* Faulty CMP Sensor: The most likely cause is a failing CMP sensor. When unplugged, the engine defaults to a pre-programmed limp mode. This mode uses a basic fuel and timing strategy. Since it runs better in limp mode, the sensor is likely sending inaccurate signals to the Engine Control Module (ECM), leading to improper fuel delivery and ignition timing. The inaccurate readings may be intermittent, explaining why sometimes it runs fine (when the sensor is giving a momentarily accurate reading) and other times it runs poorly.
* Wiring/Connector Problems: Check the wiring and connector to the CMP sensor for damage, corrosion, or loose connections. A short, break, or corrosion in the wiring can send erratic signals to the ECM, mimicking a faulty sensor.
* ECM Issue (Less Likely): While less likely, a problem with the ECM itself could misinterpret the CMP sensor's readings. This is less probable given the improvement when the sensor is unplugged. An ECM issue would likely cause poor running regardless of the sensor's connection.
* Other Sensor Problems (Possible but less likely): Other sensors (crankshaft position sensor, throttle position sensor, mass airflow sensor) can influence the ECM's calculations. A faulty sensor in this group could indirectly cause the symptoms you describe, however, the consistent improvement with the sensor disconnected points more strongly towards the CMP sensor or its wiring.
Troubleshooting Steps:
1. Inspect the CMP Sensor and Wiring: Carefully examine the CMP sensor connector for corrosion, broken wires, or loose connections. Clean the connector with electrical contact cleaner if necessary. Inspect the wiring harness leading to the sensor for any visible damage.
2. Check the CMP Sensor for Proper Operation: Ideally, you'd use a scan tool to check the sensor's voltage output. This requires a scan tool capable of accessing live data. A faulty sensor often shows erratic readings or readings outside of the specified range.
3. Replace the CMP Sensor: If inspection reveals no obvious wiring problems and the sensor's signal is faulty, replacing the CMP sensor is the most likely solution. Ensure you get a sensor from a reputable source.
4. Have the Vehicle Scanned: Get the vehicle scanned with a professional OBD-II scanner. This will retrieve any diagnostic trouble codes (DTCs) stored in the ECM. These codes can pinpoint the source of the problem more precisely.
Don't continue driving with the sensor unplugged, as the engine is running in a 'safe' mode, not optimal conditions. This can potentially cause additional damage in the long run.
