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Mystery: fuel gauge rises after starting the engine

On my early 90’s Corolla, when I turn the key to “on” but don’t start the engine, the fuel gauge will – for example – read 1/4 full. It will stay at 1/4 indefinitely as long as the engine isn’t running. Then when I start the engine and drive the car for a few minutes, I notice the gauge has gone up, so it will be reading 3/8 full within 5 minutes of driving. And I don’t have to be moving, once it gets to 3/8, it says at 3/8 whether moving or idling at a stoplight. It has always done this, even since the car was new. It’s not a problem, but I’ve always been curious what causes the fuel guaue to rise like this. Any ideas?

I would suggest simple movement. You have a float connected to a potentionmeter. Just moving the fuel around in the tank will on average yeild a higher reading than dead still on a flat. Likely the float does not always float exactly but sticks in a range until it settles down. Most gauges are not really like the speedometer. Things like fuel and temp are lazy. They kinda average. Fuel is the worst for this. I go up and down hills each day on my drive to work. I start up a hill the gauge goes down slightly each mile. I go down a hill the gauge again changes slowly higher. I get bigger changes on long gradual hills than steep ones due to time averageing or some other electronic issue.

The voltage level in the electrical system increases and that affects the fuel gauge. Key on and not running you’re looking at about 12 volts. Once running that same gauge is being provided about 13-14 volts, all depending. It’s all about different voltages being provided to a system where the resistance remains the same.

Some cars over the years (European mostly) used a voltage stabilizer on the instrument cluster which would maintain a steady, and lower, voltage supply to the fuel and temp gauges no matter what the system voltage was.

Your fuel gauge is actually a ammeter…On one side of the gauge, a hot wire (12V). on the other side, a wire runs to the gas tank sending unit where a float moves a variable resistor according to the fuel level. As the resistance changes (the float moving) the currant in the circuit changes and that change is registered on the gas gauge…When you start the engine, the voltage jumps to around 14 volts and this higher voltage also changes the amount of currant flowing through the circuit. Today, most cars have a little solid state voltage regulator that provides regulated power to the instrument cluster and prevents the changes you observe. Your car may have such a regulator, but it must have failed…

I offer two theories in combination:

  1. in order to prevent the gage needle from vibrating and wobbling as the gas sloshes around, the sender is mechanically “damped” through a section below and abovethe actual fluid level. That stabilizes the needle in use, but also prevents the needle from reaching its true reading unless the mechanism is vibrated (the fluid around it agitated).

  2. The fluid around the sender float becomes agitated when you drive and that manifests itself as (increased fluid volume - agitated fluid takes up more space} and enabling the sender float to reach its final, mechanically damped, height.

It could be that the regulator is supposed to put out - say - 10 V and with the car not running it only has 12V on its input. Lots of electronic regulators need a certain potential between in and out to maintain regulation. If they see less potential than specified, the regulator is said to be ‘starving’ and its output voltage is off by half an axle handle. The voltage it outputs could be way less than it needs to be when the car isn’t running so that’s reflected in the readout.
When you start the car, the input voltage goes up and the regulator has enough potential to maintain regulation, causing things to works fine.

Don’t look a gift horse in the mouth. Toyota engines run so economically that your engine is putting gasoline back in the tank.

LOL!

My VW’s fuel gauge also does this; has since new. I don’t know why it does that. It could be attributed to a bimetal voltage regulator for the gauge cluster supply but I tend to believe that it would have a solid state voltage regulator, if any, as the cluster has other electronic components such as LEDs, a digital clock and also can be found in the glow plug timer relay. The temp gauge does not behave similarly. The lag is not a problem; just wait for about a block or two and then read the fuel gauge.

All great comments, thanks. It remains a mystery, but now at least there’s some theories for the cause.