A timing value of 13 degrees is likely what the PCM is commanding under a specific set of operating conditions. If you believe the timing is incorrect, you need to troubleshoot the underlying problem rather than attempting to force a static timing value. Possible causes for an issue that might *appear* as incorrect timing include:
* Faulty PCM: A malfunctioning PCM is the most likely culprit if you're observing a consistently abnormal timing value. This requires professional diagnosis and likely replacement of the PCM.
* Crankshaft Position Sensor (CKP) issue: A faulty CKP sensor provides inaccurate signals to the PCM, leading to improper timing calculations.
* Cam Position Sensor (CMP) issue: Similar to the CKP sensor, a malfunctioning CMP sensor will cause timing problems.
* Vacuum Leaks: Vacuum leaks can affect the engine's operation and could indirectly influence the PCM's timing adjustments.
* MAF Sensor issue: A bad Mass Airflow Sensor provides inaccurate air readings, causing the PCM to miscalculate fuel and ignition timing.
* Throttle Position Sensor (TPS) issue: An inaccurate TPS reading also affects the PCM's calculations.
What to do:
1. Obtain diagnostic codes: Use an OBD-II scanner to read trouble codes from the PCM. These codes can point directly to the faulty sensor or component.
2. Consult a repair manual: A factory service manual for your specific year and model Tahoe will provide detailed information about testing sensors and components.
3. Professional diagnosis: If you're not comfortable diagnosing these issues yourself, take your vehicle to a qualified mechanic or auto repair shop. They have the tools and expertise to properly diagnose and repair the underlying problem causing the apparent timing issue.
Trying to manually adjust the ignition timing on this system is not only futile but extremely risky and will likely cause significant damage. Focus on addressing the root cause through proper diagnostics.
