System for controlling idle speed of an engine

A system for controlling idle speed of an engine having a bypass around a throttle valve and a solenoid operated idle speed control valve provided in the bypass, comprising:

• first means for generating driving pulses for driving the idle speed control valve in dependence on a coolant temperature and an old learning correction value;
• second means for converting temperature of a solenoid of the control valve to voltage;
• a map storing normal temperature duty ratios;
• third means for deriving a normal temperature duty ratio from the map in dependence on duty ratio of the driving pulses and the voltage;
• a first calculator for producing a difference between the duty ratio of the driving pulses and the derived normal temperature duty ratio;
• a memory storing the difference as a correction value;
• detecting means for detecting turning off of an ignition switch and for producing an off signal;
• a second calculator responsive to the off signal for adding the correction value to the old learning correction value to produce a new learning correction value and for storing the new learning correction value for a subsequent engine operation instead of the old learning correction value.