Non-ECM controlled EVAP systems typically use the following components:
• Fuel tank cap (with vacuum check valve)
• Charcoal canister (with vacuum & pressure check valves)
• Ported vacuum purge port (port P; on throttle body) EVAP System Operation
Under some conditions, the fuel tank operates under a slight pressure to reduce the possibility of pump cavitation due to fuel vaporization. Pressure is created by unused fuel returning to the tank and is maintained by check valve #2 in the charcoal canister and the check valve in the fuel tank cap.
Under other conditions, as fuel is drawn from the tank, a vacuum can be created in the tank causing it to collapse. This is prevented by allowing atmospheric pressure to enter the tank through check valve #3 in the charcoal canister or the fuel tank cap check valve. The EVAP system is designed to limit maximum vacuum and pressure in the fuel tank in this manner.
When the engine is running, stored fuel vapors are purged from the canister whenever the throttle has opened past the purge port (port P) and coolant temperature is above a certain point (usually around 129' F). Fuel vapors flow from the high pressure area in the canister, past check valve #1 in the canister, through the Thermo Vacuum Valve (TVV), to the low pressure area in the throttle body. Atmospheric pressure is allowed into the canister through a filter located on the bottom of the canister. This ensures that purge flow is constantly maintained whenever purge vacuum is applied to the canister.
When coolant temperature falls below a certain point (usually around 95'F), the TVV prevents purge from taking place by blocking the vacuum signal to check valve #1.
Was this article helpful?