Throttle valve: How it works and its possible malfunctions

Throttle body
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The throttle valve controls the air supply to the engine. It is used by gasoline engines but also by some modern diesel engines. The throttle valve is located in the intake system and is mechanically or electronically connected to the gas pedal.

When the gas pedal is pressed, the throttle valve opens. In the case of a mechanical control system, the damper reacts immediately. In contrast, the electronic control is the responsibility of the ECU (Engine Control Unit), which controls the throttle valve based on the position of the gas pedal.


If your car is equipped with this device, it's good to know more about it, as it may be the main reason the EPC warning light appears on your dashboard.

Throttle valve in the case of gasoline engines

As said above, the throttle valve controls the air supply to the engine and opens when the gas pedal is pressed. It can be controlled in two ways:

  • Mechanically

  • Electronically

Mechanically controlled throttle body

The throttle is controlled by a cable from the gas pedal, which is connected to the moving mechanism of the throttle body. The mechanical version of the throttle valve control is more efficient because the response to the throttle is instant.

Electronically controlled throttle body

The signal from the electric accelerator is processed by the engine control unit, which tells the flap how many degrees to turn. The more the gas pedal is pressed, the more the throttle valve opens.

However, if the gas pedal is not pressed, the throttle idle position is activated, and the engine control unit maintains the engine at idle speed.

Replacing the throttle valve with variable valve timing

BMW Vanos

In some engines, the throttle valve is replaced by a variable valve timing system, such as BMW Valvetronic, Honda i-VTEC, Toyota VVTC-i, and others.

Replacing the throttle valve with a variable valve timing system has several advantages, such as reduced intake manifold flow resistance with less engine load, lower fuel consumption and more engine power.

The throttle valve of the petrol engine consists of the following:

  • rotating plate controlled mechanically or electronically

  • a potentiometer that informs the control unit about the flap position

  • idle speed switch

  • stepper motor (only with an electronically controlled damper)

  • a spring-loaded mechanism that resets the throttle valve to its original position after disconnection

The throttle valve is not a problem-free component, so problems and malfunctions occasionally occur. The most common problem is its clogging with carbon and various impurities, or it can be a malfunction of the throttle electronics.

Throttle body

A throttle valve malfunction is manifested as follows:

  • irregular engine run

  • fluctuation of revs

  • engine shutdown

  • speed fluctuation

  • problematic engine starts

  • illuminated engine warning light

It is possible to remove the malfunction by disassembling and then cleaning the throttle body using a suitable cleaning agent or replacing it altogether. However, you can even clean the throttle body without disassembly by applying a cleaning agent to the throttle body with the engine turned on.

How to prevent problems with the throttle valve?

An effective way to extend the life of the throttle valve is its regular maintenance. In addition to a defect in the electronics of this part, it can also become clogged with dirt - in this case, regular cleaning of the throttle body helps.

Throttle body

If cleaning does not help, the throttle valve needs to be replaced. You can have it replaced at your service center, but if you are one of the more experienced and daring drivers, you can order or buy the throttle and perform the replacement yourself.

Throttle valve in the case of diesel engines

In diesel engines, the throttle valve is mainly used for a gentler engine shutdown. Before turning off the engine, the valve closes and thus prevents air from entering the cylinders, which causes the engine to not jerk after turning off.

Throttle valve malfunctions are manifested in diesel engines:

  • the engine light warning light is on

  • self-ignition after turning off the engine

You can apply the same rule of thumb for troubleshooting a diesel engine throttle body as for a gasoline engine throttle body. The same applies to cleaning the part or throttle body replacement.

Check out a video by Engineering Explained of how the throttle body works: