With emissions controls getting tighter and tighter, car manufacturers always have to find a way to deal with fumes and exhaust gases they would have otherwise dumped into the environment. This has led to many innovations that though good, make vehicles more complex than they used to be.
One such innovation is the EVAP canister. This is a rectangular plastic container filled with activated charcoal and mounted somewhere in your fuel system (mostly on the tank vent line). Its work is to absorb any gasoline vapor in the fuel system and store them until you start the engine.
When the engine runs, the Powertrain Control Module (PCM) opens the canister’s purge valve to the intake vacuum. This will suck any vapor in the canister and burn it in the combustion cycle.
Tip: Some evap canisters don’t need the PCM to vent. A pressure sensing switch activates the purge solenoid, opening the EVAP system to the intake manifold vacuum, which will then suck and inject the vapor into the engine.
This ensures you recapture and re-use that dangerous vapor that would have otherwise vented into the atmosphere or sat. In the evaporation line until it condenses and flows back into the tank.
Signs and Symptoms of a Failed or Failing Evap Canister
A failed EVAP canister is hard to miss. However, since these symptoms could also mean something else, people always chase other fixes in vain before thinking of the EVAP canister. Don’t forget to check your canister if you experience some of the following symptoms.
Check Engine Light With Error Code P0455
If you have a check engine light on the dash, start pulling up the error code behind the light. If you get P0455, your Evaporation Emissions Control System (EVAP) is at fault.
While the error code could also come up when you drive with a loose, missing or damaged fuel cap, it mostly means there is a leak or failure in the EVAP system. This could also include the EVAP canister.
The Engine is Hard to Start
A ruined evap canister that is stuck open robs the engine of the vacuum. This is essentially an internal vacuum leak that is hard to detect.
Vacuum leaks make the engine harder to crank. You will notice this more during a cold start. If you get the engine running, it will still have all the problems associated with a vacuum leak.
This includes but isn’t limited to a rough idle and the associated loss of power.
A Strong Fuel Smell from the Cabin
You will get a strong fuel smell in the cabin when the EVAP canister isn’t capturing all those vapors. The vapors will make their way into the environment, and the first person to discover the smell is you because you are seated on the fuel lines and gasoline tank.
The smell will be most noticeable when you start the car after it sat for a couple of hours. The smell is annoying and can cause nausea interfering with your driving focus. It could also increase the risk of a fire.
The Gas Pump Clicks Before the Tank is Full
A clogged EVAP canister prevents vapor from escaping the fuel tank. This could lead to a pressure differential that makes it harder to fill the tank to the brim since it will trigger the lock on the fuel dispenser nozzle early.
Note that this happens when filling up with high octane gas as regulations state that more space should be left unoccupied in the tank than when filling with regular gas.
This should only concern you if you fill up with the regular gas and the pump shuts the flow earlier than usual.
Bad Fuel Economy
If the vapor in the tank ends up venting into the environment, you will lose a portion of your fuel to the environment.
The effective distance you get from your tank of gas or any portion you fill up will be less than what you should since you are venting away so much.
Failed Emission Test
While some emission tests won’t specifically look into your emissions system, you will still fail if:
- The check engine light is on and has an error related to the emissions and exhaust control
- You get more than acceptable compounds in the exhaust system
What Causes a Charcoal Canister to Go Bad?
A damaged evap canister isn’t unheard of. There are actuation solenoids and sensors that could go wrong. Sometimes, you could even have a clogged evap canister if some solids get into it.
Top on the list of things that could leave you with a bad charcoal canister include:
Overfilling Your Fuel Tank
This is the biggest and perhaps honest mistake that will ruin your EVAP system and block the canister.
The evap canister is designed to handle fuel vapor, not liquid fuel. The vent hole sits at the very top of the tank, meaning fuel will never make its way into the system.
However, under very rare conditions, when you overfill the fuel tank, liquid fuel could slosh up the evaporation line and make its way to the EVAP canister. If this happens, it will soak up and ruin the charcoal.
Never fill your fuel tank to the literal brim. Stop pumping gas as soon as the pump disconnects for the first time. This feature was designed to help you stop filling the tank before it goes beyond the stipulated ‘full level.’
While you could squeeze in a few quarts by pulling the nozzle out and pulling the trigger again, you will overfill the tank to the neck hence compromising your evap canister.
A clogged or blocked evaporation system could overpressurize your fuel tank or force it to hold onto fumes with a considerably higher flashpoint. Overfilling is never worth the extra few drops of fuel you store in the nozzle. After all, you will still pay for them!
Degradation over Time
EVAP canisters degrade over time. While most will outlast your vehicle’s warranty, they will eventually give up the ghost.
The failure will be in:
- Gaskets and rubber grommets that make the charcoal canister’s attachments leakproof
- Damage to the fuel line connecting the canister to the engine or fuel tank
- Damage to actuating solenoids or pressure differential switches
- A stuck or damaged purge valve
- Brittle plastic components
Moisture in the Fuel System
Moisture is always bad news in the fuel system. The same applies to the EVAP system. If there are cracks in the plumbing or you have got a bad batch of fuel, moisture can evaporate and make its way to the EVAP canister.
This will ruin the canister as it wasn’t designed to deal with water. However, a ruined evap canister will be the least of your worries if you have water or considerable moisture in your fuel system.
Unexpected Contamination
Contamination will mostly happen if something gets into the evaporation system. Since it is designed to deal with fuel vapor, it should always be 100% solids-gunk free.
However, if something gets in, perhaps a chunk of plastic, brittle rubber, or rust carried up when you overfill the tank, it will clog your charcoal canister rendering it less effective or useless.
How to Test a Charcoal Canister
Since the EVAP canister consists of different working parts, you will need tests to determine if it is working.
- Check for continuity and the mechanical click when you supply the unit with power
- Take ohm settings and cross-check them against recommended stock readings
- Do the bubble, smoke, and close test
After locating the EVAP canister, you can try the following tests.
How to Do a Continuity Test
Start by disconnecting the purge valve from the engine side. Set the multimeter to continuity mode and use the problems to check for continuity between the two terminals on the purge valve.
If everything is okay and the solenoid works, you should get a beep (or any other audible noise) from your multimeter to indicate the continuity.
Get the Ohm Readings
Turn the dial of the multimeter to select resistance or ohm readings. With the probes still on the purge valve terminals, take a reading of the resistance in ohms.
On average, a healthy EVAP canister reads between 14 and 30 ohms. You can refer to your user manual for your vehicle’s evap canister recommended readings.
How to Fix a Clogged Evap Canister
While you could service parts of some evap canisters, most mechanics agree that replacing the entire canister is simpler and faster. They are not that expensive after all. There is no point in spending hours fixing a ruined part of the canister – unless the evap canister is rare and hard to come by.
What Happens if I Don’t Fix My Evap System
While your car could keep running with a bad evap canister, you will carry all the side effects of a faulty charcoal canister.
This ranges from bad fuel economy to the check engine light, rough idle, hard cold starts, and bad fuel economy.
Can You Drive With a Faulty Evap System
While you could drive with a faulty evap system, you shouldn’t. First, the constant check engine light will mask any future errors since you will assume it is the normal check engine light prompt.
This, coupled with the other problems associated with a bad EVAP system, makes your car less efficient. It will even be more dangerous if it vents into the atmosphere, causing pollution and possible fire hazards when parked in a confined space for long.