Owner's Problem Description
The owner reports that their 1991 Buick Skylark with a V6 3300 engine won't run after sitting for two years. The car cranks but immediately shuts off. They have replaced the fuel pump, coil pack, crankshaft sensor, and installed a new battery. The owner is uncertain about the fuel pressure specifications and is seeking guidance on potential causes for the stalling issue.
Professional Technical Analysis
The 1991 Buick Skylark with a 3.3L V6 engine typically requires a fuel pressure of around 55 PSI for optimal operation. Given the vehicle's history of sitting for two years, fuel system issues are highly likely, including potential clogging of the fuel filter or contamination in the fuel lines. The symptoms of cranking and immediately shutting off suggest that while the ignition system may be functioning to some extent, there are likely problems with fuel delivery or engine management system signals. Notably, the owner has already replaced the fuel pump, coil pack, and crankshaft sensor, indicating a focus on addressing primary ignition and fuel delivery components. A weak yellow spark, as noted in earlier discussions, may point to issues within the ignition module or coil pack, both of which are critical for generating sufficient spark. The ignition module, located in the coil pack assembly, may also be suspect, especially if it has not been replaced or verified for proper operation. Additionally, the PCM (Powertrain Control Module) needs to be checked for error codes that could indicate sensor failures or wiring issues that affect engine startup. Given these factors, thorough diagnostics focusing on fuel pressure verification, spark quality assessment, and signal integrity from the crankshaft position sensor and ignition module should be prioritized. Safety is paramount, as a no-start condition can also affect vehicle emissions control systems.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pressure Insufficient: The fuel pressure should be maintained at approximately 55 PSI while cranking. If the pressure is lower, it could indicate a faulty fuel pump or a clogged fuel filter, especially after the vehicle has been sitting for an extended period. Use a fuel pressure gauge to verify pressure at the fuel rail. Normal operation requires consistent pressure, and any significant drop below this threshold indicates a fuel delivery issue. - Fuel pressure gauge reading below 55 PSI, possible fuel filter blockage, or fuel pump malfunction.
- Weak Ignition Spark: A yellow spark indicates a weak ignition, which could suggest issues with the ignition coil, ignition module, or wiring. The ignition module, located within the coil pack assembly, may fail to deliver sufficient voltage to the plugs, leading to poor combustion. A proper blue spark is necessary for efficient engine operation. Testing the ignition system with a spark tester is recommended to verify spark quality. - Weak yellow spark observed during cranking, indicating potential ignition system failure.
- Crankshaft Position Sensor Failure: Although the crankshaft sensor was replaced, issues may still arise if the sensor is not calibrated properly or if there are wiring problems. The PCM relies on the crankshaft position sensor to determine the engine's position and timing. If the signal is weak or intermittent, it may cause the engine to shut off immediately after cranking. - Possible intermittent signal from the crankshaft position sensor, leading to immediate shutdown after cranking.
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 - Verify Fuel Pressure: Using a fuel pressure gauge, connect it to the fuel rail and measure the pressure while cranking the engine. It should read around 55 PSI. If it is significantly lower, inspect the fuel pump and filter for blockages or malfunctions. Ensure that the ignition switch is in the ON position to allow the fuel pump to prime.
- STEP 2 - Check Ignition Spark Quality: Use a spark tester to check the quality of the ignition spark. A good spark should be a strong blue color. If the spark is yellow or weak, further investigate the ignition coil and ignition module for proper operation. Test the resistance of the ignition coil (should be within specification) and check for any damaged wiring.
- STEP 3 - Inspect Crankshaft Position Sensor: Verify that the crankshaft position sensor is properly installed and connected. Check the sensor's signal output using a multimeter, which should show a consistent voltage when cranking the engine. If the signal is erratic, consider replacing the sensor again or checking the wiring for shorts or opens.
- STEP 4 - Scan for Diagnostic Codes: Use an OBD-II scanner to check for any stored diagnostic trouble codes (DTCs). This can provide insight into potential sensor or circuit issues affecting the engine's ability to start and run. Document any codes found and investigate the corresponding systems.
