How to Replace the Charcoal Canister

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In modern automobiles, the engine control module (ECM) does more than run the engine. Using numerous sensors and actuators, the ECM fine-tune’s engine operation to extract the most power from every drop of fuel. Besides improving power output and overall efficiency, this also reduces emissions – an efficient engine is cleaner. Still, there is more to emissions reduction than fuel economy.

The evaporative emissions control system (EVAP) controls hydrocarbon (HC) emissions, that is, raw fuel vapors. The charcoal canister is a major component of the EVAP system, working with various tubes, sensors, and valves to prevent fuel vapors from escaping into the atmosphere. In the presence of sunlight, HC emissions react with nitrogen oxides (NOx), forming ozone (O3). Ground-level ozone irritates lungs and eyes and is a major component of smog. Such emissions have also been linked to various cancers. The EVAP system uses the canister to limit HC emissions during refueling. What is a charcoal canister? What does it do and why is it important? Finally, how do you replace it?

What is the Charcoal Canister?

refueling a car, gasoline or diesel fuel pump nozzle in modern sedan
Evaporative Emissions Occur Most Commonly During Refueling, but the Charcoal Canister Eliminates Much of It. http://www.gettyimages.com/license/668193284

The charcoal canister is a sealed container filled with “activated carbon,” or “activated charcoal.” Activated carbon is processed to give it an incredibly disproportionate surface area for its size – it’s basically a sponge for absorbing fuel vapors. Depending on how it’s prepared, a single gram of activated charcoal can have a surface area of between 500 m2 and 1,500 m2 (5,400 ft2 to 16,000 ft2). (For comparison, a dollar bill weighs about a gram and only has a surface area of 0.01 m2 or 0.11 ft2).

To prevent HC emissions escaping into the atmosphere, valves control air flow through the charcoal canister. While refueling, the canister vent valve opens, allowing air and fuel vapors to flow through the canister to the atmosphere. The activated carbon strips the air of fuel vapors. After refueling, the canister vent valve closes, sealing the system.

Under certain operating conditions, such as low-load highway cruising, the ECM will command the canister purge and vent valves to open. As the engine pulls air through the charcoal canister, fuel vapors are flushed out, to be burned in the engine. As a result, harmful HC emissions are significantly reduced, replaced by harmless carbon dioxide and water (CO2 and H2O) vapor in the exhaust.

Why do you Need to Replace the Charcoal Canister?

Check Engine light
The "Check Engine" Light Could Indicate a Charcoal Canister Problem. Photo © Aaron Gold

There are at least a few reasons you might need to replace the canister. Symptoms you may notice from a faulty charcoal canister might include check engine light (CEL), difficulty refueling, poor engine performance, excessive fuel odor, or the reduced fuel economy.

  • Check Engine Light – A few DTCs (diagnostic trouble codes) may indicate canister replacement, usually EVAP leaks – the ECM checks EVAP system integrity by running a pressure test – or valve, pressure, or flow problems. A cracked charcoal canister could leak, setting off one or more leak DTCs. A faulty valve might result in a flow or circuit DTC, but canister replacement might be the only option if the valve is not available separately.
  • Refueling Problems – The most common cause of refueling problems is faulty valves, but could also be caused by a saturated charcoal canister. The most common cause of canister saturation is regularly over-filling the tank, or “topping off” after the fuel pump nozzle has already clicked off. When the canister is saturated, air doesn’t flow through it as readily, backing up the fuel system and clicking off the fuel nozzle before the tank is full. Canister saturation usually leads to excessive fuel odor problems, as it cannot absorb more fuel vapors, and repeated purge cycles cannot remove “wet” fuel.
  • Other Problems – Poor fuel economy or engine performance could also be the result of a faulty charcoal canister, again because of saturation. A ruptured canister could allow charcoal dust to escape into the vapor lines, clogging purge or vent valves. A saturated canister could result in an overly-rich fuel mixture in the engine, leading to running problems.

How to Replace the Charcoal Canister

man on creeper working under a car, auto repair
The Charcoal Canister Might be Under the Car, Near the Fuel Tank. http://www.gettyimages.com/license/547435766

Once you’ve determined the charcoal canister to be the source of your problems, replacement is a simple matter of disconnecting hoses and electrical connectors, swapping out the canister, and reconnecting everything.

  1. The canister may be under the hood or near the fuel tank. If you must lift the vehicle, use jack stands – never put any part of your body under a vehicle supported only by the jack.
  2. Electrical, vapor, and mechanical connections likely haven’t moved in many years. Spray mounting nuts and bolts with penetrating oil to ease removal. Also, some have found spray silicone lubricant beneficial in removing electrical and vapor lines.
  3. Remove any hose clamps and disconnect all vapor lines. Use a paint marker or masking tape to help you remember where they connect. Disconnect any electrical connectors.
  4. Removing the charcoal canister usually requires only basic hand tools, such as a ratchet and socket set. If rust is a problem, a hammer and punch might come in handy to shock a nut or bolt loose. Wear safety glasses to prevent dirt or rust from getting into your eyes.
  5. On removing the canister, if you note charcoal dust in the EVAP purge line, you should blow the line out with compressed air to prevent it from clogging the purge valve and creating another problem down the road.
  6. Bolt the new canister in place, then apply a small amount of spray silicone to vapor line and electrical connections. This will ease installation and ensure a good seal.
  7. If replacing the canister to address a CEL condition, clear all DTCs before restarting the vehicle.

A Final Thought

Replacing the charcoal canister isn’t a particularly difficult job, but determining that the canister is the faulty component can be frustrating. If you aren’t 100% sure that the canister is at fault, consult with the professionals to determine the cause of the problem you are experiencing. This is especially true of finding EVAP system leaks, which may be impossible to find without a smoke machine, too expensive for the typical DIYer.