Hot Idle Compensator

A hot idle compensator (fig. 4-42) is a thermostatically controlled device that prevents engine stalling or a rough idle under high engine temperatures. The temperature sensitive valve admits extra air into the engine to increase idle speed and smoothness.

At normal engine temperatures, the hot-idle compensator valve remains closed, and the engine idles normally. When temperatures are high (prolonged idling periods, for example), fuel vapors can enter the air horn and enrich the air-fuel mixture. The hot idle compensator opens to allow extra air to enter the intake manifold. This action compensates for the extra fuel vapors and corrects the air-fuel mixture.

Altitude Compensator

An altitude compensator is used to change the air- fuel mixture in the carburetor with changes in the vehicle height above sea level. Normally the compensator is an aneroid device (bellows device that expands and contracts with changes in atmospheric pressure).

As a vehicle is driven up a mountain, the density of the air decreases. This condition tends to make the air- fuel mixture richer. The reduced air pressure causes the aneroid to expand, opening an air valve. Extra air flows into the air horn and the air-fuel mixture becomes leaner. The opposite occurs when the vehicle descends from the mountain. The greater air density and pressure

Figure 4-42. - Hot idle compensator.

tends to make the carburetor mixture too lean. The increased air pressure collapses the aneroid and the air valve closes. This action enriches the mixture enough to compensate for the low altitude.

COMPUTER-CONTROLLED CARBURETORS

A computer-controlled carburetor uses a solenoid- operated valve to respond to commands from the microcomputer (electronic control unit). The system uses various sensors to send information to the computer that calculates how rich or lean to set the carburetor air-fuel mixture. The system is also known as a computer controlled emission system which consists of the following: oxygen sensor, temperature sensor, pressure sensor, electromechanical carburetor, mixture control solenoid, computer, and idle speed actuator. The function of each is as follows:

The OXYGEN SENSOR, or exhaust gas sensor, monitors the oxygen content in the engine exhaust. The amount of oxygen in the exhaust indicates the richness (low oxygen content) or leanness (high oxygen content) of the air- fuel mixture. The sensor voltage output changes with any change in oxygen content in the exhaust gases.

The TEMPERATURE SENSOR detects the operating temperature of the engine. Its resistance changes with the temperature of the engine. The change in resistance allows the computer to enrich the fuel mixture during cold engine operations.

The MANIFOLD PRESSURE SENSOR (MAP) measures intake manifold vacuum and engine load. High engine load or power output causes intake manifold vacuum to drop. The pressure sensor then signals the computer with a change in resistance and current flow. As manifold pressure drops, the computer increases the air-fuel mixture for added power. As the manifold pressure increases, the computer makes the carburetor setting leaner for improved economy.

The ELECTROMECHANICAL CARBURETOR has both electrical and mechanical control devices. It is commonly used with a computer control system.

The MIXTURE CONTROL SOLENOID alters the air-fuel mixture in the electromechanical carburetor. Electrical signals from the computer activate the solenoid to open and close air and fuel passages in the carburetor.