Bit of a dumb question, but is it normal for the fuel needle to drop 1/4 tank worth while driving? I'd imagine there would be some sloshing but is it that much? Is this indicative of anything major?

 

That is weird. The gauge works with heating wire wrapped on a bimetallic strip. As the resistance changes on the fuel sender, the power flowing to the slightly resistive wire affects the temperature on the strip and moves the needle.

Because of this, the fluctuation can't occur frequently enough to witness swinging just during a glance. I suppose it's possible that the gauge itself is loose internally.

I've seen mine change over time when waiting at a light uphill or downhill but not for high frequency sloshing while driving.

 

The fuel sender in the tank is a float so sloshing will change the reading. Right turns will make the gauge level go up and vice versa.
Mine doesn't fluctuate by a quarter tank though, more like about an eighth. This is on an '82 dash gauge.

Electrical contacts could also be worn causing erratic readings.

 

It's not a rapid fluctuation as far as I can tell. More so just having 3/4 when parked, and then noticing it says I have 1/2 a tank while driving around. I guess its just sloshing or electrical gremlins then?

 

Do you park at an angle or perfectly level? If the latter, that is odd. If you do feel like it, you could remove the sensor and run it through its motion while connected to a resistance meter, it should read linearly from about 10 ohms to about 110 ohms.

In this thread I ended up having to calibrate the gauge itself but there's a lot of good information on the sender etc. maybe the first thing to do would be to make sure you have good grounds.

https://www.rx7club.com/1st-generati...-good-1106492/

 

The fuel sender is at the front extreme left side of the tank. The level reading changes a lot while driving.

New Mazdas only read the fuel level when the car is first started, and then they move the needle based on how much fuel the injectors have injected, probably so the needle doesn't go up and down while driving.

 

If it only is dropping while driving and then recovers stopped then check for loose connections or bad ground at the sender or on the back of the dash. Vibration might be causing a loose connection that resolves while stopped. The other thing is on older dashes the screw and bolted connections on the back become corroded and may need to be cleaned/tightened. Older fuel senders also get high resistance spots in them that cause the reading to drop. Only other item on old dashes is there is a voltage regulator (7 volts on many models) on the back of the dash that may be going bad.