What is purpose of Vacuum Vent Valve in Chrysler Evap Emission System using LDP Pump

Question

The vacuum vent valve is located in the hose next to the front evap canister. It does not show up in any description I have found for the Chrysler LDP evap emission testing function nor does it show up in the schematic for the Chrysler evap emission system using the LDP. I have only found the Vent Valve in pictures of the front canister with no description of its purpose. This vacuum operated vent valve works every time you start and stop the engine and therefore it has a lot of cycles and failure of the diaphragm is a common cause of \"Gross Leak\" detected by the OBDII code P0455. Chrysler TSB NUMBER: 25-001-02 REV A gives a vacuum test procedure for the vent valve but no discussion of its purpose. This vacuum vent valve appears to have redundant function to the vent valve in the LDP.

Answer 1

Every time you start the engine the Engine Control Module closes the valve to perform a pressure test on the EVAP system. If there is a leak or the valve doesn't close (faulty valve or circuit), the system will not pressurize and the ECM will triger the P0455 code.

Here is the information from the service manual:

"The canister vent valve will unseal the system after completion of the test sequence as the pump diaphragm assembly moves to the full travel position"

 

Comments

OBDITech,
Thanks for the feedback but your explanation is addressing the vent valve in the LDP.  Yes, I understand that the LDP vent valve is closed via the ECM and engine vacuum after the engine is started cold in order to pressure test the system and then reopens when the LDP diaphragm assembly moves to the full travel position at the end of the test.  However my question is about a second vent valve, a vacuum vent valve located in the evap emission hose next to the front canister.  The vacuum line to this valve is connected directly to the intake manifold and the vent valve is closed only when the engine is running.  As I said in my original query, it seems to me that the second vacuum vent valve is a backup to the LDP vent valve but I have not been able to find out why it used.  Also, it is not shown on the Chrysler Evap Emission System Schematic with LDP.  So, why is this second vent valve used?
Joe