Fluctuating Fuel Gage/fuel injection
 

Is there a reason my fuel gage seems to fluctuate so much? The last quarter tank is the worst...although my computer might read 130 miles remaining in range my needle stays largely near the red. I can watch it fall when i accelerate on to the freeway. Is this related to my fuel injection, or my acceleration lag? Any insight greatly appreciated...

on early models they used an analog fuel sender and it registers faster as the float (in the tank) moves. BTW, the "range" feature is an estimate/average - it's not "gospel"

Yeah the range is just the comps. best guess at how many miles you can get with the gas you have left.

How is it fluctuating? Does it rapidly move around?

yeah i take the "range" with a grain of salt...but i also felt i wasn't getting the gas mileage i should be. time to run a full tank all the way through to get a better idea of my mpg. thank you for the response.

when i accelaerate i can watch the needle move, especially in the last quarter tank. but my gas light has only come on once...and that after driving in the red for a while. i know it works though. i might be near the red with what i think is 100 miles left in my tank. it is not a rapid movement though. i just feel like i have more fuel than my gauge reads.

the float in the tank get damaged when things like fuel pump has been repaired or it's been rearended - and even with age they don't travel "freely" they start "gumming" up.

doesn't do it with my E36 but I have owned quite a few cars that as the tank of fuel gets lower the more the gauge fluctuates it seems to depend on the type of device used to measure the amount of fuel in the tank and were it is positioned in the tank, you state that you can see the gauge move as you accelerate, it could be as you accelerate the remaining fuel is pushed to the rear of the fuel tank causing your gauge to show a low reading until the fuel in your tank settles once again.
I hope thats not too confusing.

Tap on the instrument cluster. Does the needle fluctuate?
The contacts are notorious for poor contact with age.

Pull the Instrument cluster.
CAREFULLY.
Breakdown the cluster and clean the contacts. Wd 40 or contact cleaner is all you need. Plactic safe cleaners.
Replace all the bulbs while you are at it.

Wear a wrist strap for static.

This can be a cheap fix!

John

BTW

I have to say that I have never seen a fuel sending unit in the E30 go bad. Lift pumps yes.
The sending unit is a simple and reliable design. A float moves a wiper on a potentiometer which translates to the needle swing on the guage. At approx 1.5 gal US a switch closes and illuminates the low warning light.
The OBC is pretty accurate if it is getting reliable inputs. It does not guess.

Well hefe is a first for you, 1990 325ic and the sending unit went, gauge all over the place, replaced sending unit and solved the problem. As you said, and easy fix.