303-13A Evaporative Emissions- 4.0L: Description And Operation (page 6 of 6)

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-energised 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 energises 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 (70kPa (10.15 lbf/in

2

) derived from engine load calculations) is below 3047 m (10,000 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 :  

 
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-energises) 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. 
 

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