Canister purge valve schematic

Canister Purge Valve: What It Does And Why It Matters

When talking about automotive emissions harmful to the environment, we’re mostly referring to the toxic fumes coming out of the tailpipe. However, cars using internal combustion engines can hurt the environment in many other ways. For example, the fuel we put in our vehicles can evaporate, ultimately polluting the air around us. Vapors created from gasoline and diesel fuel can cause respiratory problems, but also damage the environment as a whole. That’s why car manufacturers use EVAP (Evaporative Emission Control) systems paired with a canister purge valve to control vapor emissions.

The EVAP system consists of several components and safely takes care of the gasoline and diesel vapors created inside the fuel tank. Without such a system, the fuel vapors would evaporate and toxic fume would be released into the atmosphere. Furthermore, the high pressure inside the tank might damage it in the long-term.

In this article, we’ll dive most specifically into the canister purge valve, on of the most critical component of the EVAP system and see what it does, how it works and what happens when it goes bad. A faulty canister purge valve can create many issues that could easily be confused with other malfunctioning components.

Please note that in some textbooks and car repair manuals, the canister purge valve is known as the canister solenoid or EVAP solenoid. Nevertheless, the solenoid actually only takes care of closing and opening the purge valve.

What is the Role of the EVAP System?

The fuel contained in your car’s fuel tanks continuously evaporates – it’s fluid after all. That happens even when your vehicle is parked for days, and especially on hot summer days. Oh, and gasoline and diesel are already very volatile – much more than water, for example. Therefore, the fuel tank is continuously filled with vapor.

The Evaporative Emission Control system manages these vapors inside the fuel tank. It does that by redirecting them into the engine, where they burn and go through the catalytic converters before being released into the atmosphere. This way, fuel vapors can be used to power the vehicle, just like the fluid inside the tank and, most importantly, removed of the most harmful chemicals it contains by the catalytic converters. The EVAP system also takes care that excessive pressure inside the tank can be released without harming the environment using a charcoal canister. The canister’s main job is particularly useful to allow drivers to fill their tanks without having the fuel flood back in the filler neck and spill everywhere. But that’s a whole another topic and we’ll keep it for another article.

 

 

Canister Purge Valve

What is the Canister Purge Valve For?

The canister purge valve role is used to prevent harmful fuel vapors from entering the atmosphere. It does that by literally capturing the gases inside a small canister filled with charcoal and release them vapors into the engine at the most appropriate time.

The purge valve is the component opening and closing the passage between the fuel vapor lines coming from the tank and the intake manifold. In other words, the purge valve basically acts as a switch. When it opens, the fuel vapors are injected into the air intake. When it closes, the process stops.

The Canister Purge Valve Solenoid

The solenoid is activated by the PCM and opens or closes the canister purge valve. In most cars, the canister purge valve stays closed when no electricity is applied. In this condition, the purge valve traps vapors inside the charcoal canister, released previously by the fuel in the tank. When voltage is applied to the solenoid, it opens up and the purge valve releases the trapped vapors into the engine. If the solenoid doesn’t function properly, it might not open or close, which can lead to several issues.

The PCM will only allow the vapors to be released when the car is driving at a constant speed, favorably while driving on higher gears. Releasing the vapors when the car is driving at low speed or under load could modify the air-fuel ratio to an extent where the engine might encounter misfires. A purge valve stuck open or obstructed could cause the vapors to be released or not released at an incorrect time, which could lead to an improper air-fuel ration and cause the check engine light to come up.

A properly-functioning purge valve should also be able to seal correctly and not release any vapor in the environment when closed and let vapor flow freely when opened.

Location of the canister purge valve on a 2002 Chevrolet Corvette
Location of the canister purge valve on a 2002 Chevrolet Corvette.

How to Check if the Canister Purge Valve Works Properly?

First, you’ll need to remove the canister purge valve. Usually, you can find this part inside the engine bay, near the fuel-delivery system. Most purge valves look the same: a small black plastic box with two or three small rubber hoses on either side and an electrical connector on the top. If you aren’t sure where the purge valve is located in your car, take a look into the EVAP system diagram inside your car’s repair manual.

Remove the electrical connector that provides electricity to the solenoid and remove the vapor lines. Next, remove the whole component. Most purge valves are held in place with one bolt or slid on a rubber bushing.

Testing the Canister Purge Valve Solenoid

After you’ve removed the part, you can check if it functions properly. To do that, you’ll need a car battery and jumper cables the solenoid click, and a vacuum pump for testing the sealing properties.

Connect the car battery to the control side of the solenoid to see if it closes/opens appropriately. If your purge valve has more than two pins inside the connector, check the purge valve electrical diagram to find out which pins control the solenoid. You should hear an audible click every time you apply voltage to the system. If not, the solenoid might not work.

Canister purge valve wiring diagram
On most purge valves, you’ll find only two pins controlling the solenoid. One is the positive wire coming from the fuse and the other is the ground controlled by the PCM.

Testing the Seal

To verify the proper sealing of the purge valve, use a vacuum pump to create a vacuum inside the canister purge valve and check the gauge for at least 5 minutes. If the purge valve doesn’t seal properly, the vacuum will drop. When this happens, the best bet is to replace it right away.

If the purge valve clicks just fine and you’re still having an EVAP system DTC code, it probably means that the solenoid is not getting power or ground. The next thing to do is then to test the control circuit.

