Why is my fuel gauge malfunctioning?

What you need to know:

  • I have a ‘previously enjoyed’ 10-year-old Toyota Hilux from UK (an Invincible). Recently, the car came to a stop yet the fuel gauge indicated that the tank was an eighth full. Since then, I have had to track the mileage every time I fill the tank to avoid repeat embarrassment. A friend who has the same model had a similar experience. What might explain the fuel gauge malfunction and how can this problem be addressed? Ndwiga

Almost all fuel gauges are electric and only read when the ignition is on, but you cannot put an electronic gadget inside a tank of explosive fuel. So, the electrics bit is mounted outside the tank and gets its information through mechanical linkage to a “float arm” inside the tank (a bit like the ball cock in a toilet cistern).

As its name suggests, this arm floats on the surface of the fuel inside and goes up and down as fuel is filled and used. This moves the linkage in the external electric gadgets (there are several different designs), and clever electronic tech “reads” that movement and translates it to the needle on your instrument panel.

So, there are two main reasons why you might get a false reading. Either the electric gadget is faulty (unlikely) or the float is jamming (quite likely), so it reports the decline in fuel level to a certain point and then stops moving although the fuel level continues to fall. From your description, that is the first thing I would check. It is a reasonably quick fix.

Meanwhile, I would advise all motorists to always zero their trip buttons on the odometre when they fill up their fuel tanks, and always do the simple arithmetic of how much fuel has been added and what mileage has been covered to know the regular kilometres per litre figure.

That not only gives you a good idea of how much fuel you probably have left if the fuel gauge fails, but also gives you a regular report on your fuel consumption rate. If the car suddenly gets much more thirsty than usual, there might be a leak somewhere or your engine might have a problem. The simple ‘zero trip’ habit will give you that useful information with no trouble and at no cost.

As a post-script, you may have noticed that the fuel gauge needle changes a bit when you drive up or down a steep hill, and even sometimes, when you corner or traverse a slope.

That means the float is mounted somewhere other than the dead centre of the fuel tank and so, moves as the fuel sloshes from back to front or side to side. Some tanks have buffers to reduce this. The most accurate reading will be given when the car is stationary on a flat surface.