Toyota COMPUTER-CONTROLLED SYSTEM - Page 75

EFI - Air Induction Syste
m
1
. THROTTLE BOD Y
The throttle body consists of the throttle valve,
which controls the intake air volume during
normal engine operation, a bypass passage
through which a small volume of air passes
during idling, and a throttle position sensor
which detects the opening angle of the throttle
valve . Some throttle bodies are also equipped
with a dashpot which causes the throttle valve
to return gradually when it is closed or with a
wax type air valve .
During idling, the throttle valve is fully closed
.
As a result, intake air flows through the bypass
passage into the air intake chamber .
The engine speed during idling can be adjusted
by the idle speed adjusting screw, which
increases or decreases the volume of air passing
through the bypass passage
. (See the illustration
of the wax type air valve at right . )
NOTE
The idle speed adjusting screw adjusts the
idle speed, just as the throttle adjusting
screw does on a carburetor .
In engines equipped with a stepper motor
type or rotary solenoid type ISC valve, the
volume of air flowing through the bypass
passage is controlled by the ISC valve .
Therefore, in some engines, the idle speed
adjusting screw is set to the fully-closed
position at the factory, while in others, an
idle adjusting screw is not provided
.
2
. AIR VALV E
The air valve controls the engine idling speed
when the engine is cold .
Some engines equipped with an ISC valve do
not use this air valve
. (For further details on the
ISC valve, see page 99
. )
WAX TYP E
The wax type air valve consists of a thermo
valve and a gate valve .
The thermo valve is filled with thermo wax . The
volume of this wax changes according to the
coolant temperature . The wax type air valve
utilizes these characteristics of the thermo wax
to open and close the gate valve in order to
control the engine idling speed .
Idle speed adjusting scre w
Bypass passag e
Coolant
Thro
tt le
valv e
BI-METAL TYPE
OHP 5 3
The bi-metal type air valve consists of a bi-
metal element, a heat coil, and a gate valve
.
Current flows simultaneously to the heat coil
and the fuel pump
. This heats the element,
causing it to change shape
. This in turn causes
the gate to close gradually
.
Heat coil (ja To air intake
Bi-metal
\
IP~ chambe r
element
Gate valv e
From air cleaner
OHP 5 3
65