up too the job. You should replace this with a short length of fuel hose with a bung (sawn off bolt) in the free end, and fit hose clips at each end.
It looks like the top of your tank may be damp with fuel around the pump. This could be a leak from the unused pipe I've just mentioned, or could be a failing, or even broken O ring sealing the pump in place.
If you take the pump out to change the O ring, then you just have to tap it about 20 degrees anticlockwise after you've removed the fuel pipe, and then wriggle it out. The level sender must be removed first.

[E30REDD]

Okay, where do I find the connector I need for the fuel pump?

[Brianmoooore]

On a scrap loom.

[E30REDD]

On a scrap loom.

Okay

About the spare pipe. Is it really necesarry to mount a plugged hosing on it?

If the current plug starts to leak it wouldn't really hurt because the fuel would flow from the top of the tank to the bottom of the tank and then down to the ground.

Why is that metal pipe even there?

[Brianmoooore]

If you're happy with fuel running down under your car in the vicinity of a hot exhaust, then fine, but if it doesn't go up in flames, the very least that will happen is that the fuel will remove paint and underseal.
It'll also be an MOT test fail if spotted.
The pipe is there because it's a pump for a 55 litre tank. The excess fuel bled off by the fuel pressure regulator on the engine goes back into the tank through it. On the 62 litre tank, this fuel is put to use, priming an internal syphon that transfers fuel from one side of the tank to the other.

[E30REDD]

The exhaust is not hot enough to ignite fuel. But I get your point, thanks.