Bosch Motronic - Component Datasheet

Idle Stabiliser Valve (N71)

The idle stabiliser valve's purpose in life is to provide a mechanism whereby a controlled amount of additional air is allowed to sneak past the throttle valve plate. This provides a corrected air volume into the engine under certain conditions when the ECU makes adjustments to idle speed by changes in ignition timing. This adjusted airflow prevents the engine from stalling during the warm up phase that follows a cold start.

The operation of the idle stabiliser is controlled by a duty cycled DC voltage that is generated by the ECU. The control voltage pulses act on a single wound electric motor which is connected to a rotory slide valve. The position of this slide valve determines the adjusted air flow through the idle stabiliser valve - this is controlled by the ECU, in conjunction with changes in idle ignition timing, so that the idle speed is stabilised in conditions such as the air conditioning being turned on or off.

Should the idle stabiliser valve fail (or have its 2-pin electrical plug disconnected) a return spring within the device will move the rotary valve to a mechanical stop position that corresponds to a fixed warm engine idle speed. In this mode, the ECU is unable to control the rotary valve's position and a constant amount of corrected air passes through N71.