jetta fuel gauge reads lower than it should (and intro)

[VTPSD]

Hi All, first post here after waiting for a long time to get approved. I am not new to the audi/vw scene, but this is my first diesel vw. I recently started a new job that required me to get another commuter and I really wanted an IDI diesel vw. I ended up with this 91 jetta ecodiesel. Its in real nice shape for the northeast. I am loving the car and the economy. Having fun tightening it all back up and adding some minor mods.

I have been working through most of its issues, but there is one nagging one that I don't know where to start. The fuel gauge seems to read low all the time. It basically goes to full when I fill up, but after I have traveled about 250-300 miles its getting near empty. I fill it back up and it only takes 6 or 7 gallons. According to my bentley the tank should be 14.5 gallons. I am wondering if there is a scaling adjustment somewhere? The gauge moves smoothly but just seems to read low. I wouldn't be to concerned but I am getting the car ready for a cross country trip in june and want to have an accurate gauge. Don't want to run out of diesel 

here is the car in question


here is my favorite mod so far, got rid of the horrible AC pump/alternator belt system and used a pulley off an old audi 80 to run a single belt.


here is one of my favorite audis from my past, its a 1990 audi 90, completely restored by me, big holset turbo from a dodge cummins, homemade manifolds, Porsche brakes, rs2 front end etc etc etc. Ran 11 second quarter miles, it was a beat. It will be missed, but I sold it and used the money to build a nice 24x32 shop 

[damac]

I have never messed with the orientation of the sender and its effects on the reading?  But I guess you should atleast check that.  Book says to line up the arrow on the unit to the tank for a reason I guess before tightning down the retainer ring.  Pop the carpet out of the trunk and take the round access cover off to reach it.

I never had issues like this on any of the vw's I have had for whatever reason so I'm no help.

[ORCoaster]

PO may have had some idea to adjust it and now it really needs to be taken out and tested with a ohm meter and for full travel of the range.  Maybe it just needs a little bit of bending to make it come down sooner.  Beware little adjustments on the position of the float will be reflected by big changes on the gauge itself.  Don't get heavy handed on this.  It comes out easy enough you can adjust often.

[VTPSD]

sounds like I will just need to pull the sender and have a look, maybe tweak it just a bit. It appears to line up correctly with the tank per the Bentley. I am correct in thinking this car has a 14.5 gal tank right? I know my mileage has been right around 43, so I can be pretty sure of my remaining fuel that way as a sanity check.

[ORCoaster]

Yep can always do the math in your head as you drive or keep a sheet numbered from 1 to 14 with amount of miles driven next to it based on average or worst MPG you have had in the past.

[damac]

I can't say my 85 swung that dramatic.

But every time I put one of these cars on the road I like to drive around for a few weeks with a couple jugs of fuel and do my own tests so I have my own reference.

The 79 rabbit I am driving right now is getting 44 mpg all the time, which pisses me off because its the same as my 85.  But I am driving lik 90% freeway at 75-85mph so not sure if it would get better driving the other route I am used to with 75% at 70mph.

Anyways I have now run that sucker out of fuel a few times.  Very interesting to me just how sudden it has been for me on level ground.  Couple chugs then dead without being able to  restart

The marks on the dash don't seem to be relative either.   Takes a while to register up top after a full tank. Looks like I can go 80 miles from the first mark past red zone all the way to empty.  So now I tell myself 40 miles to the next gas stop if it register anything under that first mark

And I can't remember what the manual to my 85 said but I'm sure its bigger than mk1 jetta which is 10.5 gallons.

[8v-of-fury]

55L in the mk2's.

[VTPSD]

well I pushed it yesterday a bit farther, made it up to 450 miles before refilling. The gauge was basically at empty, but I only put 10 gal and change back in...Its strange, but after this fill up it seems to be more accurate, went 100 miles on the first 1/4 tank, and the last tank it was reading 3/4 after maybe 30 miles. The car did sit for quite a while before I got it. I don't know if its feasible that the wipers on the sender were cruddy from sitting and now that the tank has been filled a few times its starting to clean off and reduce the resistance? I think I am going to leave well enough alone, I know my mileage is pretty consistently around 41-43 and I have a 55L tank. Its a bit unnerving driving another 100 miles after the gauge gets into the red, but I think it will be ok.  Silly old German cars, they always have to have some strange quirks. Suppose that is part of the charm.
Love this old car, cant wait to get my new control arm bushings in it so I can stay in the center of the road instead of drifting all over the place!
Thanks for the help guys.

[fatmobile]

How's accurately is your engine temp gauge working?

[shwak23]

My car does this same thing actually. I pulled the sender and swung it to full and the gauge displayed 3/4 of a tank.

Sent from Outer Space!

[VTPSD]

How's accurately is your engine temp gauge working?

seems to be accurate as far as I can tell. Runs right in the middle on the highway, one notch up from the middle at the top of the 4000foot climb that it does every day.

[wolf_walker]

It isn't exactly common but a fuel tank can be sucked in by vacuum, or bad jack placement (E30 lol) or such.
I can't even remember where the tank is on a second gen Jetta it's been so long but it's worth checking.

I'd pull the sender and see what it reads where, and clean it.  Yes diesel will gunk em up, especially
if a previous owner never let it get low or such. 

[bbob203]

That alt bracket is nice!!