Evaporative Emission Control (EVAP) System

What is the Evaporative Emission Control (EVAP) System?

The Evaporative Emission Control (EVAP) System seals the fuel system of the vehicle in order to prevent fuel vapors from the fuel tank and fuel system from escaping into the atmosphere. This is important because fuel vapors contain a variety of hydrocarbons. Hydrocarbons form smog when they react with air and sunlight. Gasoline evaporates very quickly, so if the fuel system is open to the atmosphere a vehicle can pollute 24 hours per day without even being turned on. These uncontrolled evaporative emissions account for as much as 20% of the pollution produced by a vehicle!

evap system

EVAP System Components

The major components of the EVAP system are:

Fuel Tank:

The fuel tank obviously stores the gasoline when you fill up. But, you know when people tell you not to continue filling the tank after the pump automatically stops? That is because the tank has some expansion space at the top so the fuel can expand without overflowing or forcing the EVAP system to leak.

Gas Cap:

Tighten until click. The gas cap seals off the filler neck of the gas tank from the outside atmoshpere. Damaged or missing gas caps are the most common cause of EVAP system failure codes that trigger the check engine light.

Liquid-Vapor Separator:

This prevents liquid gasoline from entering the EVAP canister, which would overload its ability to store fuel vapors.

EVAP Canister:

This canister is connected to the fuel tank by the tank vent line. The EVAP canister houses 1-2 pounds of activated charcoal that acts like a sponge by absorbing and storing fuel vapors until the purge valve opens and allows the vacuum of the engine intake to siphon the fuel vapors from the charcoal into the engine intake manifold.

Vent Control Valve:

This allows the flow of the fuel vapors from the fuel tank into the EVAP canister.

Purge Valve/Sensor:

Allows engine intake vacuum to siphon the precise amount of fuel vapors from the EVAP canister into the engine intake manifold.

Vent Hoses:

The means by which the fuel vapors flow to different components of the EVAP system.

Fuel Tank Pressure Sensor:

Monitors the pressure within the fuel tank for leaks and to make sure too much pressure does not build.

Fuel Level Sensor:

Monitors the level of fuel in the tank.

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Comments 16

J. Crowe

September 10, 2018

Where would the evap components be located in reference to the gas tank/engine?

FIXD Marketing

September 19, 2018

Hey J!
Your evap components are located in the EVAP cannister, which is towards the rear of your muffler.


May 11, 2019

What could cause rough starting, and trouble idling


May 25, 2020

Hey, the technician changed the evap and asked me to drive the vehicle for 75 miles until the car is ready for inspection. After driving 75 miles, I took the car, but the technician said it was not ready. You have to drive the car more miles until it becomes 100 miles.


September 25, 2018

The tighten gas cap message came up on my 2010 impala for the first time yesterday morning (sep 24, 2018). i opened the cap and closed it tightly. the message still came on. this morning, in addition to the message, the check engine light is on.
in the research i have done, i see where people have replaced the cap multiple times and they still have the issue.
i have 114,000 + miles on my car, so i’m suspicious that the issue is not the gas cap.
what should be my next move? I don’t want to take it to the dealer only to have them replace the gas cap and charge me an arm and a leg. any suggestions are much appreciated.

Anthony E

February 2, 2020

I’d say disconnect battery (resets check engine light) and reconnect another new cap (just in case new one was faulty) then reconnect battery and give time with driving to for car to acknowledge new cap install…-DIY’er


December 20, 2018

What should your Evap vapor pressure be?


January 17, 2019

I replaced the evap canister on my 2001 Chevy Tahoe then cleared the service engine light drove it around for two weeks and now the service engine soon light came back on reading code P0440 again. What else can the problem be. I also changed the gas cap

Anthony E

February 2, 2020

Could be that fuel pressure sensor unit behind back passenger seat/or trunk connected to fuel tank

Howard Duhaime

January 28, 2019

I have a 70 camaro that sat for many years and I plan on changing the gas tank and I am going to run a fast efi fuel injection system. I see gas tanks on ecklers one with the evap and one without ,which one should I buy .


September 13, 2019

If liquid-vapor separator protects the charcoal canister from liquid gas entering, then why is it bad to overfill the gas tank?

Terrill Danner

May 3, 2019

Can a catilac converters make a transmission go bad. I have a p0446 eva emissions system vent circut on. I changed the canister. I took it to midas n they said i didn have enough pressure in the fuel pump or something but its startin to run slow up hills n back fire a lil bit. I just took it to Amaco transmission n they tellin me i need a new tranny n converters i just had those done 8 months ago. I’m lost for words

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August 4, 2019

I am very happy to read this. This is the kind of manual that needs to be given and not the random misinformation that is at the other blogs. Appreciate your sharing this best doc.


September 13, 2019

If liquid-vapor separator protects the charcoal canister from liquid gas entering, then why is it bad to overfill the gas tank?


June 10, 2020

I have a 2006 GMC Sierra sle and my diagnostic tester shows Evap. leak. How do I find the leak?

Jeevan Naik

June 21, 2020

what could be the pressure in EVAP vapor cannister ?


October 23, 2020

Most problems will be caused by a faulty gas cap or cracked and leaking rubber hoses in the evap system.

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