Description of the Enhanced Evaporative Emission Control

DESCRIPTION OF THE ENHANCED EVAPORATIVE EMISSION CONTROL SYSTEM SELF CHECK (REFER TO GRAPH # 1):

The system self check occurs 16 minutes after the engine is started. The sequence for setting diagnostic codes is described below. If a code is set during any part of the sequence, the remainder of the checks are skipped. Repairs made to the vehicle should be in the same order as the codes described below. For the self check to occur, the vehicle must be idling, not moving, and the fuel tank pressure must not be greater than 2.7 volts measured at the DPS. The self check sequence takes place as follows:

1. Canister Close Valve Check (P0446) - The canister close valve is open and the canister purge valve is closed. In a normal system, the DPS value reads close to atmospheric pressure. If the DPS voltage reads more than 1.5 volts below atmospheric pressure during this part of the check, code P0446 will set in the PCM memory. This check measures the ability of the system to read atmospheric voltage at the beginning of the check. Vacuum in the system may be caused by a number of conditions:

^ A mechanically "wide open" purge control valve in the "Purge" circuit.
^ A hose or component restriction in the "Fresh Air" circuit including the plastic tube inside the frame rail.
^ A hose or component restriction, in the "To Tank" circuit.

2. Canister Purge Valve Check (P0441) - The canister close valve closes for 4.5 seconds and the canister purge valve is closed. With the system now completely sealed, vaporizing fuel in the fuel tank raises the system pressure very slightly. A normal rise in pressure from fuel vaporization is approximately 0.02 volts. A large rise in pressure skips the remainder of the checks. If the tank is low on fuel, there may be a large rise in pressure because of fuel vaporization. If the DPS reads a vacuum drop greater than 0.64 volts during the 4.5 second period, the PCM will set fault code P0441. This condition is caused by a continuous engine vacuum source being applied to the system. The condition may be due to a "partially open" purge control valve in the "Purge" circuit.

3. Large Leak Check (P0455) - This check occurs in 2 parts. Part 1 - The purge control valve cycles (opening), exposing the system to manifold vacuum. In a normal system, within 5 seconds after the purge control valve begins to cycle, the vacuum must be drawn down at least 0.04 volts from atmospheric pressure. Part 2 - within 9 seconds after the purge control valve begins to cycle, the vacuum must drop more than 0.64 volts from its initial reading. If the system is not drawn down to the voltages described above, then the PCM will set code P0455. A "large leak" in the system may be caused by a number of conditions:

^ Broken, cracked, or punctured component(s) (i.e. cracked canister, broken two way valve, punctured hose, cracked fuel/vapor separator, etc.) anywhere in the system.

^ Loose or disconnected component(s) (i.e. loose gas cap, vapor hose not connected/clamped properly onto steel tube, fuel pump not mounted properly, etc.) anywhere in the system.

^ Restricted or pinched component(s) (i.e. fuel tank 90° vent nipple plugged with foreign material, hose completely pinched closed, etc.) anywhere in the system.

^ Missing parts, such as seals (missing 0-ring on gas cap) anywhere in the system.

4. Vacuum Stabilization Period - After the canister purge valve stops purging, vacuum in the system will stabilize. The DPS voltage should not continue to decrease 2 seconds after the purge control valve stops purging. A long stabilization period may not set a fault code immediately, however, it may indicate a hose or component restriction in the "To Tank" circuit.

5. Small Leak Check (P0442) - The system checks for small leaks for a period of five seconds after the DPS has reached its lowest voltage (maximum vacuum). During this part of the check, the canister close valve is still held closed from the previous check, and the canister purge valve is closed. After the vacuum stabilizes, the system should maintain a steady vacuum (a flat line on the HDS plot) for five seconds, since the system is completely sealed. A small leak will cause a slow rise in DPS voltage, and set fault code P0442. A "small leak" in the system may be caused by a number of conditions:

^ Cracked or punctured component(s) (i.e. cracked canister, punctured hose, cracked fuel/vapor separator, etc.) anywhere in the system.

^ Loose components(s) (i.e. loose gas cap, vapor hose not clamped properly onto steel tube, etc.) anywhere in the system.

6. Recovery Period - The system should return to near atmospheric pressure in preparation for normal evaporative emission system operation. In a normal system, the voltage rises approximately 0.1 V per second during the recovery period. A slow response in this sequence may not set a fault code immediately, however, the following conditions may exist:

^ A mechanically "open" purge control valve in the "Purge" circuit.
^ A hose or component restriction in the "Fresh Air" circuit.
^ A hose or component restriction, in the "To Tank" circuit.