Evaporative Emission Control System (EVAP)

It is our great pleasure to take a closer look at EVAP; namely, Evaporative Emission Control System, with you today.

Evap leak how to fix

Truthfully, many people do not realize how complicated their car is. Today’s cars contain several hundred systems and sensors that monitor everything from road conditions to engine performance and everything in between.

All these systems make it safer and easier for you to drive your car. That’s why it is important to maintain your car and take the necessary action when you are alerted to a problem. These systems are designed to ensure your safety and protect the environment at the same time. One of the systems designed to protect the environment is the Evaporative Emission Control System or EVAP.

Key Takeaways

  • Evaporative Emission Control System is responsible for preventing gasoline vapors accumulated in a fuel tank from escaping into the atmosphere.
  • Some of the most important components of an EVAP system include the fuel tank, fuel pump, EVAP canister, leak detection pump, and EVAP vent lines and hoses.
  • The most common issue that often triggers an engine light is an EVAP leak.
  • Most EVAP malfunctions do not cause substantial car damage, allowing drivers to continue using their vehicles safely.
  • You can perform a self-test of the EVAP system using a scan tool to check the condition of the valves, solenoids, and other EVAP system components

What Is EVAP?

Most people learn about the Evaporative Emission Control System when they forget to replace or do not tighten the gas cap after refueling. The EVAP system is responsible for capturing vapors in the fuel tank and preventing them from escaping into the environment.

Since it’s a completely closed system, any small leak will cause a fault resulting in the engine light coming on. It’s common for drivers to not even notice a problem because small leaks usually don’t affect the way the car drives. The most common signal that there’s a problem with the EVAP system is the annoying glowing check engine light.

How Does Evaporative Emissions Control System Work?

Believe it or not, the fuel we burn contains over 100 chemicals which are very harmful to the environment. The reason to add these chemicals is to enable the fuel to burn more cleanly and pollute less. Gasoline vapors are very dangerous to humans and are one of the leading causes of smog and air pollution.

For this reason, car companies had to develop a system that would prevent these harmful vapors from polluting the environment. This Government mandate was implemented in the ’70s. It stated that the EVAP system must collect, store and dispose of gasoline vapors before they could impact the environment.

The typical system is simple in design but somewhat complex in operation. It includes a canister full of charcoal, several valves, lots of hoses and vents and, of course, the sealed gas cap. The system is closed so that any excess fuel vapors are collected and stored in the charcoal canister. These vapors will eventually be added to the normal fuel/air mixture and burned during combustion.

What Are the Components of EVAP?

There are several components that comprise the EVAP system. The main component is the gas tank which is where the vapors come from. There are sensors in the gas tank that measure both the amount of fuel and the tank pressure.

On newer cars, the gas tank may also include the fuel pump, filter and pressure regulator, as manufacturers like to group these components together. The gas tank also uses a sealed gas cap which is where 99% of the leaks come from.

From the gas tank, the fuel vapors are vented to the charcoal canister, where they are stored. The canister is basically a big lump of charcoal that absorbs the vapors and prevents them from escaping into the atmosphere. 

The computer controls the canister purge valve, which enables the stored vapors to be added to the normal air/fuel mixture. There is also a canister vent valve that prevents outside air from entering the system during testing.As you can see, the system is not overly complex, but all valves and sensors must work perfectly so that the computer can control the process. Any small leak or failed sensor will generate a problem and, in turn, will illuminate the check engine light and EVAP system codes.

Since the EVAP system is a closed system, any small leak or failed sensor will generate a system fault. Obviously, the indication that there is a problem will start with the check engine light.

You typically will not notice any change in the performance of the car due to an EVAP leak. Let’s look at some symptoms, fault codes, fixes and things that you can do to prevent problems.

EVAP System Leak Symptoms

  • Check engine light is on
  • Fuel odor
  • Rough engine idle
  • Stalling
  • Poor gas mileage
  • Hesitation when accelerating

If the check engine light is on, you won’t know if it’s the EVAP system or something else until you identify the fault code. To get that code, you can go to an auto parts store or take your car to the dealer.

