Evaporative emission control system (EVAP)—Friend or foe?

Some technicians and shop owners consider EVAP problems to be a friend. Most likely it is because it is a common problem. Many in the industry believe that approximately three out of the top 10 diagnostic trouble codes (DTC) are EVAP. So, those who consider it a friend are profitable with diagnosing and repairing the EVAP system.

Others consider EVAP a foe, probably because it can be difficult to diagnose, or could it be that EVAP is a leading cause of comebacks?

Let’s review what it takes to avoid mistakes and make dealing with EVAP profitable for all.

The mission of the EVAP system:

1. Prevent gasoline fumes from escaping into the air.
2. Capture fuel vapors from the fuel tank.
3. Send vapors into the engine to burn.
4. Activate a DTC and check engine light when leaks are detected.

Components and common EVAP system problems:

• The vehicle's power of engine control module (PCM/ECM) operates each component at the correct time using factors like engine speed (RPM), outside temperature, and fuel tank pressure.
• The gas cap keeps fuel vapors from escaping from the EVAP system.
  • The gas cap is constantly loosened and tightened, therefore the gasket on the cap has a high potential to fail. Not tightening the cap triggers a code and light. 
  • A P0442 fault code means that the control module has determined a small leak in the EVAP system. A diagnostic starting point is to check the gas cap to make sure it is sealed properly. 
• The fuel tank pressure sensor measures the pressure inside the tank and reports it to the vehicle’s control module.
• The vapor lines carry the fuel vapors in the system. 
• The purge valve allows a vacuum from the engine to pull fuel vapors into the intake system to be burned up in the combustion chamber
  • The purge valve is a mechanical device. Over time, it can become stuck or fail completely. If this valve happens to be stuck open it can cause a hard start, especially after adding fuel, allowing raw fuel into the combustion chambers.
  • When the purge valve has been stuck open, I always recommend changing the oil because it is most likely fuel-saturated and can cause engine damage.
  • A P0455 fault code means that the control module determined that there is a large leak in the EVAP system. A diagnostic starting point is to check the purge valve and the vent valve for proper operation and ensure that there is not a massive leak in a fuel vapor line.
• The vent valve allows fresh air into the EVAP system and prevents an excessive vacuum build-up.
  • EVAP parts like the vent valve generally fail because of their moving parts. 
  • A P0446 fault code is triggered by a vent valve not opening or closing correctly. 
• The charcoal canister is a device filled with activated charcoal, which is used for trapping fuel vapors. When the vehicle is operating at correct conditions, the control module opens the purge valve and allows fuel vapors from the charcoal canister to flow into the intake system on the way to the combustion chamber.
  • The charcoal canister can come apart internally. The activated charcoal inside can clog the fuel vapor lines. Clogs happen because of foreign objects entering the EVAP system, such as bugs, dust, or dirt.
  • Another cause of canister problems is if you are one of those folks who squeeze as much gas in the tank as possible trying to “top off” to the next dollar or even more. Where do you think that gas goes after the pump kicks off and you force-feed it more? You guessed it, the carbon canister.
  • When you overfill a gas tank, it can damage the EVAP system by flooding the vapor intake points with liquid fuel. This can overwhelm the charcoal canister that is designed to capture fuel vapors, potentially causing it to become damaged. That causes a check engine light and potential repair costs. Essentially, the system is not meant to handle liquid fuel, only vapors, so overfilling disrupts its normal function.

Understand that many components of the evaporative emission system span the car from end to end. Also, vehicle manufacturers have different names for the parts and different designs. However, the goal remains the same - to reburn fuel vapors instead of allowing them to escape into the atmosphere.

Other common EVAP problems:

• Leaks in the EVAP are usually due to failed gaskets or O-rings. EVAP leaks allow fuel vapors to escape into the atmosphere and sometimes be smelled outside the vehicle. 
• Software updates to the control module could potentially need an update (reflash). An update could provide new specifications for the EVAP system and/or how the control module manages it. 

Tools needed to diagnose the EVAP system:

• Many scan tools provide a diagnostic direction by providing fault codes. The best scan tools use special functions to directly test EVAP components, even a service bay test to run a self-test on the entire system.
• A smoke machine fills the EVAP system with smoke and allows the technician to see leaks, small and large. Some brands can even measure the size of the leak.
• Information is vital when diagnosing the EVAP system. Each manufacturer has a specific way they control the EVAP system. An information system like ShopKey Pro can provide useful information to diagnose EVAP problems quickly and accurately.

Having the right tools and a solid understanding can make EVAP systems profitable and one of your good friends.