Whenever I'm traveling up or down a hill, accelerating/decelerating, my fuel level loves to bounce around. I'll be sitting at a half a tank and the next thing I know my gauge drops to almost empty or maybe its the exact opposite.
This has bugged me for years and with all the work that has gone into the car this one small thing has kept me from being completely satisfied with what I've done or had done to the car.
On a level road it's normal, it only acts up where the tank is bouncing around.
I'm thinking it's a broken sender?
Anyone know what might be causing these symptoms?

 

gas tank baffle.

 

I'm sorry I don't understand.
Are you saying add some or are mine gone or something?

 

Oooohhhhhhhhh, I see what you're saying.
If they are broken how would I get them reattached?

 

sorry i was on my fone when i wrote that leaving work. i believe it is welded in place, so a new tank might be in order for you. MAYBE try spot welding it back on BUT you gotta have ***** to do that!

 

Alright, that's not what I wanted to hear. I'll see what I can do.

 

How quickly does it register these changes in movement? Some movement is normal for our
gauges as you change the cars orientation or G load.

If your seeing immediate changes, like less than a few seconds and its reading full or empty
I would suspect the sender is botched in some way. Its very rare for the baffles to come
loose.

 

It's close to immediate.
I can take a video if necessary so you can see what I'm dealing with.

 

Not a bad idea. Video might show it the best. Don't crash while producing it

 

If the gauge always 'drops' from real level, rather than reading artifically higher, odds are there's a loose connection in the circuit. Anything that interrupts the flow of current through that circuit will cause the reading to drop.

Could be the sender (more likely - they do wear out from endlessly moving), could just be a loose connector at the sender or at the back of the gauge.

 

Well here's the video.
Sorry for it being a tad bit shaky, old shocks and construction around town don't help.
Looking at the beginning to 1:07 pretty much shows what I deal with regularly.
But even from the beginning to end, I have more fuel at the end.

 

So in that video how full was your tank? Pretty much half or a little less?

From what your showing I unless you have a full tank, the movement your seeing is absolutely
normal. Mine does the same thing and my guage works fine. In fact my guage has done this
since I got the car in 1983.

 

Yeah the tank was about half.

This is what I normally see happen.
So, whenever my fuel level drops a significant amount it's most likely due to the sender getting stuck right?

My dad kept saying the same thing, when he bought it new it was like this.
Well at least I got some confirmation.

 

Dude, that's your clock - it is supposed to go round and round.

Love the driving gloves. Serious business.

 

Thanks. I picked them up off of Bavarian Autosport for 50$.
Really helps when the inside of your car is 103 degrees.

 

Looks like pretty normal fluctuation based on terrain to me; I live at the top of a fairly long and steep hill, and with a half-tank of gas I will often see a difference of 15% or more between the driving downhill and driving uphill indications. And my tank and sender are both in fine shape. Can change notably based on cabin temperature, too.

The guage system in these cars is pretty archaic; the sender is a wire-wound variable resistor and the guage is actually a bimetal thermometer with a heating element wound around one leg.

The resistance in the sender varies based on the position of the float arm, which varies the amount of current (not voltage) that flows through the heating element in the guage, which makes the bimetal bend and moves the needle. The time lag involved in heating and cooling the bimetal (the thermal inertia) damps down the fast changes in resistance due to the constant sloshing in the tank, but it still moves within seconds of a consistent change in float level.

Typical design for cars of this era, and pretty inaccurate.

Sender:


Gauge internals:




Modern cars still use the variable-resistance sender & float in some form, usually with a carbon-compound rather than wire-wound resistor, but the output goes into the computer and it in turn feeds signal to a stepper motor that moves the needle. The computer is much better at damping down short-term variations.

 

That's interesting to see. Thanks for the pics.