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.