General Motors TBI
The fuel system is under pressure even when the engine is off. Be sure to release the pressure before disconnecting any fuel line, or working on the Throttle Body. You can do this by removing the fuel pump fuse at the fuse block. That disables the pump. Run the engine until is dies for lack of fuel.
Gasoline burns best when mixed with air in a ratio of roughly 14.7 parts air to1 part gasoline. Thirty years ago that mixing took place in the carburetor, which was then replaced with throttle body injection. (The latest engines use direct injection where the gasoline is sprayed into each cylinder individually.)
Throttle body injection gave improved control over fuel-air mixture and helped with emissions. These systems are found on many older vehicles and as they age can cause idling and acceleration problems.Here we’ll explore what the throttle body does, the main components,and their function.
The throttle body is a metal casting mounted at the furthest end of the inlet duct from the filter, at the point where air enters the inlet manifold. Inside, there’s a butterfly valve that looks like a flat circular plate. This is fixed to a shaft that extends outside of the throttle body. On the end of the shaft is a semi-circular pulley, and over this is fixed the throttle cable from the gas pedal. Pushing down on the gas pedal pulls the cable which opens the valve, allowing more air in. Lifting off the gas let’s butterfly valve close off the inlet duct, restricting airflow and engine speed.
As airflow changes the quantity of gasoline injected must also varied to keep the air:fuel ratio at that 14.7:1 number. To make this possible the engine control unit, (ECU) monitors various engine sensors and determines how long to keep the injector open.
Inaddition to the throttle body casting and butterfly valve, there are four important components of the system. These are:
- Fuel injector
- Throttle position sensor
- Idle air control valve
- Fuel inlet and return
Here’s a description of what each does.
Fuel inlet and return
The fuel pump delivers gasoline at high pressure, ensuring there’s always plenty available when the injector opens. As the pump supplies more fuel than is needed, the excess must be returned to the tank, which is the purpose of the return line.
The system is designed to keep fuel under pressure behind the injector even when the engine is off. That allows it to start quickly when the key is turned. It’s also why you should never disconnect the high pressure inlet line without first relieving the pressure!
Inside the housing is a fuel pressure regulator. This is typically a spring-loaded diaphragm that opens when pressure reaches a pre-set limit, allowing excess fuel to back to the tank through the return line.
The pump and regulator ensure gasoline is held behind the injector at high pressure. On a signal from the ECU a solenoid moves inside the injector, opening a valve. This allows the gasoline to spray through very fine holes in the injector into the incoming air. (Some newer injector designs are piezo-actuated rather than using a solenoid.)
Overtime the injector will wear or can become partially or even completely blocked. In both cases replacement is recommended.
(Note that while the photo shows a single throttle body injector, some larger engines will have two or more.)
Throttle position sensor (TPS)
This works in conjunction with the mass airflow (MAF) sensor, the manifold absolute pressure (MAP) sensor, and the O2 sensor to send the ECU information about how the engine is running. The role of the TPS is to report how far the butterfly valve is open. As the valve opens wider and more air is drawn in, (which is also reported by the MAF,) the ECU commands the injector to stay open longer, allowing more gasoline to mix with the air and maintaining the all-important 14.7:1 ratio.
There are two designs of TPS. The older type works like a potentiometer on changes in electrical resistance while newer designs incorporate a magnetic “Hall-effect” sensor for non-contact operation.
With the potentiometer design, electrical contacts slide across a circuit board as the position of the butterfly valve changes. This changes the resistance in the circuit, so varying the voltage. (Ohms Law: V=IR, and the I remains constant at a nominal 12 Volts.) The ECU reads that voltage, and based on that plus the MAF signal, is able to determine how long the injector should be opened for.
Contact-based sensors tend to wear, which can cause a low speed stutter or hesitation under acceleration. Hall-effect sensors don’t wear although they can be sensitive to temperature changes. In general Hall-effect sensors have higher resolution than the potentiometer style designs, so provide more precise control. However, it’s not possible to retrofit a contact-based TPS with a non-contact version as the ECU will be looking for different signals.
Idle air control (IAC) valve
At idle the throttle butterfly is almost completely closed and very little air flows past. However, the load on the engine can vary while idling,as when the a/c cycles on and off. To maintain a constant idle speed under varying load the IAC valve provides a bypass for the incoming air.
Typically this consists of a solenoid that opens and closes a diaphragm in response to signals from the ECU, allowing more or less air to flow through the bypass ports. This way airflow can increase in response to engine load without a change in throttle position.
If you suspect a faulty IAC – uneven idling or stalling are typical symptoms – a simple check is to let the engine idle then disconnect the electrical connector. The idle speed should drop noticeably. If not,the valve was probably not allowing air past. (This will not reveal a stuck diaphragm though.)
In summary …
The throttle body allows engine speed to be changed by pressing the gas pedal and allowing in more air. Connected to the throttle body are the throttle position sensor, idle air valve, and in engines without direct injection, the fuel injector or injectors. A high pressure line takes gasoline from the tank to the injector, with a low pressure line returning the excess. A rough idle or stumbling on acceleration can indicate a problem with one of these components.