4.3.2 Description

The evaporative emission 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 vapor 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
• Reference current low
• Reference leak
• Noise fault
• Change over valve stuck open
• Change over valve stuck closed
• Rough leak (0.040" or larger)
• Small leak (0.020" or larger)
• Pump electrical high
• Pump electrical low
• Change over valve electrical high
• Change over valve electrical low
• Pump heater high
• Pump heater low