Katerina Swanson
All modern cars are equipped with an Evaporative Emission Control (EVAP) system, which prevents fuel vapors from the fuel tank from escaping into the atmosphere. The EVAP vent valve, usually located underneath the vehicle near the fuel tank, controls the flow of outside air in and out of the charcoal canister. The most common problem with the vent valve is when it sticks open or fails, creating an EVAP system leak and triggering the Check Engine light. Testing the vent valve involves checking the power and ground at the electric connector, and the integrity of the valve can be verified by applying vacuum pressure and observing any leaks.
| Characteristics | Values |
|---|---|
| Location | Underneath the vehicle, close to or at the charcoal canister near the fuel tank |
| Function | Controls the flow of outside air in and out of the charcoal canister |
| Testing | Test the power and ground at the vent valve electric connector |
| Vacuum-based system | Uses the canister purge solenoid (CPS), canister vent solenoid (CVS), and fuel tank pressure sensor (FTPS) to purge gas vapors and test for leaks |
| EVAP system | Prevents fuel vapors from the fuel tank from escaping into the atmosphere |
| Common issues | Sticking open, failing, clogging due to dirt and dust, corrosion |
| Failure indicators | Check Engine light is on, difficulty starting the vehicle, hard running of the vehicle |
| Replacement cost | $150-$320 for parts and labor |
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What You'll Learn
Test the power and ground at the electric connector
To test the power and ground at the electric connector of a fuel tank vapor valve, you can perform a voltage drop test. This test can be done from both the power and ground sides.
Voltage Drop Test from the Power Side
Wear safety glasses for protection. You will need a digital multimeter (DMM) to perform the test. Set the multimeter to 20 volts D-C scale or D-C if your multimeter has auto-ranging capabilities. Connect the multimeter positive probe to the battery positive terminal. With the fuel pump wire harness connected to the fuel pump, connect the multimeter negative probe to the power feed wire at the fuel pump connector. Turn the ignition to the “on” position. The pump will only run for about two seconds while the relay is in prime position, or until it gets an RPM signal. The reading should be less than 0.2 volts. If the reading is greater, check for resistance in the power supply circuit for the fuel pump.
Voltage Drop Test from the Ground Side
The process is similar to the power side test. Set the multimeter to 20 volts D-C scale or D-C if your multimeter has auto-ranging capabilities. Connect the multimeter negative probe to the battery negative terminal. With the fuel pump wire harness connected to the fuel pump, connect the multimeter positive probe to the ground wire at the fuel pump connector. Turn the ignition to the “on” position. The pump will only run for a short time. The reading should be less than 0.2 volts. If the reading is greater, check for resistance in the ground supply circuit for the fuel pump.
Additionally, you can test the fuel tank vapor valve by connecting the fuel pump to a ground, which will turn on the fuel pumps continuously. However, do not run them for too long. Just long enough to perform your fuel system test.
Remember, before replacing any fuel pump based on pressure and flow test results, always follow the vehicle manufacturer's recommended procedure for testing all electrical connections and the electrical system charge. A minor issue, such as a loose ground, can cause problems resembling more serious malfunctions.
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Check for leaks with a smoke test
To check for leaks with a smoke test, you must first determine where to seal the system. You can close the system at the vent valve using a scan tool, or shut it at the purge valve with a pair of hose clamps. The purge valve should be closed with no power to it.
After closing the system, you can perform a smoke test on the fuel tank, the EVAP tank, and the lines connecting the fuel tank to the engine. To do this, connect the smoke machine to the vehicle's EVAP system. If your smoke machine has a \"Smoke\" or \"Visible Vapor\" setting, turn the selector to the appropriate setting and activate the machine. Remove the fuel cap until smoke can be seen coming out of the filler neck. This allows the smoke to fill the system faster and ensures that visible vapour will escape at the location of the leak. The vapor often has a UV compound that makes it more visible. By using a bright light or an ultra-violet light, inspect the system for leaking vapors.
If you do not have access to a scan tool, you can put ground and power to the vent valve and it should close. If it doesn't, the vent valve is faulty. If it does close, make sure the system seals all the way. A partially closed vent will trigger an evap code.
It is important to note that the EVAP vent solenoid needs to be in the closed position for smoke testing. Most vehicles have the EVAP vent solenoid open with the engine off, but some vehicles allow a handheld scan tool to interface with the vehicle's PCM to close the vent solenoid for leak testing. If the vehicle does not have this capability, a voltmeter can be used on most models to close the vent solenoid. Another option is to simply plug the vent hose temporarily while smoke testing, but this will not reveal a faulty solenoid.
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Observe the vacuum when the engine is off
To test a fuel tank vapor valve, you need to observe the vacuum when the engine is off. This is because a vacuum-based system goes through several phases of operation, and the vacuum behaviour when the engine is off is indicative of the vapour valve's functionality.
