ISC Solenoid Valve Control System: Notes

In this system, ECM controls ISC solenoid valve according to the signals from various sensors and switches to control the bypass air flow which plays the following three roles as required by the circumstances.

1. Idle-up Air Flow To Stabilize Engine Idle Speed  - When head lights & small lights, heater fan, rear defogger (if equipped) or air conditioner (if equipped) and/or power steering system (if equipped) is operating (that is, when electric load and/or engine load is applied) and/or vehicle is at a high altitude, higher than 8,200 ft or 2,500 m (barometric pressure is lower than 550 mm Hg), the bypass air is supplied to stabilize the engine idle speed. How much bypass air is supplied varies depending on the load. When only head lights and small lights are ON, the idle speed rises only a little higher than the normal specified idle speed (920 r/min).
2. After start air flow to improve engine start  - When and after the engine is started, the bypass air is supplied to prevent the engine from stopping when the throttle opener is OFF and to improve the engine performance at its start. How much bypass air is supplied varies depending on the engine cooling water temperature and it is decreases at time passes when engine is cool. When engine is hot and 15 seconds have passed after engine start, a given amount of bypass air is supplied till either the idle switch is once turned OFF from ON or engine speed exceeds 1,500 r/min. once.
3. Dashpot air flow to correct air/fuel mixture ratio  - A sudden change in the throttle valve opening (especially when decelerating) causes the air/fuel mixture ratio to change also suddenly. To correct it, the bypass air flow is increased or decreased. (That is, by increasing or decreasing the air, the air/fuel mixture ratio is changed gradually and dashpot effect is thus obtained.) The air is increased when the throttle valve is opened, and decreased gradually when closed.