Evaporative emissions (EVAP) purge valve

By Reinier (Contact Me)
Last Updated 2021-07-23
Automobile Repair Shop Owner

What Does the Evaporative emissions (EVAP) purge valve Do?

All EVAP valves (Evaporative Emissions Purge Valve(s)) have two functions. The first is to allow for accumulated fuel vapors in the fuel system to be purged from the fuel tank/fuel system, and the second is to assist in sealing the fuel system during leak testing procedures that are performed by the PCM (Powertrain Control Module).

Why is the Evaporative emissions (EVAP) purge valve Needed?

Numerous studies have shown and confirmed that vapors arising from fuel that evaporates from cars’ fuel tanks account for more than 60% of the total harmful emissions a car generates during its lifetime.

Therefore, to address this issue, carmakers have developed systems to contain fuel vapors and to keep them contained until they can be transported to the engine through hoses to be combusted along with the regular air/fuel mixture. Below is a generic description of evaporative emission control systems-

All EVAP systems contain the following components-

• Sealed fuel filler cap
• Charcoal-filled canister to absorb and contain fuel vapors
• Vent valve to allow for easy purging of the collected fuel vapors
• Purge valve to allow for the periodic purging of collected fuel vapors from the system
• Hoses and vacuum lines to transport fuel vapors to the engine
• Fuel vapor pressure sensor
• Position sensors to relay the status of both the vent and purge valves to the PCM

Depending on the vehicle and the EVAP system control system, some EVAP systems also contain the following components-

• Dedicated air pump to pressurize the fuel/EVAP system
• Dedicated vacuum pump to evacuate air from the EVAP system
• Sensors to measure the temperature of either (or both) the liquid fuel and fuel vapors

As a practical matter, in their present form, EVAP purge valves offer the simplest, and most efficient method to not only purge accumulated fuel vapors from any fuel system but also to direct accumulated fuel vapors to the engine to prevent them from escaping into the atmosphere. In fact, a fully functional EVAP purge valve is a critically important component in any vehicle’s overall emission control system, which explains why these valves will always set fault codes and/or illuminate warning lights when they fail.

At the risk of putting too fine a point on it, the fact is that without a working EVAP purge valve, the entire EVAP system becomes inoperable in terms of containing and controlling fuel vapors.

How Does the Evaporative emissions (EVAP) purge valve Work?

This simplified schematic shows the basic layout of the EVAP system on a Ford application. In this example, the EVAP purge valve is circled in red, while the green arrow indicates the direction of the flow of fuel vapor toward the engine.

Although there are some differences in the design specifics between EVAP purge valves, all EVAP purge valves are simple solenoid valves that open when the PCM energizes their control circuits and closes under spring tension when the PCM removes power from the control circuit.

Note though that while some EVAP purge valves incorporate a position sensor that relays the position (open or closed status) of the solenoid pintle, other designs use electrical parameters such as the current drawn by the solenoid coil to determine whether the valve or open or closed.

In practice, the fuel system is sealed during the normal operation of the vehicle. However, as the fuel moves around the fuel tank, or when fuel is returned to the tank from the injectors, the fuel generates large volumes of vapors that are absorbed by the charcoal in a dedicated canister.

When the pressure in the fuel tank generated by the fuel vapors approaches or exceeds a predefined threshold, the pressure sensor communicates this to the PCM, which then initiates a process to purge the collected vapors from the fuel system. Here is how it works-

• The pressure sensor alerts the PCM that the pressure in the tank has reached its maximum allowable threshold
• The PCM opens the normally closed vent valve and opens the normally closed purged valve
• Atmospheric pressure forces air into the EVAP system through the now-open vent valve, which “pushes” collected fuel vapors from the charcoal canister
• The fuel vapors then pass through the now-open purge valve and are transported to the intake manifold through dedicated lines and hoses to be combusted along with the regular air/fuel mixture
• When the fuel vapor pressure sensor detects that the excess pressure in the fuel tank had been vented, it relays this information to the PCM
• At this point, the PCM closes both the vent valve and the purge valve to reseal the fuel system, to allow the charcoal canister to begin absorbing another load of fuel vapor

In a fully functional EVAP system, the purge valve will always be closed, except for when vapor purging is in process, or when the PCM on some systems opens it to allow the engine vacuum to create a vacuum in the fuel system. On these designs, the PCM monitors the vacuum thus created to check for leaks in the system. Thus, if the vacuum decays after the purge valve is closed again, the PCM measures the rate of decay via the fuel vapor pressure sensor and sets one or more fault codes, with the actual fault code being set depending on the rate of decay.

If the vacuum holds steady for a predefined period, the PCM concludes that the fuel system has no leaks, and will allow the EVAP readiness monitor to run to completion.

Where is the Evaporative emissions (EVAP) purge valve Located on the Engine?

This image shows the location (arrowed) of the EVAP purge valve mounted on the firewall of a Ford Explorer. Note though that while the actual location of EVAP purge valves varies greatly between vehicle makes and models, all EVAP purge valves are always located in a vapor line that runs between the EVAP charcoal canister and the engine, and more specifically, between the charcoal canister and the intake manifold.

