Lucas Hotwire MPi
Stepper Motor Idle Speed Control


The idle speed on this system is controlled by the Fuel ECU via a stepper motor actuated throttle by-pass valve. This maintains a minimum idle speed of 950 RPM (900 RPM on cars with automatic transmission) which is 200 RPM above the base idle speed when the engine is fully warm. The ECU will raise the idle speed above this if the engine is cold or the fuel temperature is excessively high, it may also raise the idle speed if a fault condition is detected.

Frequently high or fluctuating idling speed problems are caused the ECU not going into to normal closed loop mode, the following should be checked before attempting to adjust the idle speed.

If these causes have been eliminated and the stepper motor itself is suspected of sticking it may be cleaned and lubricated but may only be unscrewed after it has been full retracted.
To retract the stepper motor plunger:
  1. With the engine off disconnect the stepper motor.
  2. Switch the ignition on.
  3. Count to 10 then reconnect the stepper motor.
  4. Switch the ignition off
  5. Count to 10 again then disconnect the stepper motor
  6. Switch the ignition on
  7. Count to 10 again  then reconnect the stepper motor.
  8. Switch the ignition off.
  9. After counting   to 10  the stepper motor may be unscrewed and removed.
The stepper motor plunger and throttle body may now be cleaned using carb cleaner. I always use Redex  or fuel injector cleaner to lubricate the parts on reassemble. The base idle speed will probably now have to be reset for full procedure see   Setting the Base Idle Speed.

For full procedure to set idle speed mixture see High Idle Speed CO Emission Test Failure

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