With smoke applied, it didn’t take long to find the cause of this BMW’s leak.
The Engine Control Module (ECM) is in charge of both the operation and testing of the EVAP system. It checks the system’s ability to purge HC vapors that are stored in the charcoal canister, and checks the system for leaks as small as 0.010 inch up to 0.040 inch. These tests fall under the “non-continuous” monitors, meaning that the system is tested only once during a given drive cycle. Codes set are “two trip” codes, meaning that faults discovered must be confirmed before the ECM commands the Malfunction Indicator Lamp on. In addition, many EVAP monitors have very specific enabling criteria that can make it difficult to run a monitor manually. This includes factors like how long the car has been idle (cold soak time), the amount of fuel in the tank (usually needs to be between ¼ and ¾ full), and others that you should be aware of when troubleshooting EVAP system problems. Because Mode $06 has a listing of the specific tests used by the ECM, it can be a helpful tool in your troubleshooting of intermittent EVAP faults and for verification of the final repair.
Here are a few other EVAP system basics to remember when you begin your testing:
• The Canister Purge Valve is normally CLOSED.
• The Canister Vent Valve is normally OPEN and needs to be closed for leak testing. Global OBD II Mode 8 will perform this function on some models, and you may be able to use your scan tool’s bi-directional features for those that don’t.
• The service port Schrader valve has reverse threads (on systems that use service ports)
Notice the location of the charcoal canister. It wouldn’t be too hard to accidentally damage it if you weren’t careful lifting the vehicle.With smoke applied, it didn’t take long to find the cause of this BMW’s leak.
• Fuel soaked charcoal canisters equal a bad liquid fuel separator. There are different names for this component but it still has to do the same job, allowing only fuel vapors to enter the charcoal canister. If the canister becomes fuel soaked you will need to blow out the lines in a well-ventilated area with an inert gas (not air) besides replacing the liquid fuel separator and charcoal canister.
• Line and hoses are the number one areas that need to be checked for leaks.
• Vent and purge solenoids need to be energized to the open/closed or closed/open positions at least ten times. We have experienced that just simply testing the solenoids by open and closing them a couple of times may prevent you from finding a defective valve that is sticking mid way. Try using your scan tools bi- directional control or use a Power Probe to exercise the valve to uncover one that sticks.
The Three EVAP Systems
EVAP systems can be distinguished by how the ECM performs the leak test. It is interesting to note that many systems use some form of vacuum decay method for self-tests, while most of us tend to use some form of pressure test for our troubleshooting. As with other systems, EVAP leaks might be missed relying on pressure testing alone.
The first one we’ll overview are systems that use engine vacuum as the source vacuum for testing system integrity. GM, Ford and many imports use measurements from sensors giving feedback concerning the vapor management or purge control valve, solenoid-operated canister vent, and fuel tank pressure sensor. They self-test in the following order:
1. Purge valve is closed and the canister vent valve is open. No engine vacuum is reaching the canister and with the canister vent open system pressure equals atmospheric pressure.
2. Purge valve is opened and the canister vent is closed. This should increase EVAP system pressure by 6 to 8 inches of water.
3. Purge valve and canister vent remain closed while the PCM monitors how long the system retains sufficient vacuum.