Leak Diagnostic System: Notes

Fuel vapor from fuel tank is routed to and stored in EVAP canister. Vapor is then guided into combustion chamber via canister purge valve. Leak diagnostic system ensures that EVAP system is not leaking vapor into the atmosphere. Leak diagnostic system can detect a leak or hole .020" (.5 mm) or larger.

Leak diagnostic system uses a vacuum-driven pump. Vacuum comes from intake manifold via a non-return valve. ECM controls a three-way valve in leak diagnostic pump and pump action uses a spring-loaded diaphragm. A position sensor is mounted on the diaphragm and the ECM uses this sensor to determine whether three-way valve should be activated. Position sensor acts on a contact breaker. Breaker transmits a high or low signal to ECM. Leak diagnostic pump diaphragm is activated if ECM determines that it is in its resting position. The three-way valve is activated creating a vacuum to operate leak diagnostic pump. Three-way valve is deactivated when ECM determines that leak diagnostic pump diaphragm is in activated position. Atmospheric pressure is allowed into diagnostic pump and diaphragm returns to resting position.

Leak diagnostic system operates once per operating cycle under the following conditions:

• Battery voltage is correct.
• Engine coolant temperature and intake air temperature are 39-140°F (4-60°C).
• Engine coolant temperature must have dropped by at least 59°F (15°C) since previous operating cycle.
• Atmospheric pressure must remain constant.