Diagnostic Module Tank Leakage (DMTL) Pump - (NAS Only)
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 change-over valve, which seals the EVAP system. The Evaporative emissions canister 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 change-over 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, 15 seconds after the engine has started, the ECM energizes the heating element in the pump to prevent condensation formation and possible incorrect readings. The heater remains energized until either the engine and ignition are off (if no DMTL test is running) or until after the DMTL test is completed.
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 a 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 atmospheric pressure is above 70 kPa (10.15 lbf/in), i.e. the altitude is less than approximately 3047 m (10000 feet).
- The ambient temperature is between 0 and 40°C (32 and 104°F).
- The Evaporative emissions canister vapor concentration factor is 5 or less (where 0 is no fuel vapor, 1 is stoichiometric fuel vapor and greater than 1 is rich 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.
The ECM performs a check for minor leaks after every 2nd major leak check.
When the leak check is complete, the ECM stops the DMTL pump and opens (de-energizes) the change-over valve.
If the fuel filler cap is opened or refueling 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 MIL (Malfunction Indicator Lamp) in the IC (Instrument Cluster) on the next drive cycle.
The duration of a leak check can be between 60 and 900 seconds depending on the results and fuel tank level.
The Diagnostic Module Tank Leakage (DMTL) pump 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 Evaporative emissions canister and incorporates an electric air pump, a Positive Temperature Coefficient (PTC) heating element, a normally open change-over 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 change-over 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 head and is connected to the DMTL pump by a vapor pipe.
The DMTL test is performed after the engine has stopped following a run of 10 minutes or more, providing that the vehicle fuel tank is between 15 and 85% full, the ambient temperature is above 0°C (32°F) and less than 40°C (104°F) and the vehicle was not started for at least 180 minutes prior to this run.
The DMTL pump is driven to pressurize the fuel tank and the current is measured with the change-over valve in different states.
A comparison of the current draw in each state indicates the degree of any leak, and the ECM then sets the appropriate Diagnostic Trouble Code (DTC).