{"id":3569,"date":"2020-04-16T11:33:26","date_gmt":"2020-04-16T18:33:26","guid":{"rendered":"https:\/\/www.emanualonline.com\/blog\/?p=3569"},"modified":"2025-05-08T04:01:08","modified_gmt":"2025-05-08T09:01:08","slug":"what-is-evap-system","status":"publish","type":"post","link":"https:\/\/www.emanualonline.com\/blog\/what-is-evap-system","title":{"rendered":"What is the EVAP System and How Does it Work?"},"content":{"rendered":"

Modern vehicles contain many systems that take care of things you didn\u2019t even know existed. Take the EVAP (Evaporative Emission Control System), for example. Aside from experienced mechanics, most people have no idea what the EVAP system does and how it works. And even if you know what it does, repairing a potential EVAP problem can be a nightmare.<\/p>\n

Like most other systems in your vehicle, the EVAP is crucial for proper operation. If not correctly taken care of, many issues can arise regarding the performance of the car. Even worse, the same problems can occur when other systems are faulty, which makes it even harder to find the culprit. Moreover, vehicles with faulty EVAP systems will often fail emission tests.<\/p>\n

Fortunately, you are just in the right place to learn everything there is to know about the EVAP system. In this article, we will explain the role of the EVAP system in modern cars and each of its components. Furthermore, you will learn how the system works and the potential issues that may arise from a faulty EVAP component.<\/p>\n

If you\u2019re an enthusiast that wants to work on your own vehicle, we will also give you the usual EVAP-related OBD-II error codes and where to start the troubleshooting process. If it’s your first time dealing with an EVAP system problem, we strongly recommend you get your hands on your car’s repair manual first<\/a>. You’ll find a bunch of information that should help speed up the troubleshooting process. Repair manuals often include troubleshooting charts and procedures to test each component to quickly locate where the problem is.<\/p>\n

What Does an EVAP System do?<\/h2>\n

But first, to be able to find a problem with a specific system, you need to understand what it does and how it works. The EVAP system is used to prevent fuel vapors from the fuel tank from escaping into the atmosphere. However, that is only a short explanation \u2013 there is much more to it. For example, it is also crucial that the system redirects the vapors into the combustion chamber at the right time. That way, instead of polluting the atmosphere, the EVAP system uses the fumes to power the engine partially.<\/p>\n

On top of that, any vapor that isn\u2019t used in the combustion chamber is wasted fuel. For the vehicle owner, that means increased fuel consumption and possibly some performance issues. By re-routing the vapors inside the cylinders, the EVAP extracts even the last droplet of gasoline from the fuel used.<\/p>\n

Emission Issues Related to EVAP Systems<\/h2>\n

The reason why the EVAP system needs to keep the vapors from escaping is to reduce harmful emissions. As always, the main culprits for polluting the atmosphere are fossil fuels. That\u2019s why every vehicle produced from 1970 onwards must come with an EVAP system installed.<\/p>\n

Gasoline, for example, contains many substances that can severely damage the atmosphere. Yes, fossil fuels harm the atmosphere even after burning in the combustion chamber, but at a much lower rate.<\/p>\n

Unburned fuels contain aldehydes, which can instantly create smog in the surrounding air. Moreover, gasoline and diesel also contain aromatics, olefins, and higher paraffin, which can further harm the environment. Some of these substances are poisonous if inhaled, another reason why it\u2019s essential to limit the emissions as much as possible.<\/p>\n

If our vehicles didn\u2019t have EVAP systems, every major city would have major smog issues by now. Furthermore, a faulty EVAP system will pollute the air even when you\u2019re not driving your vehicle. Why? To keep it simple, the EVAP system can’t just re-route the fuel vapors into the intake system at any moment. Fuel vapors can only be injected back into the system when specific conditions are met. For example, the engine needs to be at normal operating temperature, the vehicle must be driving at a constant speed and without load, etc. To do so, the charcoal canister stores fuel vapor for future use. If the EVAP system leaks<\/a> or doesn’t store the vapors properly, the harmful chemical will be released into the atmosphere while the engine is turned off.<\/p>\n

How Does The EVAP System Work?<\/h2>\n

The EVAP system consists of several parts used to keep the fuel vapors from escaping, store them, and re-route them back into the engine. Let\u2019s take a detailed look at each one of those components, what they are used for and what happens when they go bad.<\/p>\n

\"Gas<\/p>\n

Gas cap<\/h3>\n

Everybody who owns a car knows what’s a gas cap. However, the gas cap is the most often neglected component on this list. Contrary to popular beliefs, it’s not only used as a lid for the filler neck. It’s also essential to the proper function of the EVAP system.<\/p>\n

Each gas cap has a seal that keeps the vapors from escaping feely out of the tank. Besides, it also forms a tight seal, closing the EVAP system and allowing positive pressure to build up in the system. Naturally, it\u2019s vital that you properly close the gas cap each time you refuel your vehicle. Omitting to correctly screw the gas cap will cause the check engine light to come up, and a DTC code will be recorded.<\/p>\n

Fuel tank<\/h3>\n

Another integral part of the EVAP system is the fuel tank. It stores not only gasoline or diesel fuels but also a large quantity of fuel vapor. For optimal operation, it is crucial to never overfill the fuel tank. Otherwise, the EVAP system might be overloaded with fuel and emit vapors in the atmosphere.<\/p>\n

\"Fuel<\/p>\n

Fuel tank pressure sensors<\/h3>\n

These sensors are used to measure the pressure inside the tank and the amount of fuel. Apart from notifying the driver of the amount of fuel, these sensors give the ECU information on the pressure inside the tank. They are also the sensors responsible for notifying the driver if an anomaly or a leak is detected in the system.<\/p>\n

\"Evap<\/p>\n

Charcoal canister<\/h3>\n

The charcoal canister is usually a small plastic box located near the fuel tank or inside the engine bay on older cars and filled with activated charcoal used to store the fuel vapors. The activated charcoal acts as a sponge here, capturing the gases and storing them for later use.<\/p>\n

\"Charcoal<\/p>\n

Canister purge valve<\/h3>\n

The canister purge valve, also called the canister vent valve depending on the manufacturer, is used to trap the vapors inside the charcoal canister or release them into the air intake. The purge valve is essentially a solenoid that opens up the fuel vapor lines when electricity is applied. The PCM controls the opening and closing of the canister purge valve and opens it up when the right conditions are met. The solenoid is by far the most frequently replaced part of the EVAP system<\/p>\n

Vent control valve<\/h3>\n

The vent control valve, sometimes also called the canister close valve, is used, as its name implies, to close the charcoal canister. this function is mostly used by the PCM to test the EVAP system for leaks.<\/p>\n

Liquid-vapor separator<\/h3>\n

Located on top of the fuel tank, the liquid-vapor separator splits the vapor from the liquid. If it wasn\u2019t for the separator, the charcoal canister might be overflown with liquid fuel and stop working properly.<\/p>\n

EVAP-related Error Codes<\/h2>\n

As you can see, many parts need to work in tandem to ensure the proper operation of the EVAP system. If any of the parts don\u2019t work correctly, many issues can arise, often related to the performance of the car. That said, some of the symptoms can often refer to other faulty systems or parts.<\/p>\n

Fortunately, you can easily detect a problem with the EVAP system with an OBD-II reader. Here are all the error codes that are EVAP-related as well as a brief description and the most common causes:<\/p>\n

EVAP system codes<\/h3>\n