How to Find & Fix an EVAP Leak

by Amy RodriguezUpdated July 07, 2023
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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.

So here’s how to identify an EVAP leak – a small leak or a large leak – in your automotive. There are a few options, including using a scan tool, doing an emissions test, using a leak detection pump, or even troubleshooting by doing a diagnostic trouble code of your fuel system. Even just doing a visual inspection of your EVAp canister and filler neck can help you spot a small EVAP leak and avoid having to take your Doge or Toyota into the repair shop. Be sure to check out the intake manifold, the connector of the control valve, and the charcoal canister.

The following will explore the most common causes of an EVAP system leak, how to do leak testing, and how to conduct auto repair on the evaporative emission control system.

Things You Will Need

  • Smoke Machine Tester
  • Ultraviolet Light
  • Wheel chocks
  • Floor jack
  • Jack stands

1. Verify Gas Cap is Tightened.

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.

2. Put Chocks behind rear wheels

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.

3. Locate the EVAP service port adapter

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.

4. Place the smoke machine tester's hose

Place the smoke machine tester's hose into the service port adapter. Turn on the smoke machine by choosing the "Test" mode in the EVAP code or fault code area. This will start the smoke test of the fuel vapors.

5. For 60 Seconds, allow the smoke to fill the EVAP system

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.

6. Run UV light across the underside

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 or vacuum leaks are very common on older vehicles.

7. Replace any leaking or cracked hose

Replace any leaking or cracked hose within the EVAP system. In addition, repair or replace any EVAP purge valve that may emit fumes during a gross leak. Be sure to check the pressure sensor when you’re done with your DIY fix.


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.

Video: How to Diagnose EVAP System Problems and Leaks

Helpful comments from the video:

  • Your videos have helped me to move forward in my automotive career and I greatly appreciate the content.
  • Also don’t buy aftermarket fuel caps. Use oem parts. Thank me later 😎

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