Owner's Problem Description
The owner reports intermittent stalling of their 1993 Buick Regal with a 3.1L engine over a two-week period, eventually leading to a complete no-start condition. Various components have been tested, including the crank sensor, MAP sensor, camshaft sensor, ECM, and ignition module. The vehicle has spark and maintains 42 lbs of fuel pressure, with the regulator confirmed to be functional. Uniquely, the car will only start when the throttle body is choked manually, running well thereafter until it fails to start again after several restarts.
Professional Technical Analysis
This analysis focuses on a 1993 Buick Regal equipped with a 3.1L V6 engine, which has likely traveled over 150,000 miles. The reported symptom of intermittent stalling, culminating in a no-start condition, suggests a potentially complex issue, likely involving the electronic control systems. The vehicle exhibits normal spark and fuel pressure of 42 PSI, which is adequate for operation, indicating the ignition and fuel delivery systems may be functioning correctly. However, the unique requirement to choke the throttle body manually to initiate starting points towards possible air-fuel mixture problems or sensor malfunctions. Testing of the crankshaft position sensor and camshaft position sensor is crucial, as failures here can affect the timing of fuel injection and spark. It is also essential to evaluate the coolant temperature sensor, as incorrect readings can lead to improper fuel mixture adjustments. Further, the ECM's adaptability when swapped with a known good unit raises concerns about potential wiring issues or intermittent faults in other related sensors, such as the MAP sensor. Analyzing the vehicle's wiring for corrosion or poor connections might reveal hidden faults affecting performance. Given the age of the vehicle, factors such as grounding issues or electrical noise must also be considered, as they can introduce erratic behavior in the engine management systems. The current condition of the vehicle shows signs of potentially unsafe operation due to the unpredictable starting behavior, warranting immediate attention.
Possible Causes
Most common causes (ordered by frequency):
- Faulty Coolant Temperature Sensor: If the coolant temperature sensor is providing incorrect readings, it can significantly affect the engine's air-fuel mixture. During testing, it should show a resistance of around 2,500 ohms at 70°F and drop to around 300 ohms at 200°F. An out-of-spec sensor could cause the ECM to either enrich or lean out the mixture improperly, leading to poor starting and stalling issues. Disconnecting the sensor should trigger the cooling fan, indicating it's not functioning correctly if it doesn't. - Check engine light may illuminate, incorrect fuel trims observed during live data analysis, with potential codes related to fuel system or mixture.
- Intermittent Crankshaft Position Sensor Failure: The crankshaft position sensor may be failing intermittently. It should produce a voltage signal of 0.5V to 5V while cranking. If the signal drops out or becomes erratic, it can lead to poor starting conditions, as the PCM relies on this signal for timing fuel injection and spark. Given the vehicle's age, sensor wear or internal shorting could be contributing factors. - Intermittent no-start condition, potential stored codes P0335-P0336 related to crankshaft position.
- Ignition Module Failure: The ignition module may not be providing consistent spark under certain conditions. It should provide a square wave signal of approximately 5V on the primary circuit when tested with an oscilloscope. If the module fails under heat conditions or shows inconsistent output, it can lead to stalling and no-start situations. Given the age of the vehicle, deterioration of internal components is common. - Symptoms include misfires, intermittent starting issues, and potential stored codes P0350-P0358 related to ignition system.
- Faulty MAP Sensor: The MAP sensor may be malfunctioning, providing incorrect vacuum readings to the PCM. At idle, it should read around 1V and approximately 4.5V at wide-open throttle (WOT). If the readings are outside these ranges, it may lead to improper fuel metering and stalling, especially under load. Testing with a multimeter will confirm the voltage output. - Poor engine performance, hesitation during acceleration, and possible lean or rich running conditions observed.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial OBD-II Diagnosis: Start with a comprehensive scan using a professional OBD-II scanner or the GeekOBD APP to retrieve any stored codes. Pay special attention to any freeze frame data, which can provide crucial context for the symptoms being experienced. Monitor live data for key parameters such as RPM, coolant temperature, and fuel trim to identify any anomalies during the starting process.
- STEP 2 - Sensor Verification: Check the crankshaft and camshaft position sensors with a multimeter. The crankshaft sensor should provide a consistent voltage signal while cranking (0.5-5V), and the camshaft sensor should show continuity. Also, verify the MAP sensor outputs during different throttle positions to ensure correct readings. Review the coolant temperature sensor's resistance values at varying temperatures to confirm proper operation.
- STEP 3 - Ignition System Testing: Test the ignition module and coils for proper output. The ignition module should provide a square wave signal of approximately 5V on an oscilloscope. In addition, check the secondary resistance of the ignition coils (should be within 0.5-1.5 ohms). Ensure all electrical connections are clean and tight to prevent intermittent failures.
- STEP 4 - Fuel System Evaluation: Confirm fuel pressure at the rail using a fuel pressure gauge, verifying it maintains a steady 42 PSI. Inspect the fuel pump for proper operation and listen for any unusual noises. If possible, perform a fuel volume test to ensure adequate flow. Additionally, check the fuel filter for clogs, which can restrict flow and cause starting issues.
