Not sure what NAS stands for but this is what my manual says for an evap pump---
DIAGNOSTIC MONITORING OF TANK LEAKAGE (DMTL) - NAS ONLY
The DMTL system is a legislative requirement for NAS vehicles. The DMTL system periodically checks the EVAP system and the
fuel tank for leaks when the ignition is switched off.
The DMTL system comprises the previously described components of the EVAP system with the following additional components; a
DMTL pump and a DMTL filter.
The DMTL pump is connected to the atmospheric vent of the charcoal canister and incorporates a Positive Temperature
Co-efficient (PTC) heating element, a normally open valve and a reference orifice. The DMTL pump is only operated when the
ignition is switched off and is controlled by the ECM. The ECM also monitors the electric air pump operation and the normally open
valve for faults.
The DMTL filter protects the pump from dust being drawn into the system when the pump is being operated. The filter is located
on the fuel filler pipe and is connected to the DMTL pump by a hose.
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 pressurise 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:
The vehicle speed is zero
The engine speed is zero
The pressure altitude (70 kPa (10.15 lbf/in2) derived from engine load calculations) is below 3047 m (10000 feet)
The ambient temperature is between 0 and 40°C (32 and 104°F)
The charcoal canister load factor is 2 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 10 minutes
The battery voltage is between 10 and 15 volts
The last engine off time was more than 180 minutes
No errors are detected with the EVAP components, the ambient air temperature and the fuel level
High range must be selected on the transfer box.
• 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 2nd 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 60 and 600 seconds depending on the results and fuel tank level
