Evaporative System Monitoring
Evaporative System Monitoring
Leakage Diagnosis Procedure
The leakage diagnosis procedure is a pressure check of the EVAP system.
In order to perform the check, the EVAP system will be sealed and pressure applied by the leakage diagnosis pump (LDP). The pressure variation time is analyzed by the ECM.
Monitoring Structure
Part 1:
Part 2:
Monitoring Structure, Explanation
The complete diagnosis procedure consists of the following steps:
1. Tank pressure check
The first step of leakage diagnostics is the pressure check of fuel tank system by testing the reed switch. In case of an open reed switch, the fuel tank system has sufficient pressure for the sealed check and no further pressure has to be supplied to the fuel tank system by the LDP. The diagnosis is waiting until the EVAP purge valve is opened in order to purge the carbon canister. In case the reed switch is still open either the reed switch stuck open or the EVAP purge valve stuck closed. A stuck closed EVAP purge valve will be detected during the EVAP purge system diagnostic. If the EVAP purge valve passes the diagnostic, then the reed switch is defective, otherwise the leak check of the fuel tank system is passed and the EVAP purge valve is defective.
In the case the reed switch is closed, the LDP is switched on in order to supply pressure to the fuel tank system and the diagnostic is continued with the step 2 to 3 (as described below)
2. LDP Self-check procedure
a. Closed check
LDP control is disabled and the reed switch has to be closed otherwise the reed switch is defective.
b. Close to open check
LDP control is switched on once and the diaphragm has to move to the upper position. The time is measured between closed and open position of diaphragm detected by the reed switch. When the final upper position of diaphragm is reached in a certain time, then the check will be passed.
3. Leak check of EVAP system
a. Fast pulse
After the self check procedure, the LDP control supplies pressure to the fuel tank system with a pressure dependent number of compression strokes in a certain time. In order to supply pressure to the fuel tank system, the LDP can perform compression strokes in several attempts.
b. EVAP system sealed check, measure stroke and measure phase
The decrease of fuel tank pressure is measured via time of diaphragm movement followed by a compression stroke. Within a certain time, the LDP control is determined within at least four measurement strokes. The averaged time is a measure for the tightness of fuel tank system.
Evaporative Purge System Monitoring
The purge flow through the EVAP Purge Valve is checked when the vehicle is at rest during an idle condition and the Lambda controller is active.
Monitoring Structure
Monitoring process:
The EVAP Purge Valve is opened while monitoring the Lambda controller and the airflow through the throttle unit.
For rich or lean mixture through the EVAP Purge Valve:
Flow through the EVAP Purge Valve is assumed as soon as the Lambda controller compensates for a rich or lean shift.
After this procedure the EVAP Purge Valve is reset and the diagnosis is completed. Monitoring Structure (cont.)
For stochiometric mixture flow through the EVAP Purge Valve:
In this case, the Lambda controller does not need to compensate for a deviation. However, when the EVAP Purge Valve is completely opened, the cylinder charge increases significantly. Therefore, flow through the throttle unit must be decreased in order to maintain the desired idle speed. Flow through the EVAP Purge Valve is assumed when the flow through the throttle unit is reduced by idle control. The EVAP Purge Valve is reset and the diagnosis is completed.
Schematic diagram:
MIL-illumination
If both mixture compensation and reduction of the airflow through the throttle unit does not occur for two diagnosis cycles, then a defective EVAP Purge Valve is assumed and the MIL is illuminated.