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How to Find and Fix an EVAP Leak

Time:2014-05-22 Click:1956
Vehicles driven on public roadways must typically pass emissions tests before the car can be registered with the state of residence. Emissions exude from the car's exhaust pipe. The fumes are by-products of the gasoline combustion within the engine. Each vehicle is equipped with a system called EVAP (Evaporative Emission Control System) for reducing the emissions emitted into the Earth's atmosphere. However, the EVAP system may require troubleshooting if it detects a leak.


1. Verify that the gas cap is firmly tightened onto the gas tank entry point. The EVAP system includes the fuel tank. As a result, a loose gas cap is translated by the EVAP system as a leak, causing the "Check Engine" light to illuminate.

2. Lift the vehicle's front end with two car jacks. Chock the rear wheels for preventing any slippage.

3. Locate the EVAP service port adapter within the engine's compartment. Typically, the port is near the engine's front on the passenger side. The port has a valve and supply hose protruding outward.

4. Place the smoke machine tester's hose into the service port adapter. Turn on the smoke machine by choosing the "Test" mode.

5. Allow the smoke to fill the EVAP system for approximately 60 seconds.

6. Visually inspect the EVAP system by running the UV light across the vehicle's underside, following the system's path from the engine compartment to the rear fuel tank. The UV light will illuminate any leaks along the system since it will react with the smoke. As the smoke exits through a leak, the UV light will show an obvious plume of air.

7. Replace any leaking or cracked hose within the EVAP system. In addition, repair or replace any EVAP purge valve that may emit fumes.

Tips & Warnings

Some leaks can be large or as small as a needle prick. Carefully inspect the EVAP system during the smoke testing. A tiny leak can easily go unnoticed, eventually becoming worse as the vehicle is driven.

Use caution while locating the leak underneath the vehicle. Do not bump the chocks or jacks. Fatal injury can occur if the vehicle falls off of the supporting tools.