EVAP System Pressure Test Diagnosis

The Enhanced I/M Evaporative Pressure Test is performed by filling the EVAP vapor line and fuel tank with nitrogen to a pressure of 14 inches of water (approximately 0.5 psi). If the system maintains at least 8 inches of water pressure after 2 minutes, it passes the test.


Because of difficulty regulating compressed air, and the hazardous nature of introducing oxygen to stored fuel vapors, never attempt to pressurize the EVAP system using shop air!

If the EVAP system fails the pressure test, a leak exists either in the vapor vent line between the canister and tank, the fuel tank itself, or the fuel cap. Visual checks may or may not identify the source of leak(s) in the system; however, you should never pressurize the EVAP system with shop air! Doing this would introduce oxygen into the EVAP system were it could combine with fuel vapors and create a very explosive condition. Secondly, the system is tested at very low pressure which would make accurate, pressure regulation difficult. If the system was accidentally pressurized beyond this point, severe damage to the system may result.

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