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.
* The Coolant Temperature Sensor (CTS)
* The thermostat operating temperature (the thermostat should be a 88 C type not the more common 82 C type as listed in some parts catalogues) or the base idle speed being set too low by attempting to adjust the idle speed setting without following the correct procedure.
* A faulty Fuel Temperature Sensor (FTS) may also give rise to high idle speed.
* The idle speed may also be high if the battery has been disconnected however this will usually return to normal by itself after the car has been driven for 20 miles or so.
* The Base Idle Speed may have been set too low forcing the ECU to raise the controlled idle speed above the nominal 950/900 RPM. This is frequently the cause of the engine stalling at parking speeds when the steering is turned to full lock.
* Check for vacuum leaks in the inlet manifold, servo and vacuum pipe to the ECU
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