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.
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
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.
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
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 CARBURE-
TOR has both electrical and mechanical control
devices. It is commonly used with a computer control
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