Operation

DMTL Operation
To check the fuel tank and the EVAP system for leaks, the ECM operates the DMTL pump and monitors the current draw. Initially, the ECM establishes a reference current by pumping air through the reference orifice and back to atmosphere. Once the reference current is determined, the ECM closes the normally open valve which seals the EVAP system. The purge valve remains de-energized and is therefore closed. The output from the air pump is diverted from the reference orifice and into the EVAP system.

When the normally open valve is closed, the load on the air pump falls to zero. Providing there are no leaks, the air pump will begin to pressurize the EVAP system and the load and current draw in the pump increases. By monitoring the rate and level of the current increase, the ECM can determine if there is a leak in the EVAP system.

During normal vehicle operation, the ECM energizes the heating element in the pump to prevent condensation formation and possible incorrect current readings.

Leaks are classified as:
^ Minor - equivalent to a hole diameter of 0.5 to 1.0 mm (0.02 to 0.04 in)
^ Major - equivalent to hole diameter of 1.0 mm (0.04 in) or greater.

The ECM performs a check for major leaks each time the ignition is switched off, providing the following conditions are met:
^ ECM is in power down mode for more than 3 seconds after the ignition is switched off
^ The vehicle speed is zero
^ The engine speed is zero
^ The pressure altitude (derived from engine load calculations) is below 1800 m (6000 feet)
^ The engine coolant temperature is more than 2.25 degree C (36 degree F)
^ The ambient temperature is between 0 and 40 degree C (32 and 104 degree F)
^ The charcoal canister load factor is 3 or less (where the load factor is a measure, between -1 and +30, of the fuel vapor stored in the charcoal canister. Where -1 is 0% fuel vapor, 0 is stoichiometric fuel vapor level and +30 is 100% saturated with fuel vapor.
^ The fuel tank level is valid and between 15 and 85% of nominal capacity
^ The engine running time during the previous cycle was more than 20 minutes
^ The battery voltage is between 10.94 and 14.52 volts
^ The last engine off time was more than 150 minutes
^ No errors are detected with the EVAP components, the engine coolant temperature, the ambient air temperature and the fuel level.

NOTE: A leak test can be performed using T4. This overrides the above conditions and is useful for checking correct system and component operation.

The ECM performs a check for minor leaks after every 14th major leak check or after refuelling is detected.

When the leak check is complete, the ECM stops the DMTL pump and opens (de-energizes) the normally open valve.

If the fuel filler cap is opened or refuelling is detected during the leak check, by a sudden drop in the current draw or a rise in the fuel level, the ECM aborts the leak check.

If a leak is detected during the check, the ECM stores an appropriate fault code in its memory. If a leak is detected on two consecutive checks, the ECM illuminates the Malfunction Indicator Lamp (MIL) in the instrument cluster on the next drive cycle.

The duration of a leak check can be between 40 and 270 seconds depending on the results and fuel tank level.