Some of the common fault codes associated with the EVAP system are as follows:

  • P0440 Evaporative Emission Control System Fault diagnostic trouble code means that a leak has been detected or the vapor pressure sensor in the fuel tank has malfunctioned. One indication might be a faint odor of gasoline originating from the gas tank.
  • P0445 EVAP Purge Control Valve Circuit Shorted code means that the EVAP purge valve is malfunctioning by either being stuck open/closed or shorted out. Some of the symptoms of this problem are fuel odor, rough engine idle, hesitation or stalling.
  • P0449 EVAP Emission Control System Vent Valve/Solenoid Circuit diagnostic trouble code means that the EVAP vent control valve is malfunctioning due to an electrical issue (high resistance) or mechanical issue (sticking valve). One obvious symptom may be a noticeable fuel odor but usually no change in car performance.
  • P0455 EVAP Emission Control System Leak Detected (gross leak) code means that there is a huge leak in the system. There may be a noticeable odor but usually no performance issues. When this error code is triggered, it usually means that there is a large hole somewhere. Something like a hole in the canister or a missing gas cap would be good examples.
  • P0441 Evaporative Emission Control System Incorrect Purge Flow code means that the computer has detected that the flow through the purge valve is incorrect. Again, this could be due to a faulty or punctured evaporative system hose or an electrical connection to the purge valve. It could even be caused by a faulty vacuum switch, so all possibilities need to be considered.
  • P0442 EVAP Emission Control System Leak Detected (small leak) code means that there is a small leak in the EVAP system. The gas cap is usually the culprit, but a pinhole in a hose or a loose-fitting hose could also cause this. One symptom of this type of leak might be reduced gas mileage.
  • P0446 Evaporative Emission System Vent Control Circuit diagnostic trouble code means that the computer has detected a short in the vent control valve circuit. A symptom of this type of leak is reduced gas mileage.
  • P0440 Evaporative Emission Control System Fault code means that a vapor pressure sensor has malfunctioned or there is a leak in the fuel tank vapor system. A common problem is a leak in the fuel tank vapor lines or a loose gas cap that doesn’t align well with a fuel tank filler neck. You may also notice a fuel smell if the leak is large enough.
  • P0452 Emission Control System Pressure Sensor Low Input code means that the computer has detected a voltage issue with the EVAP system. The measured voltage must be within a certain range, or else system components could be affected. The major component that would be affected by low voltage is the fuel tank pressure sensor.

How To Fix an EVAP Leak?

As far as we know, EVAP leaks are usually identified by a computer error code. The truth is any car code reader can reveal those codes. Unfortunately, the error code may point you in the right direction but will not usually pinpoint the exact problem.

Finding and fixing the source of the problem requires a process of elimination. In some cases, an accident will damage one of the components of the EVAP system. This type of damage may not appear right away but may take some time to show itself. Finding and fixing an EVAP system leak often involves patience and experience by a trained technician.

Causes of EVAP Leaks

Some of the problems that will cause an EVAP system problem include the following:

  • Loose-fitting, wrong or missing fuel cap
  • A leak in the charcoal canister
  • A leak in any of the hoses that interconnect the system components
  • The leak in the fuel tank
  • Sticking purge valve or vent valve
Check the gas cap if it’s loosed.

What to Do With EVAP Leak: EVAP Maintenance Tips

Due to the design of the system, eliminating every source of leaks is a daunting task. As we have pointed out, the gas cap is the cause of 99% of the problems. Many people do not realize the importance of this simple component. Just making sure it is right and tight will go a long way to keeping the system operating correctly.

Obviously, components will fail over time, so ensuring that your car is maintained on a regular basis is important. For example, at your next oil change, ask the technician to do a visual inspection of the EVAP system. They may spot a cracked hose or crack in the canister that can easily be fixed on the spot.

If the visual inspection does not provide conclusive results, a technician may also perform something called a leak test. There is more than one way to conduct a leak test. Some prefer to use smoke to detect any leaks in the structure of the system, while others may turn to more traditional means and spray EVAP components with a soapy solution in order to locate potential leaks.

Bottom Line

As with any system in your car, the EVAP system has an important job to do, protecting the environment.

From the moment fuel enters the tank until it is burned and discharged into the environment, your car is in control. For this reason alone, it is important to maintain your car’s systems and not ignore any warning signs of a problem.We are all in this together, and we need to do our part to protect the environment, one car at a time.

Evaporative Emission Control System FAQs

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