EVAP Pressure Testing Using Special Test Equipment

The best way to test and identify leak(s) that cause a pressure test failure is to use special EVAP pressure testing equipment available from aftermarket suppliers. This equipment allows you to perform an actual pressure test, in addition to having features that help you locate the leak. There are many variations and differences between test equipment and procedures, but for the sake of example, here is the test procedure for an EVAP pressure tester that uses pressurized nitrogen gas:

1. Disconnect the fuel tank vapor line from the canister and attach the pressure tester to this line.

Note: The tester may have an adapter that allows you to connect the pressure line between the tank filler neck and the fuel cap.

2. Activate the tester and pressurize the line until 14 inches of water pressure is maintained.

3. Observe the pressure gauge and note if the pressure begins to drop.

Note: It is normal for pressure to initially rise or fall slightly then stabilize after a few seconds. This is caused by the initial temperature variation between the nitrogen and EVAP fuel vapors. Once temperatures stabilize, the pressure will equalize if no leak exist.

4. If the pressure drops dramatically, listen for leaks from the fuel cap, tank seams, and hoses.

5. Check for frayed or cracked hoses, poor connections, damaged fuel tank seams, faulty fuel cap gasket or check valve.

6. The leak may be found by spraying the suspected area with soapy water and looking for bubbles.

7. Special ultrasonic leak detectors are now available that can "listen" for the exact frequencies caused by these low pressure leaks. Another method uses the exhaust analyzer to check for the escape of fuel vapors (HC) from the leaky part/component.

Note: The drawback to using the exhaust analyzer is the limited amount of fuel vapors that exist in EVAP system (fuel tank). If the leak is not quickly identified, all HC vapors will escape leaving only a nitrogen (inert gas) leak to locate.

8. If the leak cannot be identified by the completion of the test, select the manual mode that provides a constant pressure on the system.

9. Once the leaky part/component is identified, perform the needed repair or replacement.

Do It Yourself Car Diagnosis

Do It Yourself Car Diagnosis

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