How does the Evaporative Emission System (EVAP) work?
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Here is a list of tools I use every day at the shop. Let me know if you use them too in the comments!
What is the Evaporative Emission System (EVAP)?
The Evaporative Emission System (EVAP) is a system in your vehicle that stores fuel vapors, which then are directed into the engine to be used. Inside the fuel tank, fuel vapors build due to motion and heat. Instead of letting the fuel vapors escape into the atmosphere, which is harmful to the environment, the EVAP system disposes of them in a proper way.
Frequently Asked Question:
Where do I find wiring diagrams and step-by-step repair procedures for EVAP Systems?
What are the different parts of the Evaporative Emission System (EVAP)?
The different parts of the Evaporative Emission System (EVAP) are:
The gas cap
This is where fuel enters the vehicle. The gas cap has a gasket that aids in keeping fuel vapors from escaping the Evaporative Emission System (EVAP).
The purge valve
The purge valve is usually located on or near the engine. It allows a vacuum from the engine to pull fuel vapors into the intake system, to then be used.
The vent valve
The vent valve is usually located under the vehicle near the charcoal canister. The vent valve is a component that allows fresh air into the EVAP system and prevents an excessive vacuum from building up.
The charcoal canister
The charcoal canister is a device filled with activated charcoal, which is used for trapping fuel vapors. When the vehicle is ready, the control module will open the purge valve and allows fuel vapors from the charcoal canister to flow into the intake system.
The vapor (or EVAP) lines
These lines carry the fuel vapors in the system.
Fuel tank pressure sensor
The fuel tank pressure sensor measures the pressure inside the tank and reports it to the vehicle’s control module.
The vehicle’s control module (PCM/ECM)
The control module operates each component at the correct time using factors like engine speed(RPM), outside temperature, and fuel tank pressure. It also monitors the system with tests while the vehicle is running and also after it has shut off.
Many parts of the Evaporative Emission System (EVAP) span the whole car. Vehicle manufacturers have different names for the parts and different designs, but all follow the basic principle.
What are some common problems of the Evaporative Emission System (EVAP)?
Leaks
Leaks in the EVAP are usually due to failed gaskets or o-rings. EVAP leaks will allow fuel vapors to escape into the atmosphere and can usually be smelled outside the vehicle.
Part failures
Over time, EVAP parts like the gas cap, purge valve, and vent valve generally fail because of their moving parts.
Overfilling your tank
“Topping off” your tank can damage the EVAP components. The system is designed for fuel vapors, not fuel. If the fuel level is too high, it could overflow into the EVAP system. It is best to fill your tank until the pump shuts off automatically.
Software updates to the control module
The control module’s software could potentially be in need of an update. A new update could provide a new tolerance for the EVAP system and how the control module monitors it.
Is safe to drive with an EVAP problem?
No, unless a trained professional has inspected the vehicle and reported it safe.
Depending on your vehicle and the size of the leak or problem, it could be hazardous. Fuel vapors can escape or other components could be damaged.
Check out this case study of a 2015 Ford Mustang with an EVAP purge valve malfunctioning. It caused multiple other failures including a hard start. It also pulled the fuel tank into an excessive vacuum. In some cases, vehicles with a plastic tank can be damaged from an excessive vacuum.
What are the most common Evaporative Emission System (EVAP) failures?
Loose or worn gas cap
The gas cap is constantly loosened and tightened. The gasket on the gas cap has a high potential to fail. This can cause a leak to range in size from small to large.
Purge valve
The purge valve is a mechanical device. Over time, it can become stuck or fail completely.
Vent valve
Just like the purge valve, the vent valve is a mechanical device. It, too, can become stuck or fail completely.
Leak
Leaks can be a result of worn gaskets or cracked/broken fuel vapor lines. In most cases, the fuel vapor lines run underneath the vehicle. There is a high chance for fuel vapor lines to becomes damaged due to the road conditions.
Clog
Clogs can happen because of foreign objects entering the EVAP system, such as bugs, dust, or dirt. The charcoal canister can also come apart internally. The activated charcoal inside can clog the fuel vapor lines.
What are some symptoms of a failed Evaporative Emission System (EVAP)?
Fault codes
The vehicle’s control module (PCM/ECM) runs various monitors (tests) that evaluate the EVAP system. It tests the electrical integrity of components and also monitors for leaks. When there is a problem with any part of the system, the control module will set a fault code and illuminate the check engine light.
Fuel odor
In some cases, a fuel odor will be present when there is a leak or a component has failed.
Hard starts after filling up
A common problem with EVAP issues is a hard time starting the vehicle after the fuel tank has been filled.
What are some common fault codes of the EVAP system?
P0442 - Evaporative Emission System (EVAP) Small Leak Detected
A P0442 fault code means that the control module has determined that there is a small leak in the EVAP system. A diagnostic starting point would be to check the gas cap to make sure it is sealed properly or to check the fuel vapor lines for a small leak.
P0455 - Evaporative Emission System (EVAP) Large Leak Detected
A P0455 fault code means that the control module has determined that there is a large leak in the EVAP system. A diagnostic starting point would be to check the purge valve and vent valve for proper operation and that there is not a massive leak in a fuel vapor line.
P0446 - Evaporative Emission System (EVAP) Vent System Performance
A P0446 fault code can be triggered by a vent valve not opening or closing correctly.
What does an Evaporative Emission System (EVAP) diagram look like?
The picture below shows a diagram of an EVAP system.
Fuel Fill Neck and Gas Cap
Vapor Recirculation Line
Vapor Line
Fuel Tank
Vent Valve
Vent Valve Line
Charcoal Canister
Purge Valve Line
EVAP Service Port
Purge Valve
What are the tools used to diagnose the EVAP system?
Scan tool
Many scan tools can provide a diagnostic direction by providing fault codes, but the best scan tools have special functions to directly test EVAP components.
Smoke machine
A smoke machine can fill the EVAP system with smoke and allow a mechanic to see small to large leaks. Here is a link to Amazon to the smoke machine I use for every EVAP leak.
Information
Information is key when diagnosing the EVAP system. Each manufacturer has a specific way they control the EVAP system and where they place the components. A great information system like Alldata and Identifix can provide useful information to diagnose EVAP problems.
Common Hand Tools
Just like most components on a vehicle, the EVAP system can be taken apart with common tools like a screwdriver and socket set. Here is a list of tools I use every day at the shop.
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