What is a throttle valve?

Located in the carburetor, the throttle valve of a gasoline engine regulates the intake of fresh air. This allows fuel and air to form a combustible mixture that powers the engine.

In a carburetor gasoline engine, the throttle valve is located in the carburetor. In fuel injection engines, the throttle valve is located in a separate housing called the throttle body. Throttle valves are simple mechanical components consisting of a round sheet metal disc mounted on a rotating shaft. Depending on the type of engine, a central throttle valve prepares the mixture for all cylinders. Each cylinder has its own throttle valve. On older engines, the throttle is controlled mechanically by a cable or linkage. In modern engines, the throttle is controlled electromechanically by the engine ECU, depending on the position of the accelerator pedal and a number of other parameters.

How does the throttle work?

When the throttle is open, air enters the combustion chamber unimpeded. It is drawn in by the pistons through the intake valves. In addition to fuel, atmospheric oxygen is also important to the combustion process. The fuel itself is injected into the cylinder. When the piston moves back up, the air-fuel mixture is first compressed and then ignited by the spark plug.

The throttle valve controls the amount of fuel burned with each stroke of the piston, ensuring that combustion is as efficient as possible. When the throttle is closed, no air or oxygen enters the engine and the combustion process is interrupted. Pressure on the accelerator controls all throttle valves, whether electric or electronic.

Causes and symptoms of throttle valve failure

Because of their protected location in the engine intake system, it is virtually impossible for throttle valves to suffer sudden damage, such as mechanical stress. Damage usually occurs over time. Possible causes include deposits on the edge of the throttle plate, dirt in the intake passage, and wear on the throttle shaft or shaft bearings. All of these causes can prevent the throttle from sealing the intake port properly.

Symptoms of throttle valve failure include:

• Variations in idle speed
• A slow, delayed drop in engine speed when the accelerator is released
• Reduced engine power and increased fuel consumption
• Jerky driving
• Jerky acceleration

The different types of throttle valve

Installed in the engine's intake manifold, the throttle valve in older vehicles was operated by the accelerator pedal using a cable. In more modern vehicles, there are several ways to operate a throttle;

Electric throttle valve

On electric throttles, the throttle cable mechanically controls the throttle position. An electrical signal indicates the current throttle position to the engine control unit, and the engine management system processes the information and compares it with data from other sensors. The ECU continuously monitors the throttle position and adjusts it using an actuator to optimize fuel consumption and exhaust emissions.

Electronic throttle valve

An electronic throttle is no longer mechanically linked to the accelerator pedal. When the engine speed needs to be increased, the electronic throttle receives the signal and transmits it to the engine management system. The engine management system calculates the optimum throttle position and controls it using a servo motor.

Cleaning and repair

A visual inspection of the throttle valve is required to determine if repair is necessary or if cleaning is sufficient. This does not necessarily require disassembly of the throttle. On older carburetors, the air filter and air filter housing must be removed. The throttle valve will then be visible inside the carburetor.

On modern fuel injected engines, the throttle body and actuator must be removed and opened. The condition of the throttle can then be checked. If the throttle valve is clogged or has deposits, it's usually sufficient to clean it with a special cleaning agent. If the throttle shaft or bearings are lapped or the throttle is otherwise damaged, the throttle, including shaft and bearings, should be replaced.