There are two ISC valves; the ISC valve #1 for the front
cylinder, and the ISC valve #2 for the rear cylinder. When
the engine is cold, each ISC valve is opened according to
the coolant temperature by the ECU, providing good starta-
bility in cold weather and allowing the engine to run fast for
quick warm up (fast idle).
Each ISC valve has a solenoid valve. When the solenoid
[A] is energized by the ECU, the solenoid valve [B] opens
fully and inlet air [C] flows into [D] the ISC pipe.
The air flows through the ISC pipes [A], and hoses [B]
(bypass) of the throttle assy [C] into the cylinder head.
When starting the engine, the ISC valves always open
fully, often causing too high fast idle. So the ECU controls
ISC valves according to the coolant temperature. The ECU
keeps the solenoid valves open and close repeatedly since
the engine starts so that proper fast idle speed is obtained.
When the engine has warmed up, the solenoid valves close
and ISC valves finish operation.
When the coolant temperature is low, the ratio (duty ratio)
of valve opening time to total time (valve opening time and
closing time) is set high, making the fast idle speed high. As
the temperature goes up, the ratio is gradually decreased
to lower the fast idle speed.
Coolant Temperature [A]
100% [D] (Duty ratio at full open)
Duty ratio (0 ∼ 100%) [E]
Generally, when a device is used for intermittent duty like
starting, running, stopping, and idling, the device is oper-
ated under "duty control". The ratio of running, or working
time to total time is called "duty ratio" ("duty" means the work
or operation done by a device).
FUEL SYSTEM (DFI) 2-63