Purge Valves (EVAP Systems)

The purge valve is the part of the vehicle Evaporative Emission Control (EVAP) system.

The EVAP system prevents fuel vapours in the fuel tank from escaping into the atmosphere. The EVAP system traps fuel vapours from the fuel tank and temporarily stores them in the charcoal canister.

A purge valve solenoid

EVAP system diagram

When the engine is running under certain conditions, the fuel vapours are purged from the canister and burned inside the engine. The purge valve precisely controls the amount of fuel vapor that is purged from the charcoal canister.

In modern cars, the purge valve is an electrically-operated solenoid, that is controlled by the engine computer. When the engine is off, the purge valve is closed. When the engine is running and fully warmed up, the engine computer gradually opens the purge valve to allow some amount of fuel vapours to be moved from the charcoal canister and burned in the engine. The purge flow is monitored by a number of sensors. If the purge flow is less or more than is expected under the conditions, the computer illuminates the "Check Engine" light.

Toyota Corolla 2009-2012 Purge Valve Solenoid

Purge Valve Problems

The most common problem with the purge valve is when it sticks (fails to open) or does not close fully. This may cause the "Check Engine" warning light to come on. 

In some cars, a stuck-open purge valve can cause difficulty starting right after refuelling at a gas station: for the first few seconds the engine may run rough and stumble. And all of a sudden, it runs fine.

Purge valve problems are common in many cars. They are fairly easy to replace.

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