Are fuel gauges accurate?

Many car enthusiasts complain about the inaccurate level of the fuel level by the fuel gauge sensor in the tank. This is partially true because the fuel gauge is not that reliable in showing the true amount of fuel left in the car’s tank, because of its type, and also due to the problems that it can have.

In this article we will explain in depth what the fuel gauge is, and what problem it can have.

What is a fuel gauge sensor?

A car fuel gauge (FLS) is a sensor that measures the amount of fuel. It is installed directly in the tank and works in conjunction with the fuel gauge located on the car’s dashboard. This sensor works with a digital or analog signal. Fuel gauge sensors based on analog signals are not that accurate and because of that, they are rarely used.

Digital fuel gauge sensors are more accurate because they can turn the signal into digital information, thus measurement errors are corrected, some of which are caused by the tank circumference and uneven fuel distribution. The most important stage in their work is the stage of physical measurement.

Types of fuel gauge sensors and their accuracy

Float fuel gauge

Also called a potentiometric sensor. It is the standard sensor installed by the car manufacturer in the fuel tank. The float and potentiometer are connected to each other. When the fuel level in the tank decreases or increases, the location of the float will also change. 

Since it is connected to a potentiometer, the resistance of the resistor and the voltage coming out of the FLS will change. Information from the sensor will show in the dashboard using a separate wire or an analog signal on old cars and on newer models, you will see the remaining fuel thanks to the CAN bus or a digital signal.

The accuracy of the float fuel gauge sensor can reach errors between 10% and 30%

The ultrasonic fuel gauge

This type of fuel measuring sensor is considered to be highly accurate, having an error rate of just 1-2 %. It is installed outside the fuel tank in its center at the bottom creating an ultrasound pulse that enters through the bottom and fuel, is then returned to the emitter. The time between sending and returning the signal serves as the basis for calculating the height of the fuel column in the tank.

Although is accurate, the ultrasonic fuel gauge has some disadvantages, the first one is its installation which is extremely difficult. The sensor is attached to the fuel tank with a special glue or fixed with a metal tape wrapped around the tank. It is necessary to achieve its perfect fit to the surface. If the contact is not very tight, the ultrasound waves are distorted, which leads to the transmission of incorrect data.

If the fuel tank is uneven, this will also lead to incorrect data regarding the fuel level. Also, if foreign objects are in the tank, like dirt and water this will interfere with the ultrasonic signal and will indicate fluctuations of the fuel level.

Experts recommend using ultrasonic FLS only in cars that are equipped with LPG installations.

Capacitive fuel gauge

This type of sensor has an accuracy of 98%. It functions on the principle of an electric capacitor, in which the measuring part is represented by two non-touching tubes – one inside the other. The tubes serve as capacitor plates, and from above are attached to the FLS board.

An electric current is transmitted to the tubes, and gasoline, diesel or any other fuel serves as dielectrics for the electric capacitor. Since the surfaces of the plates do not touch each other, the gap between them is filled with fuel when the sensor is lowered into the tank. This indicates that the fuel level in the tubes and tank is identical.

As the tubes are filled with fuel, the capacitor’s electric capacity changes. For example, given that gasoline has a lower electrical resistance than air, it is obvious that the charging period of the device will be reduced. The more fuel in the tank, the sooner the capacitor is charged.

Bad symptoms and problems of the fuel gauge sensor

Because this sensor runs in rough conditions, at least chemically, its parts are subjected to failure in time. You will see the following symptoms:

  • After starting the car, the fuel level indicator does not move. It is worth checking the connecting wires for oxidation, and the fuses.
  • The sensor readings are constantly fluctuating. This can happen because of worn-out contact tracks due to the constant movement of the slider;
  • The indicator periodically shows zero, although the fuel tank is full. The most common reason is that the float stop was not correctly positioned. And if the arrow often points well to a zero value and twitches, you need to check the current collector, it often touches the resistor too weakly. It also makes sense to check the resistor winding;
  • The low flammable material warning light has stopped working. In this case, it is worth replacing the resistor;

Most of the problems regarding the fuel gauge lie in the rapid wear of the tracks, less often the wiring is worn out. In this case, the readings of the devices are either not accurate, or are not displayed at all on the car’s dashboard. In this case, the wires have been damaged.

Fuel level can be measured incorrectly not only due to wear of the magnetic and current-carrying parts. Here’s what can also affect the sensor’s performance:

There is a hole in the float, which is why it is not properly sinked in the fuel;

A deformed fuel tank which interferes with the float movement;

If after checking the fuel gauge and its wiring, you don’t find any problems you will need to check the fuel tank. Although problems can be rare, there is a slight possibility that the fuel tank is damaged.

How to check the fuel gauge

If the sensor is working, but the indicator is fluctuating between 0 or E and 1 or F (low to high), first you need to adjust it. Here’s how:

The float pin moves to extreme positions alternately, at the same time the indicator should follow to the corresponding positions, 0 for an empty tank and 1 for a full tank. If there is no correspondence between the position of the pin and the arrow, the pin itself must be bent and the adjusting continued.

Next, you need to remove the panel located in the car’s dashboard, detach the wires, remove the indicator, and mount the wires back. Start the car and let the engine idle for about 10 minutes and then adjust the indicator axis to the 1 position and put the indicator back.

When the sensor often is at 0 (E), you will again have to disassemble the dashboard, but this time in order to clean up all possible places of oxidation. You will also need to check all the masses on the body.

Get a multimeter. The fuel gauge indicator has a resistance of about 7 Ohms when the tank is full, and when it’s half-filled, the rage is 108-128 Ohms. When the fuel tank is simple, the resistance fluctuates between 315 and 345 Ohms. Using a multimeter you just need to measure its resistance as you fill up with fuel.

If you decide to dismantle the device, the float will have to be moved by hand. If changing the position of the float does not affect the resistance, the sensor is definitely faulty and will have to be replaced.

Conclusion

The fuel gauge is an important car part, without it, drivers will definitely be much more uncomfortable. Like any other sensors, if you suspect or see that the sensor is inaccurate, that is more likely because of its problems and bad symptoms, but, in general, they are rare. The good news is that you can change the sensor or repair it.

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