Evaporative Emission Control System (EVAP)

It is a 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 Emissions Control System or EVAP.

What is EVAP?

Most people learn about the EVAP system when they forget to replace or do not tighten the gas cap after refueling. The EVAP system with an aim to capture vapors in the fuel tank and prevent 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. You probably will not even notice a problem because a small leak won’t affect the way the car drives. About the only thing you will notice 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.

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.

Symptoms of bad evap or evap leak?

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

If the check engine light illuminates 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 into the dealer.

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

  • P0440 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 code means that the EVAP purge valve is malfunctioning by either being stuck open/closed or has shorted out. Some of the symptoms of this problem are fuel odor, rough engine idle, hesitation or stalling.
  • P0449 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 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 code means that the computer has detected that the flow through the purge valve is incorrect. Again, this could be due to faulty hoses 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 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 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 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. You may also notice a fuel smell if the leak is large enough.
  • P0452 code means that the computer has detected a voltage issue with the EVAP system. The measured voltage must be with 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. To find and fix 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 may lead to EVAP Leaks

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

  • Loose-fitting, wrong or missing gas 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.

Recommendation of 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.

Bottom line

As with any system in your car the EVAP system has an important job to do, protecting the environment. If you look at the systems and processes that make up the design of your car, you will see that most are focused on 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 out part to protect the environment, one car at a time.

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