(Updated on May 28, 2020)
The Evaporative Emission Control (EVAP) system features a vapor canister purge valve which manages the quantity of fuel vapor coming out of the charcoal canister.
The purge valve used to be controlled with a vacuum but now, newer cars allow the engine control unit to electronically control it.
The purge valve and charcoal canister work together to recycle the emissions back into the engine so that they’re eliminated as the internal combustion process takes place. You could say that one cannot work without the other.
What is a Vapor Canister Purge Valve?
A vapor canister purge valve is a component of the evaporative emission control system and its main function is to bring those stored vapors into the engine from the charcoal canister.
There is a vacuum line that connects the intake manifold with the charcoal canister that contains the fuel vapors. The purge valve lies within the vacuum line to allow these vapors into the intake manifold at the appropriate time.
Older vehicles use vacuum operated purge valves while newer vehicles used electronically operated ones.
How Does a Vapor Canister Purge Valve Work?
When the purge valve opens, the fuel vapors can access the intake manifold. From there, the vapors are sent into the internal combustion chamber where they will be ignited with the fuel and air mixture.
If you have an electronic purge valve, then the engine control unit manages it. This is the central computer of the vehicle which will calculate the exact amount of vapor that needs to go into the engine.
Once it allows enough vapor through, the purge valve will close so that no mater vapor gets through. The vapor will just sit in the charcoal canister until it is needed again.
Bad Vapor Canister Purge Valve Symptoms
If you have a bad vapor canister purge valve, the symptoms that you experience will potentially be worse than the symptoms of a bad charcoal canister. You need to understand what these symptoms are if you are going to tell the difference between the two possible problems.
Here are five things to look out for:
1) Check Engine Light
There are dozens of reasons for the Check Engine Light to illuminate on your dashboard. If you have a damaged or worn out purge valve, this could possibly cause the Check Engine light to come on.
There are sensors which detect when the purge valve is working. But if there are no signals coming from the purge valve anymore, this will indicate to the sensors that the valve must be damaged.
The sensors will then relay this information back to the engine control unit, which will result in the Check Engine warning being given to you.
2) Rough Idle
If your car engine has a rough idle, then it is no longer running smoothly. It will run at an inconsistent rate as you’re driving, especially when stopped at a stop sign or red light.
The longer you continue to let this problem persist, the more consistently your engine will run. The worst point will be when your engine stalls altogether.
Try not to let it get this out of hand. Usually, when the purge valve is to blame for the rough idle, it will cause a vacuum leak which will impact the idle speed.
3) Trouble Starting Car
If you have a vacuum leak that was brought on by a bad vapor canister purge valve, then you will probably have difficulties starting your vehicle. This will cause outside air to uncontrollable enter your engine and interfere with the overall internal combustion process.
If you have unmetered air mixing with fuel in the cylinder chamber, then it will cause engine problems for sure. The most obvious problem will be an engine that does not start.
4) Poor Engine Performance
If you are able to start your vehicle with a bad vapor canister purge valve, don’t expect it to be a smooth ride. You will almost certainly experience weaker performance from your engine.
In other words, your engine won’t be able to generate as much power to provide the acceleration that you are demanding as you press down on the gas pedal. This will be most obvious when going uphill or attempting to pass another vehicle.
5) Emissions Test Failure
As you know by now, the vapor canister purge valve is responsible for redirecting fuel vapors back into the engine. This is preventing toxic hydrocarbons from being emitted from your tailpipe.
So, if you were to have a failing purge valve, then it wouldn’t be able to redirect those fuel vapors and prevent them from escaping your vehicle. You will find this out the hard way when you go for your emissions test and end up failing.
If this happens, have the mechanic check your purge valve to see if it’s the part responsible for the failed emissions test. If it is worn or damaged, then you will need to replace it promptly. Then you should be able to pass the emissions test.
Vapor Canister Purge Valve Replacement Cost
The replacement cost of a vapor canister purge valve will be somewhere between $130 to $230 on average. The main expense will be for the part itself, which should cost around $75 to $120.
The amount of time a mechanic needs for replacement isn’t too long, so you are only looking at around $50 to $100 for the labor cost. It’s best to skip the dealership service area for a simple task like this.
You’ll save yourself a lot of money by going to an independent auto repair shop or simply do the replacement yourself if you have even a little bit of auto repair or maintenance experience.