Evaporative Emission Control System Vent Control Circuit Malfunction
This indicates that a portion of the EVAP control system is no longer functioning properly. The EVAP system consists of components including (but not limited to) the gas cap, fuel lines, charcoal canister, purge valve, system pressure and flow sensors, electrical wiring and connectors, vacuum lines, the fuel tank, and fuel vapor hoses.
Code Set Parameters
Variations in evaporative emission control system pressure or flow are received by the PCM in degrees of voltage. If the PCM detects a voltage reading that does not coincide with the manufacturer’s reference voltage, a code will be stored and a malfunction indicator will be illuminated. Some models require multiple failure cycles for service engine soon lamp illumination but a code is normally stored on the first failure. Stored codes presented without malfunction indicator lamp illumination may be read as pending codes.
Normally, no symptoms other than a stored code and a service engine soon lamp will be exhibited. However, other evaporative emission control system codes are likely to be present.
These may include a faulty or improperly installed fuel cap, a faulty purge control solenoid, clogged or broken charcoal canister, broken or cracked vacuum or fuel vapor hoses, a faulty pressure or flow sensor, or a bad PCM. PCM failure is rare.
Evaporative emission control system components are often replaced in error when a vacuum leak is present or the fuel cap is not properly tightened.
- The evaporative emission control system is responsible for capturing fuel vapors (from the fuel tank and lines) before they escape into the atmosphere
- This is usually accomplished using a combination of engine vacuum, a charcoal storage reservoir, and electrically powered solenoids
- Flammable fuel vapors are directed to the charcoal canister via plastic or metal hoses where they are stored until a signal from the PCM activates the purge control solenoid
- The purge control solenoid opens the purge control valve (with the engine running) and allows engine vacuum to draw the fuel vapors into the engine where they are burned just like liquid fuel.
The charcoal canister is typically located near the fuel tank, which is underneath the vehicle
- The fuel vapor and vacuum hoses can be quite long and subject to damage from the elements and road debris.
Several tools may be needed to successfully diagnose this code if no exhaust leaks are detected
- A suitable scanner (or code reader), a digital volt/ohmmeter, and a smoke machine will be most helpful in performing a successful diagnosis
Begin with a visual inspection of all wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by clearing the code and operating the vehicle to see if it returns
- This will help to determine whether or not the malfunction is intermittent
After the codes are cleared, test drive the vehicle to see if the code returns
- If the code fails to immediately return, you may have an intermittent condition
- Intermittent conditions can prove to be quite a challenge to diagnose and in extreme cases may have to be allowed to worsen before a correct diagnosis can be made
Proceed with your diagnostic process by removing and reinstalling the fuel cap and test driving the vehicle (multiple drive cycles may be required to activate a service engine soon lamp) to see if it returns
- If other evaporative emission control system codes are present, diagnose and repair them as required
- Reset the codes and test-drive the vehicle.
If an evaporative emissions system leak code is present (and the fuel cap is good), visually inspect vacuum and fuel vapor hoses for cracks, splits, kinks, or breakage, and repair as necessary
- Inspect the charcoal canister and purge valve for damage or corrosion, replace faulty parts as needed
- Start the engine and listen for vacuum leaks near the purge control valve or the hose from the purge valve to the intake
- If no leaks are detected, a smoke machine can be very helpful for locating leaks in the extensive network of vacuum and fuel vapor lines that are required for the evaporative emissions control system
- Place the vehicle on a lift so that you can observe it from underneath
- Start the engine and allow it to enter closed loop operation
- Listen carefully to the purge control solenoid
- Once the solenoid begins to make a clicking noise, the purge valve should be opened
- Place the nozzle of the smoke machine hose into the vacuum supply hose for the purge solenoid and allow the system to fill with smoke
- Visually inspect all lines, hoses, and system components for escaping smoke
- Pay particular attention to the area around the fuel cap, as faulty gas caps are common
- Remember that the charcoal canister usually has a vent which will allow smoke to escape slowly
- This is normal
Evaporative emission control system codes that pertain to flow can be best diagnosed beginning with the charcoal canister
- A charcoal canister that has become contaminated with liquid can get clogged and restrict purge flow
- Check for suction at both sides of the charcoal canister when the purge control solenoid is opened
- If there is suction to the canister, it is probably faulty
- Remove the canister and shake it, listening for broken or loose particles
- Replace the canister as necessary
- If the canister proves to be functional, check fuel vapor hoses for kinks from crushing
- If the canister and lines appear normal, test the purge control solenoid for proper operation
- It should act like a “on/off” valve that is activated by completing a circuit that is normally rated for battery voltage
- Check manufacturer’s specs before applying voltage to the solenoid
- If the solenoid is faulty, an evaporative emissions control circuit code should also be exhibited
If an evaporative emission control system circuit code is present, inspect electrical connectors and wiring for signs of corrosion or damage and repair as required
- If wiring and connectors appear to be operational, unplug the connector from the PCM and perform a resistance check on the purge control solenoid, the system pressure and flow sensors, and system wiring
- Compare your findings to manufacturer’s specifications and repair as necessary
- Reset the codes and retest the vehicle
- Keep in mind, that following repairs, multiple drive cycles may be necessary to verify that the fault condition has been rectified.