1.4.2.1 Monitoring Function - Leak Detection
P0442, P0456
The evaporative system monitoring detects EVAP system leaks with a diameter of 0.02 inches and up.
By means of a Diagnostic Module Tank Leakage (DMTL), an electrical actuated pump located at the atmospheric connection of the evaporative canister, a pressure test of the EVAP system is performed in the following order:
While the reference leak is being measured, the electrical actuated pump delivers through the reference restriction. The engine management system measures the pump's electrical current consumption in this section.
During the leak measurement process, the electrically actuated pump delivers through the charcoal canister into the fuel-tank system. The pressure in the evaporative system may reach up to 2.5 kPa, depending on the fuel level in the tank. The engine management system measures the pump's electrical current consumption. A comparison of the currents of the reference leak measurement and the leak measurement is an indication of the leakage in the tank.
0.02 inch Diagnosis, Very Small Leak:
P0456
The first step of this diagnosis is reference measurement. The result of the pump reference current is stored (picture in chapter a). After the solenoid switches, the venting system is pressurized (picture in chapter b). The small leak threshold for the small leak measurement is reached when the leak is smaller than 0.04 inches. The small leak measurement phase then follows. When the DMTL current reaches the reference current within the very small leak time limit, the system is tight, meaning that the leak is smaller than 0.02 inches. Otherwise, a very small leak, between 0.02 and 0.04 inches, is detected.
0.04 inch Diagnosis, Small Leak:
P0442
The first step of this diagnosis is also the reference measurement. The result of the pump reference is stored (picture in chapter a). After the solenoid switches, the venting system is pressurized (picture in chapter b). During the small leak phase time, the pump current must reach the small leak threshold 1:
Small leak threshold 1 = idle current pump + K1 x (reference current - idle current). Factor K1 is between 0.16 and 0.28. Depending on the characteristic current value of the pump (reference current - idle current), this value can vary between every pump.
If the small leak threshold 1 is not reached within the small leak time limit, the small leak threshold 2 must be reached within an additional time limit.
Small leak threshold 2 = reference current pump - K2 x (reference current - idle current). Factor K2 is between 0.60 and 0.80, depending on the characteristic current value of the pump (reference current - idle current).
When the small leak threshold 2 is also not reached, a leak > 0.04 inches is detected.
The typical current of a tight system, a 0.02 inch leak, and a leak > 0.04 inches is illustrated in the diagram below.
- After the test, the remaining pressure in the evaporative system is bled off through the charcoal canister by switching off the pump and solenoid.