Lucas  Hotwire Mpi
High idle CO  readings  -- Rich Idle Mixture

The Lucas Hotwire system is prone to over rich mixtures particularly at idle which may cause an Emission Test failure  due to high CO readings.  The idle mixture is solely adjusted by an air bleed screw which by allows air into the inlet manifold  by-passing the  Hotwire Sensor.  The idle mixture is adjusted the car fully warm (allow the electric fan to cut in then cut out again) and  at the specified  stepper motor controlled idle speed; 950 RPM Manual Gearbox, 900 RPM Automatic Transmission. The mixture adjustment  screw is located on the throttlebody adjacent to the small brass elbow. To weaken the mixture the screw is turned outwards.  I use a Gunson Gastester for monitoring the exhaust gas CO content.

However the amount of adjustment available this way is very  limited and may not be sufficient without further work to ensure an emission test pass. This may be due to a number of reasons so before starting any work on the fuel system check the plugs, plug leads, distributor cap, rotor arm, air filter, cylinder compressions. The thermostat and  Coolant Temperature Sensor (CTS) should also be checked as these have a vital role in the control of the fuel injection system.  The output voltage setting of the throttle position sensor should also be checked.
In addition test the fuel rail temperature sensor by simply disconnecting it. If the CO content of the exhaust drops or idle speed  slows this may be defective and it and its associated connections to fuel ECU should be checked following the procedure given in the page  Fuel Temperature Sensor (FTS)Testing.

If the engine is in good general health the next stage is to clean out the throttle body and associated pipe work using carburettor cleaner. If you remove the stepper motor you must follow the procedure for retracting the plunger described in the stepper motor page.

The small bore passages and pipe work  connected to the by-pass screw often clog and restrict the volume of air passing. The little pipe elbow where the connection is made to the air filter casing  appears particularly restrictive  and should be cleaned out  carefully, the small bore rubber pipe also becomes restrictive due to dirt build up and the rubber swelling with age. On one car an over rich mixture was cured by the simple and highly unofficial expedient of disconnecting this tube from the air filter and allowing the tube to draw air through a wire mesh filter of the type fitted to the pick up pipe in windscreen washer systems.

If these simple measure fail the  next stage is to check the output from the hotwire airflow meter. The output from the hotwire sensor tends to increase with age from the nominal 1.4 volts at idle. If the test returns a value  above 1.7 volts at idle the hotwire sensor may need replaced. For details of how to carry out the test see Hot Wire Air Mass Flow Meter.   If the hotwire output  is within acceptable limits often removing the fuel injectors and having them professionally ultrasonically  cleaned  may reduce the CO content of the  exhaust to within emission test limits.

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