Diesel Fuel Guage Problems

My '87 300SDL, which I just purchased about a month ago, had an erratic fuel gauge. When it got down to about 1/4 tank it would fall to zero and I mean zero, then bounce back and forth to what I assumed was the correct fuel level. I ran it way down once and the fuel light did not come on.

So, thanks to Tom Hansen, I got a new float unit and it was installed yesterday. The fuel light came on for the mechanic and that was good, but it registered only about 1/8 of a tank and was approximate 1/2 full. I filled it up tonight thinking you might have to fill the reservoir the first time???? But, nothing changed...it hangs at 1/8 tank.

Ideas?

Thanks for your help.

[MBenzNL]

I guess that the float unit is not floating that well.
I presume that the fuel gauge functioned well (till 1/4 of the filled tank) before the float unit was exchanged...

I think the float unit is stuck at 1/8 and does not float on top of the fuel at all! Maybe the tech made a mistake or just didn't install the float unit in the right way.

greetingz,

Thank you for responding.

Yes, it worked fine before until it reached 1/4 tank.

Maybe I should have him look at it again...not that hard.

['79 Super Turtle]

I'm not sure if this will apply to you but you may want to read this link to see how a fuel sender and gauge are tested. This was for a W123/W116.

Fuel Gauge Sending Unit

I am looking at the link now...thanks.

Earlier today, out of the blue, the guage went from the proverbial 1/8 reading to about 1/2 tank, which is where it was. Then I drove it down to the point where the fuel light did come on and filled the tank. It went back to just under half tank reading, then as I drove it about 20 more miles before stopping just now, it gradually moved up to 3/4 tank reading.

I am hoping that once it goes to the top, hopefully, the diesel will lubricate the float mechanism enough to allow it to function normally. Is this an unusual problem?

Thanks.

[MBenzNL]

No, that is not a normal problem. I would have the tech that changed the float unit to look at it again. It is definitely not functioning properly.

greetingz,

Thank you, kind sir. I talked to the technician and we agreed to let it be for the next two tanks and if it is still acting up, he will put another one in.

Again, thank you.