Fuel consumption too high: Reasons for too high consumption

The fuel consumption of a combustion engine depends on many factors. It can sometimes be higher and sometimes lower without there being a defect. Loaded or empty, short or long distances, summer or winter, all of this affects fuel consumption. This applies to both petrol and diesel engines, even if the absolute fuel consumption of petrol engines is subject to greater fluctuations than that of diesel engines. After all, the driver has the matter in his own hands, or rather in his feet. Because driving style is one of the most important factors when it comes to the fuel consumption of the car. This is also where corrective action can be taken most quickly in order to reduce consumption.

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It is different if there is a defect as the cause of increased fuel consumption. The search can be very difficult. Incidentally, it is not possible to determine from when a defect is actually responsible for increased fuel consumption based on the level of additional consumption. The consumption can deviate by only a few percent and is therefore hardly noticeable, but it can also be twice as high as usual. It usually becomes noticeable when nothing changes in the usual parameters. The same route, the same volume of traffic, the same driver and the same driving style – and yet consumption increases. If that is the case, you should get to the bottom of the cause.

Calculate fuel consumption: the formula

In order to determine whether your car is actually consuming too much fuel, it is necessary to determine the current consumption. A first clue – if available – is the consumption display of the on-board computer. Deviations from the actual consumption often occur here. Therefore, you should measure the fuel consumption yourself. This is the most accurate method and also very simple.

How to measure your car’s fuel consumption:

• Fill the vehicle up to the brim. A little patience is required. By shaking the car, the last air bubbles escape and the tank is really full to the brim.

• Then “zero” the trip meter and drive the vehicle as usual. The route should not be too short (at least 100 kilometers).

• Fill up your vehicle to the brim again at the same gas station and pump.

• To calculate the consumption, the amount of fuel consumed in liters is multiplied by 100 and divided by the kilometers driven. The result is the average consumption per 100 kilometers.

Example: 38.27 liters x 100 : 523 kilometers = 7.32 liters per 100 km

You can check that yourself

Before you take your car to the workshop, you can check a number of things that could be the cause of increased consumption. This includes checking the tire pressure. If it is significantly lower than specified, possibly with several tires, the search for the cause is quickly completed. Up to 30 percent more fuel consumption is possible if the air pressure is too low. Tip: Check tire pressure regularly. In addition, the tires can be pumped up more, a maximum of 0.5 bar above the factory specification. The resulting reduced rolling resistance lowers fuel consumption. (All information about saving fuel)
Measurement of tire pressure

If the tire pressure is too low, fuel consumption increases by up to 30 percent. It is better to stay slightly above the factory specification when it comes to air pressure, this saves fuel.

Air resistance plays an equally important role. This can be seen, for example, in the use of a roof box, which, however, is rarely left in the car and punctually ends up back at the rental company after the skiing holiday. The situation is different with loose paneling parts that have come loose under the vehicle or in the wheel arches – which is difficult to see. The loose parts can significantly worsen the air resistance, especially when driving on country roads and motorways, and thus increase consumption. The condition of the air filter can usually be checked more quickly. If it is dirty, the reduced air flow makes the work of the engine more difficult. The optimal fuel mixture can hardly be produced, which drives up the thirst for fuel. (Tyre pressure: find the right value)
Big consumption test

Fully loaded plus poor aerodynamics – this can increase consumption by over 50 percent.

Mechanical causes for increased fuel consumption

In general, every bearing that is responsible for the freewheeling of engine and transmission shafts can contribute to increased consumption due to increased resistance. However, if the resistance is so strong that it has a noticeable effect on consumption, the defect has usually already been announced by a significantly changed background noise (grinding and grinding noises, depending on the gear selection and speed).


Defective brake pistons can cause a stuck brake. The damage must be repaired immediately in a workshop.

The same applies to the wheel bearings of the wheels. Here, the noises are initially only announced when the defective side is loaded (most recognizable when cornering at idle). It is different with “hanging” brakes, which often do not make themselves felt so quickly. However, the free movement of the brakes can be checked by allowing the car to coast from a low speed. If the car stops jerkily, this is an indication of a stuck brake piston. It may be possible to determine which wheel the brake is stuck on by feeling the rim temperature. The wheel with the stuck brake is often noticeably warmer than the other rims. Turning the wheel on the lifting platform brings final certainty. With disc brakes, the brake pad that is constantly in contact may be visible. In this case, the vehicle should be taken to a workshop immediately.

Clogged injectors increase fuel consumption

The injectors can also be responsible for increased consumption. An engine has one injection valve for each cylinder; diesel engines are referred to as injectors. The injection nozzles atomize a very fine fuel mist either in the intake tract (port injection) or directly in the combustion chamber of the cylinder (direct injection). Over the years, the nozzles get dirty, which means that combustion is no longer optimal. As a result, the engine lacks power, while consumption increases at the same time. In many cases the addition of special additives already helps. However, before using it, owners should clarify with the car manufacturer whether the additive has also been approved for the vehicle. In the case of stubborn deposits, it is advisable to remove the injection nozzles and have them professionally cleaned. (Injectors: defects, symptoms, cleaning)

Broken ancillaries cost power

Defective ancillaries can also be responsible for increased fuel consumption, such as a defective air conditioning compressor. When switched on, the compressor is set in motion by means of a magnetic clutch via the V-belt and from then on provides increased resistance for the engine. It becomes correspondingly thirsty and, depending on the design and size, uses about 0.5 to 2.5 liters more fuel per 100 kilometers thanks to the air conditioning. If the magnetic coupling is defective, the compressor may no longer be able to be switched off, so the increased resistance is permanently maintained.

The operation of stiff ancillary units such as the air conditioning compressor or the alternator (image) costs the engine additional power.

The same applies to modern alternators, which only work when overrun and are decoupled under load. If the decoupling does not work, consumption also increases here. Even if the solenoid valve has not suffered a misfire, a broken bearing or a defective belt pulley can make it difficult for the units to run and increase the engine’s working resistance. Corresponding damage can usually be recognized by noises when the units are running. (Cars really consume this: manufacturer data vs. test consumption)

Sensors determine the injection quantity and timing

The electronic engine control is another possible source of error that should be considered in the event of unusually high consumption. In most cases, it is not the control unit itself that is defective, but one of the many sensors that supply the control unit with information. The most important of them: mass air flow sensor, crankshaft sensor, lambda sensor, camshaft sensor, throttle valve sensor, knock sensor, coolant temperature sensor, TDC sensor and the air temperature sensor. From the data provided by these sensors, the engine control unit calculates, among other things, the ignition point and the quantity and timing of the fuel injection.
If the information from one or more sensors is incorrect, fuel consumption can increase – for example if too much fuel is injected or at the wrong time. If the sensors fail, the engine usually falls into the emergency mode (engine control light comes on). Here, too, the combustion process is not optimal, which also increases consumption. In many cases, a defective sensor can be read from the error memory of the engine control unit. However, this is not always the case. It can happen that the workshop has to replace sensors on suspicion. In the picture gallery we show other possible causes that can be responsible for increased fuel consumption!

picture gallery


Fuel consumption too high: the most important reasons

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