Vent valve (solenoid): how it works, symptoms, problems, testing
Vent valve (solenoid). Click for larger photo
EVAP system diagram
All modern cars are equipped with an Evaporative Emission Control (EVAP)
system. The EVAP system prevents fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP system collects and temporarily stores the fuel vapors in the charcoal canister. The charcoal canister is filled with activated carbon pellets that can absorb the fuel vapors. When the engine is running, the fuel vapors are purged from the canister and burned in the engine.
The vent control valve (solenoid), or simply vent valve controls the flow of outside air in and out of the charcoal canister. One side of the vent valve is connected to the charcoal canister. Another side is connected to the vent hose that has a filter or screen at the end and is attached to the car body or frame. In some cars, the vent valve is attached to the canister. In others, it's installed separately near the canister. The vent valve is controlled by the engine computer (PCM). Normally the vent valve is open. It closes when the engine computer tests the EVAP system for leaks. If a leak in the EVAP system is detected, the Check Engine light will illuminate on the dash and the trouble code related to the problem will be stored in the engine computer.
Vent valve problems
The most common problem with the vent valve is when it sticks open or doesn't close. This creates an EVAP system leak and triggers the Check Engine or Service Engine Soon light. For example, a failed vent valve often caused a Check Engine light with the code P0455 Evaporative Emission Control System Leak Detected in some Nissan and Infiniti vehicles. Another issue is that some dirt and dust could be drawn into the vent valve through the vent hose and cause it to clog up. This will also trigger the Check Engine light.
Some GM trucks were known to have this problem which causes the code P0466. The repair includes vent valve replacement and some modification of the vent valve setup. A clogged up vent valve can cause trouble filling the gas tank.
How the vent valve (solenoid) is tested
The vent valve design and setup varies between different manufacturers. You can find the proper testing procedure in the vehicle's service manual. We listed a few sources where you can get access to a service manual online at the bottom of this page. In most cars, the vent valve is normally open, meaning it should be open with no voltage and close when the voltage is applied.
As an example, let's test the vent valve in this 2000 Honda Accord. Honda calls it the Canister Vent Shut Valve. Honda service manual says to test the valve while it's installed in its place, at the canister located underneath the car. For a better demonstration, we removed the vent valve and tested it separately. The first step, as per the service manual, is to connect the vacuum gauge. We tried applying the vacuum and as you can see the valve doesn't hold it because it's fully open. The vacuum gauge immediately drops to zero.
The next step as per the service manual is to activate the valve with the Honda scan tool called the PGM Tester. The PGM Tester commands the engine computer to apply 12 volts to the vent valve. We connected the extension wires and activated the vent valve with the tester. Immediately, the vent valve produced an audible click.
Typically a bad vent valve does not hold vacuum when activated, or doesn't activate at all. For example, a failed vent control valve was fairly common in early Nissan Rogue SUVs. If you would test the failed vent valve the same way, it probably wouldn't even click when the voltage was applied, because the vent valve internal parts likely would have been seized.
Replacing the vent valve is not very expensive ($150-$260 part and labor).
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