Nissan Repair: Trouble Codes, canister purge valve, fuel vapors

Question
96 Nissan 240SX stalls momentarily and sets codes PO440 and PO446. Stalls first thing in morning after driving a couple of blocks. Where to start? Thanks

Answer
Butch,

A great site to explain the On Board Diagnostic codes is http://www.obd-codes.com

Here is what the P0446

Evap vent valve has only one purpose. It closes in order to seal the vent so the EVAP system can pressurize and ensure there are no leaks. Evap vent valve is usually supplied Batt. voltage with key on. The ECM's driver controls the ground, and when grounded, activates the valve (closing it). If the ECM detects a short to ground, and open,or a short to battery voltage on the control circuit, P0446 will set. Code also may refer to ECM detecting that EVAP system is unable to achieve or maintain vacuum during test.
Symptoms

There will be no obvious symptoms to driver, other than MIL illumination.
Causes

A code P0446 could mean one or more of the following has happened:

   * Faulty vent valve
   * Open, short or excessive resistance on Vent valve control circuit
   * Blockage of vent valve
   * Bad PCM

Possible Solutions

With a P0446 OBD-II trouble code, here are some things to try:

   * Replace Vent valve
   * Repair open, short, or resistance problem in control circuit
   * Repair open, or short, or resistance problem in power circuit
   * Replace PCM

Technical Description

Evaporative Emission Control System Malfunction
What does that mean?

This indicates that a part of the EVAP control system is no longer fuctioning correctly. The EVAP system consists of many parts, including (but not limited to) the gas cap, fuel lines, carbon canister, purge valve, and other hoses. The (EVAP) emission control system prevents the escape of fuel vapors from a vehicle's fuel system. Fuel vapors are routed by hoses to a charcoal canister for storage. Later, when the engine is running a purge control valve opens allowing intake vacuum to siphon the fuel vapors into the engine.
Symptoms

You likely won't notice any drivability problems.
Causes

A code P0440 could mean one or more of the following has happened:

   * The gas cap is not installed or working properly
   * The purge solenoid has failed
   * The canister is plugged and not working properly

Possible Solutions

With a P0440 OBD-II trouble code, diagnosis can be tricky at times. Here are some things to try:

   * Remove and reinstall the gas cap, clear the code, and drive for a day and see if the codes come back.
   * Inspect the EVAP system for cuts/holes in tubes/hoses
   * Inspect for damaged or disconnected hoses around the Evap purge solenoid
   * Check and/or replace the sensor
   * Check and/or replace the purge valve
   * Have a professional use a smoke machine to detect leaks