Automotive evap system

What is the EVAP System and How Does it Work?

Modern vehicles contain many systems that take care of things you didn’t 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.

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.

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.

If you’re 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. 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.

What Does an EVAP System do?

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 to escape in the atmosphere. However, that is only a short explanation – 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.

On top of that, any vapor that isn’t 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.

Emission Issues Related to EVAP Systems

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’s why every vehicle produced from 1970 onwards must come with an EVAP system installed.

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.

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’s essential to limit the emissions as much as possible.

If our vehicles didn’t 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’re 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 or doesn’t store the vapors properly, the harmful chemical will be released into the atmosphere while the engine is turned off.

How Does The EVAP System Work?

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’s take a detailed look at each one of those components, what they are used for and what happens when they go bad.

Gas cap

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.

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’s 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.

Fuel tank

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.

Fuel tank pressure sensor

Fuel tank pressure sensors

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.

Evap system: charcoal canister

Charcoal canister

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.

Canister purge valve

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

Vent control valve

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.

Liquid-vapor separator

Located on top of the fuel tank, the liquid-vapor separator splits the vapor from the liquid. If it wasn’t for the separator, the charcoal canister might be overflown with liquid fuel and stop working properly.

EVAP-related Error Codes

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’t 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.

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 a brief description and the most common causes:

EVAP system codes

  • P0440: Evaporative Emission Control System Malfunction
    This code is a generic code letting you know that something is not right with the system. It’s usually linked with a leaking fuel tank or a fuel pressure system malfunction.
  • P0441: Evaporative Emission Control System Incorrect Purge Flow
    This code is letting you know that something is wrong with the flow of recirculated vapor coming out of the purge valve. It’s usually related to a faulty purge valve, a clogged or leaking vapor line or a bad pressure sensor.
    hints to evaporative emission control system incorrect purge flow
  • P0442: Evaporative Emission Control System Leak Detected (Small Leak)
    This is the code letting you know that the EVAP system has failed the pressure test conducted by the PCM. It’S usually related to a leak in one of the vapor lines. Start by inspecting the rubber lines going to and from the purge valve. 9 times out of 10, you’ll find the culprit right away.

Purge valve codes

  • P0443: Evaporative Emission Control System Purge Control Valve Circuit Malfunction
    This code is a generic code letting you know that the PCM has detected an anomaly in the electrical circuit controlling the opening and the closing of the purge valve. There may be a short-circuit in the circuit or a wire may be cut. In all cases, when this code is recorded, you need to get your car’s wiring diagrams out and start troubleshooting.
  • P0444: Evaporative Emission System Purge Control Valve Circuit Open
    A P0444 will be recorded when the EVAP circuit stays open at all times. It might be caused by skinned wires touching each other and causing the purge valve to stay open. Another explanation might be that the purge valve is simply faulty and is stuck open.

OBD2 scan tool

  • P0445: Evaporative Emission Control System Purge Control Valve Circuit Shorted
    A P0445 means pretty much the same as a P0443 if only that this time, the PCM was able to pinpoint that the problem was caused specifically by a short-circuit. This time, look for skinned and damaged wires touching the body of the vehicle.

Canister vent valve

  • P0446: Evaporative Emission Control System Vent Control Circuit Malfunction
    This code is also similar to a P0443 but this time it concerns the canister vent valve instead or the purge valve. The troubleshooting procedure is also the same.
  • P0447: Evaporative Emission Control System Vent Control Circuit Open
    This code and troubleshooting procedure is also fairly similar to a P0444 but for the CVV instead. The Canister vent valve could also be faulty or stuck open.
  • P0448: Evaporative Emission Control System Vent Control Circuit Shorted
    Another one pretty similar to another on this list, this code is like a P0445 for the CVV. Look for damaged wires on the CVV control circuit. Since the CVV is often located under the car on recent vehicles and not inside the engine bay like the purge valve, it’s not rare to find wires damaged by little rodents who chew on them.
  • P0449: Evaporative Emission System Vent Valve/Solenoid Circuit
    This one is also a generic code and will often come up as soon as a problem is detected with the vent valve. This code is usually related to an electrical problem or a malfunction of the valve itself.

Pressure sensor codes

  • P0450: Evaporative Emission Control System Pressure Sensor Malfunction
    P0451: Evaporative Emission System Pressure Sensor/Switch
    P0452: Evaporative Emission Control System Pressure Sensor Low Input
    P0453: Evaporative Emission System Pressure Sensor/Switch High
    P0454: Evaporative Emission Control System Pressure Sensor Intermittent
    As the definition suggests, these codes are related to a fuel pressure sensor malfunction. It could be the sensor itself or an open circuit condition. Each code is associated with a specific troubleshooting procedure to help you speed up the process.

Leak-related codes

  • P0455: System Gross Leak Evaporative Emission
    This code tells you that there’s a large leak into the EVAP system. The most common cause is an incorrectly positioned gas cap. One of the fuel vapor lines could also be completely disconnected.
  • P0456: Evaporative Emission System Leak Detected (very small leak)
    This one is the same as the P0455 if only that this time the leak is quite small. This code can easily be the hardest one to troubleshoot on this list. Small leaks are obviously a lot more complicated to find than large ones and often require the use of a smoke machine.
  • P0457: Evaporative Emission System Leak Detected (fuel cap loose/off)
    The last one on the list is one of the easiest ones to fix. It is usually related to a loose or missing fuel cap. If the seal on the cap is worn off or damaged, it can also cause this code to be recorded.

Does a faulty EVAP System Affect Performance?

A faulty EVAP system component won’t directly impact performance, or at least not right away. When a problem occurs, modern cars are smart enough to detect the malfunction and redirect more gasoline from the primary fuel delivery system. As soon as the PCM detects a problem, it will usually simply cancel the system and run without it. However, driving with a canceled EVAP system might significantly reduce the fuel-efficiency of your vehicle.

Moreover, if the system simply stops working, the PCM can just forget about it and keep running. On the other hand, if the faulty component keeps the EVAP system continually open or if a bad sensor tells the PCM to open it at the wrong time, you might experience engine issues. For example, the engine might not start instantly or it could run rough at idle. Moreover, the check engine light will illuminate, and the car will almost inevitably fail emission tests. Finally, if there is a leak in the system, the interior might start smelling like gasoline or diesel fuel. And that’s not fun – these fumes are noxious.

 In Brief…

Modern EVAP systems are very reliable, at least in the first 5-6 years of the vehicle’s lifetime. However, almost every part of the system can fail eventually. For example, the activated charcoal inside the canister might degrade over time, or one of the valves might start leaking. And usually, there is no telling when this might happen.

Fortunately, you can take care of the EVAP system by using high-quality gasoline or diesel fuel. Moreover, you should never overflow the gas tank, especially if you leave the car stationary for longer. That way, you’ll only introduce more pressure to the system, which can damage some of the parts. In other words, only top up the tank when you’re sure that you will use the car.

In all cases, if you run into a problem with the EVAP system on your car, we strongly suggest you get a copy of your car’s repair manual before starting to test everything. Using the troubleshooting procedures found in repair manuals is always the fastest and easiest way to locate and fix car problems, and especially on tricky systems like the EVAP system.

About Ivo G

Hey guys and girls, I’m Ivo, a passionate car enthusiast with a particular weakness for rear-wheel-drive and stick-shift gearbox cars. Meanwhile, I don’t mind getting my hands dirty – I have experience as a Certified Head of Service at a Toyota Dealership, and knowledge that I wish to share with you.

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