Early model fuel gauge/OBC innaccurate

[Varinn]

I've got an 84 325e that I've had fuel gauge problems with since day one.

My tank holds 55L of fuel
My cluster gauge reads 55L when full, my OBC reads 46.5L
My R light comes on with 10L of fuel remaining
At 15L remaining, my OBC reads 5 (and never drops below this)
I get 10L/100KM confirmed average consumption, econometer/OBC both read the same
Upon fillup from empty, OBC and gauge take about 3-4 minutes to respond at all

Coding plugs on both are from 88 Super Etas, cluster from 88 Seta, fuel gauge is from a 91 318is

Anyone have any ideas where the funkyness is coming from? Only the cluster gauge seems accurate but the reserve light comes on so early, I've heard it should fire up at the 6L mark, not 10L.


NOTE: I can add imperial units to the post if you guys down south are confused by metric system for this

[jlevie]

The low fuel light should come on when there is between 6 and 10l in the tank, so that appears to be working correctly. The level sensor, by design, is slow to respond to changes in fuel level (to keep the gauge from bouncing). Fuel has to flow into or out of the sensor via a small hole. If there is trash in the tank that hole could be partially plugged and making the sensor even slower to respond.

The OBC should be more accurate than the gauge, but the level sensors aren't exactly precision instruments even when new. And the A/D converter in the OBC could be slightly off. A combination of those could explain the different readings between the gauge and OBC.

[nando]

what fuel sender do you have? the early sender is not compatible with the later gauges and OBC coding plugs.

also, the 318is and early E30s have a smaller thank than some of the later M20 based cars (and all B25s). I don't know if the seta got the smaller tank or not. that could account for the difference you're seeing.

[jlevie]

what fuel sender do you have? the early sender is not compatible with the later gauges and OBC coding plugs.

also, the 318is and early E30s have a smaller thank than some of the later M20 based cars (and all B25s). I don't know if the seta got the smaller tank or not. that could account for the difference you're seeing.

The right side level sensor is the same on early and late cars. The problem with the gauge and OBC comes when you mix parts meant for the 63L tank (with two level sensors) with parts meant for the 55L tank (with only one sensor).

[nando]

his OBC may have the coding plug for the larger tank, which is what i was getting at.

[jlevie]

his OBC may have the coding plug for the larger tank, which is what i was getting at.

That might explain the lower than correct full tank reading, but then the OBC should read ~30L when the tank is empty.

[Varinn]

I have a second fuel level sender from an early e30 with VDO sender. Will be installing it and a new OBC this weekend. I tried adjusting the calibration screw on my OBC last night (and promptly lost the little grommet) but I was only able to get my reading up to 47.5 on a full tank.

[dnguyen1963]

I have a second fuel level sender from an early e30 with VDO sender. Will be installing it and a new OBC this weekend. I tried adjusting the calibration screw on my OBC last night (and promptly lost the little grommet) but I was only able to get my reading up to 47.5 on a full tank.

Wow, I just fill the tank up and drive. You must be an engineer.

[Varinn]

Wow, I just fill the tank up and drive. You must be an engineer.

that's cool bro, if you have any other insightful comments just let me know. Myself, I'll continue looking at fixing things on my car that aren't working 100%

[dnguyen1963]

that's cool bro, if you have any other insightful comments just let me know. Myself, I'll continue looking at fixing things on my car that aren't working 100%

Don't be offended. There was no intention on my part. I have many engineering friends and they treat their cars the same way you do. On my car, I took out the sender unit and completely cleaned out the rod holding the floater. There was a lot of brown deposit on the rod. The fuel gauge and OBC readouts were much more accurate after the cleaning. Give that a try.

[Varinn]

Don't be offended. There was no intention on my part. I have many engineering friends and they treat their cars the same way you do. On my car, I took out the sender unit and completely cleaned out the rod holding the floater. There was a lot of brown deposit on the rod. The fuel gauge and OBC readouts were much more accurate after the cleaning. Give that a try.

My bad if i misunderstood your comment, I'll give what you say a try