Testing the Canister Purge Valve Control Circuit

The easiest and first thing to test is the positive side. Once your car starts, the canister purge valve fuse supplies power to the control side of the solenoid. Use a test light or a multimeter to test the fuse and make sure it isn’t burned. If it’s not, replace the fuse and test it again. If everything seems good, it’s time to test the ground.

Since the ground is supplied by the PCM, you’ll need to activate it in order to test it. To do that, you’ll need an OBD2 scanner with special test features. Select the “special test” or “activation test” tab and search for the canister purge valve test mode. Click on it and activate it. Once activated, the PCM will start sending ground to the purge valve control output pin. Use your multimeter to make surge ground is supplied to that specific pin. If it’s not, multiple causes may come to mind. A sensor might not work properly, a communication circuit might be open, or the PCM might be faulty. In all cases, exhaustive troubleshooting might be required and you should probably leave that to professionals.

On the other hand, if the ground actually comes out of the PCM, the electrical circuit between the PCM and the purge valve might be open. Find the stripped or damaged wire and repair it.

Faulty Canister Purge Valve Issues and Symptoms

Be sure to check the symptoms listed below before replacing the canister purge valve. Some of the symptoms can also be related to other faulty vehicle components, though. Therefore, if you aren’t sure the problem actually comes from the purge valve, seeking help from a certified mechanic is always the thing to do.

The Engine has Trouble Starting

A solenoid stuck open might open the way for more air to enter the combustion chamber. When the engine is overflown with air, it might have trouble starting, or it won’t start at all.

Increased Fuel Consumption

When the canister purge valve isn’t functioning correctly, you might notice increased fuel consumption. This happens because the gasoline or diesel fuel simply evaporates, instead of going into the engine. In other words, you will be losing fuel without even burning it.

Rough Idling

Another issue that might arise with a faulty canister purge valve is rough idling. A defective or damaged solenoid might create a vacuum leak. When that happens, more air can enter the combustion chamber, which changes the air-to-fuel ratio.

On older cars, the engine might choke when idling, and sometimes it won’t start at all. On newer vehicles, the engine computer might try to compensate by injecting more fuel, usually causing black smoke to come out of the exhaust system. Nonetheless, the most common symptom will still be rough idle. Instead of choking, though, you might experience a change in engine RPMs.

Worse Performance

On most modern vehicles, the ECU should take care of adjusting the air-to-fuel ratio to keep your engine running. However, you might still experience worse acceleration or at least inconsistent performance.

Check Engine Light

The canister purge valve is directly related to the proper functioning of the motor. Therefore, every time it fails, the ECU will light up the “Check Engine Light” on the dashboard. That said, this light is not always related to the solenoid. The “Check Engine Light” refers to any issue inside the motor that will hinder its performance in some way.

Check engine light
Check Engine Lights can show a minor problem or indicate a larger issue.

On most vehicles, a faulty canister purge valve can trigger any of these codes:

  • P0440: Evaporative Emission Control System Malfunction
  • P0441: Evaporative Emission Control System Incorrect Purge Flow
  • P0443: Evaporative Emission Control System Purge Control Valve Circuit
  • P0444: Evaporative Emission Control System Purge Control Valve Circuit Open
  • P0445: Evaporative Emission Control System Purge Control Valve Circuit Shorted

However, a P0442: Evaporative Emission Control System Leak Detected will usually let you know that one of the vapor hoses is cracked and is leaking instead. In that case, replacing the purge valve would have no effect. Such a problem is fairly common in recent vehicles because of the purge valve being placed to close to the exhaust manifold. Excessive heat will slowly dry out rubber hoses and lead them to crack prematurely. To correct this problem, some car manufacturers have created re-routing components that will relocate the purge valve elsewhere.

For example, this problem was so frequent on older Hyundai Accent that the manufacturers came up with a re-routing kit consisting of longer hoses, a new bracket, and a longer connector allowing mechanics to place the purge valve near the front of the engine instead. If you have such a problem in your car, calling your dealer’s part department might be a good idea before trying to buy new hoses and fix everything up yourself.

Failing Emission Tests

The canister purger valve is linked with vehicle emissions, both directly and indirectly. The direct part is the fuel vapors coming out of the tank. The indirect component is the wrong air-fuel mixture, which can produce more CO2 and hydrocarbons. Naturally, a car that produces more emissions will fail the stringent emission tests performed in recent years. However, if it’s your case, the check engine light should be up in the dashboard and there’s little to no chance to pass an emission test with a check engine light on.

Last Words

The EVAP system on your car is a fairly simple one and should pose any problem for anyone with basic mechanic knowledge to troubleshoot. On the other hand, trying to diagnose an intermittent faulty purge valve condition might be a lot more complicated. In any case, make sure to get your hands on a quality repair manual before trying to measure current and resistance in the circuit. Failing to do so might damage more expensive components and end up doing more harm than good. Aside from that, any weekend DIYer should be able to diagnose and replace a canister purge valve with only basic tools and save a lot of money at the same time.

Good luck!

About Ivo G

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Hey guys and girls, I’m Ivo, a passionate car enthusiast with a particular weakness for rear-wheel-drive and stick-shift gearbox cars. Meanwhile, I don’t mind getting my hands dirty – I have experience as a Certified Head of Service at a Toyota Dealership, and knowledge that I wish to share with you.

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