Bought this car used and am really only starting to drive it more frequently so I don't know what my expected mpg should actually be. I've filled my car up and drove it approx 250 miles. The fuel indicator needle has gone all the way to near empty but no light. I'm aware that the light not working might be due to the resistance wire on the sending unit. Perhaps it is gunked up?? Regardless, after I fill my tank at the station it looks as if I've only consumed 10 gallons vs. the 17 or so. I know that the sender unit malfunction is due to build up resulting in a bouncing needle, but does my scenario seem like it could also be the culprit? Are one of wires not getting the proper resistance fooling the sender into thinking the tank is near empty when it is not?

Sorry that was so wordy.. please help. lol

__________________
1985 300d Turbo Diesel

Assuming a W123 300D it is most likely gunk in the sender.

This might help.

PeachPartsWiki: Fuel Level Sending Unit replacement

And more pictures of the internals here

W123 300D Fuel tank sender question

__________________
1992 W201 190E 1.8 171,000 km - Daily driver
1981 W123 300D ~ 100,000 miles / 160,000 km - project car stripped to the bone
1965 Land Rover Series 2a Station Wagon CIS recovery therapy!
1961 Volvo PV544 Bare metal rat rod-ish thing

I'm here to chat about cars and to help others - I'm not here "to always be right" like an internet warrior



Don't leave that there - I'll take it to bits!

Sending unit is a good first step. Just be careful in disassembling/reassembling.

MPG in a new-to-you vehicle with unknown tune and no specifics on driving habits should be low- to mid- twenties. Lots of variables there, to include iffy odometers and non OE tire sizes.

Pump shutoff with a less than full tank might be a clogged vent line, if that's what you mean by 10 vs. 17 gallons.

You should get a low-fuel test light in Position I on your cluster in an '85. This was implemented mid-W123 build, so old cars (or, more specifically, old instrument clusters) didn't ever see the light until fuel was really low.

Until you know the pecularities of your fuel system (gauge, light, strainer) and your odometer I'd err on the side of caution and not try to run it down, unless you've got a fuel can in the trunk.