The Evaporative Emission (EVAP) system in your Chevy plays a crucial role in reducing harmful pollutants. A key component of this system is the On-Board Diagnostics II (OBDII) system, which monitors the EVAP system’s performance and readiness. This article outlines the specific GM procedure to ensure your Chevy’s OBDII EVAP system is ready for emissions testing. Getting the EVAP monitor to a “Ready” state can sometimes be challenging, but understanding the process makes it easier.
Understanding the GM EVAP Monitor Procedure
The GM EVAP monitor procedure is standardized across General Motors vehicles, including Chevrolet models. This procedure verifies the integrity of the EVAP system, preventing fuel vapors from escaping into the atmosphere. The process involves a specific sequence of driving conditions and tests to ensure all system components are functioning correctly. The OBDII system uses various sensors and parameters to determine the EVAP system’s readiness. These include:
- Ignition Voltage: Must be within the acceptable range (11-18 volts).
- Barometric Pressure (BARO): Needs to be above a certain threshold (more than 75 kPa).
- Engine Coolant Temperature (ECT) and Intake Air Temperature (IAT): Both must be within a specific range (4-30°C or 39-86°F) at startup, with a minimal difference between them.
- Ambient Air Temperature: Similar to ECT and IAT, it needs to be within a defined range (4-30°C or 39-86°F).
- Fuel Level: Should be between 15-85% for accurate readings.
- Fuel Alcohol Content: Varies depending on the vehicle’s RPO code (LMG).
Chevy OBDII EVAP Readiness Drive Cycle
To set the EVAP monitor to “Ready,” a specific drive cycle, often referred to as the “I/M (Inspection/Maintenance) Complete System Set Procedure,” must be followed. This involves a series of steps, including:
- Cold Start: Ensure your Chevy meets the cold start criteria mentioned earlier. Allow the vehicle to sit for at least 5 minutes after confirming the criteria before starting the engine.
- Initial Idle: Start the engine and let it idle for at least 2 minutes, or until the engine reaches an operating temperature of 65°C (149°F).
- Engine Run: Run the engine for 6.5 minutes, maintaining specific Mass Air Flow (MAF) and engine speed (RPM) parameters.
- Idle and Acceleration: Alternate between idling and controlled acceleration periods, adhering to specific Throttle Position (TP) sensor angle, MAF signal, and RPM requirements. Avoid touching the accelerator during idle periods.
- Driving Cycle: Drive the vehicle at low speeds (24 km/h or 15 mph) for a specified duration, followed by highway driving (45-112 km/h or 28-70 mph), including a deceleration fuel cut-off test.
- Post-Drive Idle: After the driving cycle, let the engine idle for a final period. Then turn off the ignition and do not disturb the vehicle for 45 minutes.
Verifying Chevy OBDII EVAP Readiness
After completing the drive cycle, use an OBDII scanner to check the I/M System Status indicators. Ideally, all indicators, including the EVAP monitor, should display “YES” or “Ready.” If the EVAP monitor still shows “NO” or “Not Ready,” repeat steps 12-18 up to six more times, allowing a 17-hour period between attempts. If the issue persists, further diagnosis may be necessary, using the OBDII scanner to retrieve Diagnostic Trouble Codes (DTCs) related to the EVAP system.
Conclusion
Successfully running the GM EVAP monitor procedure and achieving a “Ready” status for your Chevy’s OBDII system is crucial for passing emissions tests. Following the outlined steps and understanding the underlying parameters contributes to a smoother process. If problems persist, consulting a qualified automotive technician is recommended.
