How to Find & Fix an EVAP Leak

by Amy Rodriguez
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Matt Cardy/Getty Images News/Getty Images

Of all the ways in which a vehicle can fail, all the things that can trigger a check-engine light, perhaps none are so infuriating as the "EVAP System Malfunction." It just doesn't seem fair that this system, this thing that your car doesn't even need to run, should cause so much aggravation. But evaporative emissions -- gasoline evaporating out of the fuel tank -- are amazingly expensive things. Think about how fast a small puddle of gasoline will evaporate from a hot sidewalk, and you'll have some idea of how much money is just floating away through your leaking EVAP system. Now, that really IS infuriating.

Step 1

Verify that the gas cap is firmly tightened onto the gas tank entry point. The EVAP system also monitors the gas tank, so an open gas cap can be the largest -- and perhaps only -- leak in the system. Leave the fuel filler door open after you tighten the cap.

Step 2

Kick a pair of chocks behind the rear wheels, and lift the front end of the vehicle with a floor jack far enough that you can fit underneath. Secure it on a pair of jack stands.

Step 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. You'll see a valve and supply hose protruding outward.

Step 4

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

Step 5

Allow the smoke to fill the EVAP system for approximately 60 seconds, and dim the lights in your work area. Complete darkness would be preferred, if not for the obvious danger of hitting your face on a car.

Step 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. Any smoke leaking from the system will illuminate in the ultraviolet light. Make sure to check the fuel cap; cap seal failures are very common on older vehicles.

Step 7

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

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