However, most modern vehicles have multiple lines and hoses connecting the various components of the EVAP system, and it is, therefore, easy to disconnect the wrong line or hose as a result. Nonetheless, since EVAP purge sensors are rarely located on the actual fuel tank, one aid in identifying the purge valve is to look for a line between the charcoal canister and the intake manifold that “runs through” a solenoid valve.

Note though that on some modern vehicles, the EVAP purge valve is located under the vehicle, and you may have to remove splash covers and shields under the vehicle to access all the lines, hoses, and pipes under the vehicle.

What Does the Evaporative emissions (EVAP) purge valve Look Like?

This image shows an example of an EVAP purge valve intended for use on some Ford applications. Note that the design of EVAP purge valves/solenoids does not follow a general pattern, and as a result, the appearance of EVAP purge valves varies greatly between different vehicle makes and models.

Therefore, we recommend that if you do not have access to repair manual or OEM-level service information for your particular vehicle, you seek professional assistance with the identification, diagnosis, and/or replacement of a defective or malfunctioning to prevent the possible unnecessary replacement of expensive EVAP system parts and components.

What are the Symptoms that the Evaporative emissions (EVAP) purge valve is Bad?

Typical symptoms of a bad or failing EVAP purge valve are much the same across all applications, and could include one or more of the following-

• Stored trouble code(s) and illuminated warning light, but note that the actual stored code(s) depends on the nature of the problem. Here are some examples of common fault codes relating to EVAP purge valve failures and/or malfunctions-

• P0440 – “Evaporative Emission Control System malfunction”
• P0443 – “Evaporative Emission Control System Purge Control Valve Circuit Malfunction”
• P0444 – “Evaporative Emission Control System Purge Control Valve Circuit Open”
• P0445 – “Evaporative Emission Control System Purge Control Valve Circuit Shorted

• If the purge valve is stuck in the open position, additional fault codes relating to a) fuel trims, and b), gross EVAP system leaks may be present, or the idling speed may be higher than usual. In some cases, the engine may idle roughly, or the engine may not idle at all
• If the EVAP purge valve is stuck in the closed position, excessive pressure in the fuel tank may dislodge some hoses, or even split some hoses in the fuel system. In most cases, though, excessive pressure in the fuel tank will trigger fault codes and warning lights on the dashboard long before fuel lines and hoses are damaged
• Although failed, or defective EVAP purge valves only rarely produce drivability issues, filling the fuel tank may take considerably longer than usual
• The EVAP readiness monitor may not initiate, or the monitor may not run to completion. In both cases, the vehicle will not pass a compulsory emissions control test

How do you test the Evaporative emissions (EVAP) purge valve?

While it is possible to test an EVAP purge solenoid by measuring the continuity and resistance of the coil, these values are for the most part application-specific, which means that you need access to OEM-level service information to compare your findings with.

Note also that while some EVAP purge valves require battery voltage to work, others work on reduced voltages, with 9 volts being typical. Thus, if you are sure that the purge valve on your particular vehicle runs on battery voltage, you can make up some jumper wires to supply the solenoid with direct current. If the solenoid works, you will hear a “click” sound as the valve cycles, but note that on some vehicles, the solenoid is “silent”, and you will likely not hear anything, even if the valve works properly.

WARNING: Note that in all cases, no electrical testing of the EVAP purge valve solenoid should be done while the valve is connected to the vehicles’ wiring, because this could damage one or more control modules, including the PCM. Therefore, we strongly recommend that you seek professional assistance with diagnosis and repair if you suspect you might have a defective or malfunctioning EVAP purge valve.

How do you replace the Evaporative emissions (EVAP) purge valve?

Due to their sometimes-awkward locations, it can at times be challenging even for professional mechanics to access, remove, and replace EVAP purge valves.

In practice, you may need to remove parts of the exhaust system, as well as splash guards, heat shields, and/or protective covers under the vehicle to access the relevant lines and hoses. Once you have done that, you need to identify the EVAP purge valve and disconnect its electrical connector, before removing retaining screws and or clips holding the valve in place.

At this point, you can remove the hoses on the inlet and outlet sides, but bear in mind that on most modern vehicles, you will need special tools to disconnect the lines without damaging them. In a few cases, you may be able to disconnect the lines from the valve by using standard tools, but in most cases, there is no way to disconnect the lines without special tools, and not have the connections leak afterward.

Assuming that you have access to the required tools, disconnect to old purge valve from its lines, and install the replacement purge valve in the reverse order of removing the old valve. Be sure to secure all lines you had disturbed, and double-check the routing of all electrical wiring to ensure the wiring does not rub or chafe against anything. Reconnect the electrical connector, and make sure that it locks into place securely to ensure proper operation of the new valve.

Replace all heat shields, splash covers, and protective covers after checking that all exhaust system connections are gas-tight and secure if you had to remove parts of the exhaust system.

Given the above, it might be possible for DIY mechanics with above-average skills and access to a comprehensive set of tools to replace an EVAP purge valve. However, if you are not skilled and do not have access to more than basic tools, the wiser option might be to seek professional assistance with replacing an EVAP purge valve.