Principle of Operation: Notes

Emissions Management controls evaporative and exhaust emissions. The ECM monitors the fuel storage system for evaporative leakage  and controls the purging  of evaporative vapors. The ECM monitors and controls the exhaust emissions by regulating the combustible mixture  and after treating by injecting fresh air  into the exhaust system. The catalytic converter further breaks down remaining combustible exhaust gases and is monitored by the ECM for catalyst efficiency. 

The MS42 Evaporative Leakage Detection  uses a Leak Diagnosis Pump (LDP) to pressurize the fuel tank and the evaporative emission system (approx. 25mb.). The LDP equipped system is capable of detecting a leak as small as 0.5 mm. The LDP is replaceable as a complete component. The vacuum supply line (required for pump operation) is in the wiring harness from the engine compartment to the rear of the vehicle.

The LDP is a unitized component that contains the following:

• Vacuum Chamber
• Pneumatic Pressure Chamber
• DME Activated Vacuum Solenoid
• Reed Switch - providing a switched voltage feedback signal to the ECM

In the inactive state, the LDP diaphragm is at the bottom end (of down stroke). The diaphragm pushes a rod downward against spring pressure to open the canister vent valve.

This open valve serves as the filtered air inlet path for normal evaporative "breathing".

During Leak Testing of the evaporative system, the vent valve is sprung closed to block atmospheric venting. The Evaporative Emission Valve is also sprung closed to seal the system.

During every engine cold start the LDP solenoid is energized by the ECM. Engine manifold vacuum enters the upper chamber of the LDP to lift up the spring loaded diaphragm.

As the diaphragm is lifted it draws in ambient air through the filter and into the lower chamber of the LDP through the one way valve.

The solenoid is then de-energized, spring pressure closes the vacuum port blocking the engine vacuum and simultaneously opens the vent port to the balance tube which releases the captive vacuum in the upper chamber.

This allows the compressed spring to push the diaphragm down, starting the "limited down stroke". The air that was drawn into the lower chamber of the LDP during the upstroke is forced out of the lower chamber and into the evaporative system.

This electrically controlled repetitive up/down stroke is cycled repeatedly building up a total pressure of approximately +25mb in the evaporative system. After sufficient pressure has built up (LDP and its cycling is calibrated to the vehicle), the leak diagnosis begins and lasts about 100 seconds.

The upper chamber contains an integrated reed switch that produces a switched high/low voltage signal that is monitored by the ECM. The switch is opened by the magnetic interruption of the metal rod connected to the diaphragm when in the diaphragm is in the top dead center position.

The repetitive up/down stroke is confirmation to the ECM that the valve is functioning. The ECM also monitors the length of time it takes for the reed switch to open, which is opposed by pressure under the diaphragm in the lower chamber. The LDP is still cycled, but at a frequency that depends upon the rate of pressure loss in the lower chamber. If the pumping frequency is below parameters, there is no leak present. If the pumping frequency is above parameters, this indicates sufficient pressure can not build up in the lower chamber and evaporative system, indicating a leak.

The chart represents the diagnostic leak testing time frame in seconds. When the ignition is switched on, the ECM performs a "static check" of circuit integrity to the LDP pump including the reed switch.

• On cold engine start up, the pump is rapidly activated for the first 27 seconds. This rapid pumping phase is required to pressurize the evaporative components.
• Once pressurized, the build up phase then continues from 27-38 seconds. The ECM monitors the system through the reed switch to verify that pressure has stabilized.
• The measuring phase for leak diagnosis lasts from 38-63 seconds. The pump is activated but due to the pressure build up under the diaphragm, the pump moves slower. If the pump moves quickly, this indicates a lack of pressure or a leak. This registers as a fault in the ECM's.
• From 63-100 seconds the pump is deactivated, allowing full down stroke of the diaphragm and rod. At the extreme bottom of rod travel, the canister vent valve is pushed open relieving pressure and allowing normal purge operation when needed.

The MS43 Evaporative Leakage Detection  is performed on the fuel storage system by the DM TL pump which contains an integral DC motor that is activated by the ECM. The ECM monitors the pump motor operating current as the measurement for detecting leaks. The pump also contains an ECM controlled change over valve that is energized closed during a Leak Diagnosis test. The ECM only initiates a leak diagnosis test every second time the criteria are met. The criteria is as follows:

• Engine OFF  with ignition switched OFF. 
• ECM still in active state or what is known as "follow up mode" (ECM Relay energized, ECM and components online for extended period after key off).
• Prior to Engine/Ignition switch OFF condition, vehicle must have been driven for a minimum of 20 minutes.
• Prior to minimum 20 minute drive, the vehicle must have been OFF for a minimum of 5 hours.
• Fuel Tank Capacity must be between 15 and 85%  (safe approximation between 1/4 - 3/4 of a tank).
• Ambient Air Temperature between -7°C & 35°C (20°F & 95°F)
• Altitude > 2500m  (8,202 feet).
• Battery Voltage - 11 Volts (minimum) 

When these criteria are satisfied every second time, the ECM will start the Fuel System Leak Diagnosis Test. The test will typically be carried out once a day ie:, once after driving to work in the morning, when driving home in the evening, the criteria are once again met but the test is not initiated. The following morning, the test will run again.