Evaporative Emission System Monitoring
Evaporative Emission System Monitoring
Schematic
Description
The evaporative monitoring system being used permits the detection of leaks with a diameter of 0.5 mm (20 thou of an inch) or greater.
This is achieved by means of a pressure test of the system. This is performed by the Diagnostic Module - Tank Leakage (DMTL), which is an electrically operated pump fitted to the atmospheric air intake of the EVAP canister.
The test proceeds in 2 stages:
^ Reference Leak Measurement - The pump operates against the reference restriction within the DMTL. The Engine Control Module measures the current consumption of the pump motor during this phase.
^ Leak Measurement - The solenoid in the DMTL is operated in order to shut off normal purge airflow into the EVAP canister. The pump can now pressurize the fuel tank and vapour handling system. The Engine Control Module again measures the current consumed by the pump motor and by comparing this with the reference current, determines if a leak is present or not. A high current indicates a tight system and a low current indicates a leaking system.
Fault Conditions That Can Be Identified
^ Reference current high - Small leak (0.020 inch or larger)
^ Reference current low - Pump electrical high
^ Reference leak - Pump electrical low
^ Noise fault - Change over valve electrical high
^ Change over valve stuck open - Change over valve electrical low
^ Change over valve stuck closed - Pump heater high
^ Rough leak (0.040 or larger) - Pump heater low
Typical Monitoring Results
Strategy Flowchart
Diagnosis Frequency and MIL Illumination
Evaporative Emission Canister Purge Valve
Circuit Continuity
Monitoring the commanded and actual state of EVAP canister purge valve checks circuit continuity. If the commanded and actual state do not match a timer is started. If the states do not match within the timed period the a failure is registered. If the failure occurs on two drive cycles then the appropriate DTC is logged.
Flow Check
The purge flow monitor works continuously and is designed to detect low purge flow caused by a blockage in the purge system or a malfunctioning EVAP canister purge valve.
The basis of the diagnostic is to detect the presence of intake pressure pulses caused by the 10 Hz pulse width modulated control of the EVAP canister purge valve duty (as shown in figure 1 below).
A discrete Fourier transform calculation is used to help distinguish these pulses from other noises present in the intake pressure signal.
Purge operation
Purge Flow Strategy Flowchart
If the above table does not include details of the following enabling conditions: - IAT, ECT, vehicle speed range, and time after engine start-up then the state of these parameters has no influence upon the execution of the monitor.
Drive Cycle Information
Evaporative system leak & DMTL module faults
1. Ensure that fuel filler cap is secure (minimum three clicks)
2. Ensure that fuel level is within the range of 15 > 85%
3. Ensure that normal "high range" gears are selected
4. Ensure that the ambient temperature signal is within the range of 0 > 40 degrees C (if not, a short drive may be necessary to overcome the filtering used in this signal)
5. Ensure that any other DTCs have been rectified (especially if they relate to the purge valve or DMTL heater) and then clear them from the CM memory
6. Leave the vehicle to stand undisturbed for at least 3 hours in an environment with ambient temperature within the range of 0 > 40 degrees C and atmospheric pressure above 70 kPa
7. Start the engine and allow to idle for at least 10 minutes
8. Switch the engine off and remove the key from the ignition switch
9. Allow the vehicle to stand undisturbed for at least 10 minutes
10. Switch the ignition back on, wait for 10 seconds, check for DTCs
11. If a small leak fault is being investigated this drive cycle will need to be repeated (rough leak check is every drive cycle, small leak check is every other drive cycle)
Purge valve & Purge flow faults
1. Ensure that the ambient temperature signal is above 0 degree C (if not, a short drive may be necessary to overcome the filtering used in this signal)
2. Ensure that any other DTCs have been rectified and then clear them from the CM memory
3. Start the engine and allow to idle for at least 10 minutes
4. Stop / re-start the engine and allow to idle for a further 5 minutes
5. Check for DTCs