Firstly, the canister vent (CV) solenoid seals the EVAP canister from atmospheric pressure. This allows the EVAP canister purge valve to obtain the target vacuum in the fuel tank. The vacuum should remain steady when the engine is not running. If the vacuum is not steady, this indicates a problem with the vapour valve.
The vacuum hold and decay phase is the next phase. The PCM moves to the vacuum release phase, where the CVS is de-energized (opened) and the negative pressure/vacuum is released. The FTPS should slowly decay until there is no pressure in the system, close to 0.0 in.-H2O. If the pressure does not decay, the PCM concludes that the CVS is stuck closed.
The PCM then moves to the vapor generation phase. The PCM energizes (closes) the CVS, resealing the gas vapor system, then looks for a rise in vapor generation, watching the FTPS. The natural slosh of fuel inside the fuel tank, the exhaust temperature, and the operation of the fuel pump should create a rise in vapor pressure. If the pressure does not rise above 2.5 in.-H2O, the PCM will set a P0442 (.040-in. leak).
Therefore, observing the vacuum when the engine is off is a critical part of testing a fuel tank vapor valve. It helps identify any issues with the vacuum hold and decay phase and ensures that the vapor generation phase initiates correctly.
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Test the purge solenoid with a vacuum pump
The purge solenoid is a normally closed solenoid that controls the flow of engine vacuum to the EVAP system. The EVAP system prevents fuel vapors from the fuel tank from escaping into the atmosphere. When the engine is off, the purge valve is closed. When the engine is running, the engine computer pulses the purge valve open, allowing some amount of fuel vapors to be purged from the charcoal canister and burned in the engine.
To test the purge solenoid with a vacuum pump, you can try the following steps:
- Apply vacuum to the back end of the valve and observe if the valve retains the vacuum. This test can be done with the engine off, as described by a Hyundai Elantra owner who received a trouble code P455, indicating a leak in the emissions system.
- If the valve does not hold the vacuum with the engine off, it is likely that the internal parts of the valve are seized, as seen in some Nissan Rogue SUVs with a failed vent valve.
- Another testing method involves unhooking the electrical connector on the purge valve and the vacuum line on the canister side while keeping the engine side attached. With the engine running, you should not feel any vacuum coming through the purge valve.
- Additionally, you can perform a vacuum test using a tool like an oil extractor, as mentioned by another Hyundai Elantra owner. This test helps measure vacuum and loss to ensure the system is functioning correctly.
It is important to note that a common problem with the purge valve is when it sticks or does not close fully, causing the "'Check Engine'" light to illuminate. If you encounter this issue, it may be necessary to replace the purge valve or seek professional assistance for further diagnostics and repairs.
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Check the canister vent solenoid
The canister vent solenoid is an important component of the EVAP system, which prevents fuel vapours from escaping into the atmosphere. The EVAP system collects and temporarily stores fuel vapours in a charcoal canister filled with activated carbon pellets. The canister vent solenoid is typically located underneath the vehicle, near the fuel tank, and can be identified by its two wires and a rubber tube open to the atmosphere. It is normally open, meaning it should be open with no voltage applied and closed when voltage is applied.
To check the canister vent solenoid, you can refer to the vehicle's service manual for specific testing procedures. However, here is a general overview of the process:
First, locate the charcoal canister, which is usually a big black box next to the fuel tank. The canister vent solenoid will be attached to the canister and will have two wires and a rubber tube connected to it.
Next, you will need to test the solenoid valve while it is installed in its place at the canister. This can be done by applying and removing voltage to the solenoid and observing whether the valve opens and closes accordingly. A properly functioning solenoid valve should open with no voltage applied and close when voltage is applied.
Additionally, you can observe whether the Check Engine light is on and check for any trouble codes that may indicate a problem with the canister vent solenoid. For example, a code such as P0446 or P0442 may indicate an EVAP system vent control circuit malfunction or an evaporative emission control system leak, respectively.
If the canister vent solenoid is found to be faulty, it may need to be replaced. This repair typically includes vent valve replacement and some modification of the vent valve setup. It is important to note that the design and setup of the vent valve can vary between different vehicle manufacturers.
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Frequently asked questions
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 vent control valve (solenoid), or vent valve, is a part of the EVAP system.
If your fuel tank vapor valve is faulty, the engine computer (PCM) turns the Check Engine light on and sets the code. A common problem with the vent valve is when it sticks open or fails, creating an EVAP system leak.
Testing the Purge Solenoid is fairly easy if you have an advanced scan tool. First, locate the solenoid valve. It has two or three wires attached to it, and a vapor line that goes toward the fuel tank. Disconnect the vapor line and attach a vacuum pump with a gauge to the purge solenoid inlet fitting and apply vacuum with the pump. The vacuum should remain steady when the engine is not running.
Replacing the vent valve is not very expensive ($150-$320 for the part and labor).
