fuel gauge weirdness

I too have a fuel gauge issue. For my IM the gauge stays on full for a long time and then goes down to empty fairly fast. That makes it so you almost never know the actual fuel level unless you just filled up or are close to empty. I have been told “that is the way these gauges work”, that the problem isn’t the gauge or the sensor but rather results from the shape of the fuel tank, and that I should just record the odometer reading when I fill up and use it to judge how full the tank is. None of these are very satisfying.

_*cough* Use the trip odometer ^*cough*

I have a 110 liter (29 gallons) tank in my 911. Gauge is very accurate. The general sensing mechanism is simple and in every other car I have ever owned worked.  Even in the ‘52 Jag XK-120 where the fuel gauge after pushing a button also read the oil level so one could stop at a station and top up the 13 quarts of oil it took and rapidly burned.

Never understood why inaccurate fuel gauges in these cars seem accepted and most everyone suggests using the odometer.  Is there some fundamental difference in gauges, sensor, or shape of tank that makes accurate measurement challenging?

Resetting the trip odometer is simple but can be forgotten.

Everybody's got a white whale they're chasing. This one is as good as any other, I suppose. Goodness knows I've got my own.

Go get 'em.

@jncspyder It depends on exactly which fuel sender you have, but the old VDO senders that used a "foam" float had a coating on them that "goes away" and would slowly start to saturate over time and they became "waterlogged" with fuel.  When this happens they would float to the top slower and slower, until they just sunk to the bottom.  Likewise, I've seen the plastic floats do similar, spring a leak and slowly fill with fuel and become less buoyant, but that is much less common.

@ProfHollan your issue sounds different.  If you have a sender that is a float on an arm they are adjusted in 2 ways, they have a stop that is top and bottom for the float arm, and then the arm itself can be bent between it and the float to change both travel and the upper and lower limits.  If it is bent too far down it can be at its upper limits before reaching the top of the tank.  Let's say the arm stop was set to a proper full tank, but then the arm was bent to only allow the float to reach 1/2 way in the tank.  In this case ithe float would be somewhere int he middle of the tank when you filled up, and would read full, but it could not physically move down until the tank itself was 1/2 full and that is the point it would start dropping the needle giving you full range not he needle for only the last 1/2 of the tank.  make sense?

@chines1 definitely makes sense. At a second home in New Mexico but will have a look at the sensor when back in La Jolla. Not sure what type it is.  Thanks!

There are simply bigger fish to fry here. It isn't even close to a modern car. Great analogy, Stan, about the white whale.

Having said that, on my old car with VDO gauges it worked well enough. It read full when full, then started to go down. At 1/4 tank the needle would fluctuate wildly depending on "G" level of driving. Simply get gas at 1/4 tank and forget about it again.

The new car works like a new car. The gauges are step motors, with built in hysteresis to smooth the needle. It's nice.

Prof Hollan: I'm sure Gordon will see this and turn you on to one of his gauge-smoothing electronic gizmo/thingamabobs. They do help with the jumpy needle issue.

@jncspyder follow Carey's advice, you probably need a new sender, but it could be just a bad ground or hinky wire. After all, it is a Mr. Hyde-mobile.

When it bounces around 1/4 tank it's telling you it's time to get gas!

@ProfHollan  I don't think that my gauge dampening circuit would help this much.  It softens the swing of the gauge as the sender is moving up and down with the sloshing of the fuel.  If you have that issue (gauge needle swinging wildly about) then it might help a bit - Not totally cure it, but make the swings less.  

It sounds like you have a different problem and Carey has given you some places to start looking.  Remember that the sender is slightly downhill in the tank so make sure you have the tank 1/2 full or less before you remove the sender to check on it.  It  took me 3 or 4 iterations of removing the sender, bending the float arm a bit , re-assembling and road testing it before I got mine to where I like it.  That seems about normal.  It's a pain to do, especially because you have to see where the needle is with a full tank and later on when it's almost empty.  Mine reads a needle width higher than "4/4" when full and I have gotten down to 1/8 before filling but that's tempting fate if you're way out in the boondocks.

Good luck with this, and be patient.

@chines1 @DannyP thanks guys for the input ....your explanation makes perfect sense....i'll live with it until it obviously fails completely....it's definitely another FLOW CHART scenario....everyone needs to print this out....frame it...and hang it in their shop

My gauge got wonky like that last summer and it turned out to be a loose connection at the sender.