Owner's Problem Description
The owner reported that while backing out of the driveway, the engine suddenly stopped as if the key was turned off. The engine cranks but fails to start. The fuel pump operation is audible, and starting fluid was attempted but did not yield any sputter. Previous repairs include replacing the crankshaft position sensor and engine temperature sensor. The owner is now considering replacing the computer due to the persistent no-start condition.
Professional Technical Analysis
The vehicle in question is a 1998 Saturn SC2 with a likely 150,000+ miles. The symptoms began abruptly while reversing, indicating a possible electrical or fuel system failure. The owner confirms the fuel pump is operational as indicated by the audible sound during cranking, suggesting the fuel delivery system may be intact. However, the absence of spark when checking the ignition system points to a potential failure in the ignition control components. The crankshaft position sensor was replaced, which is crucial for ignition timing and fuel injection. Typical failure patterns for this vehicle include ignition module failures or wiring issues that can disrupt the signal to the ignition coils. The absence of a spark during cranking suggests a deeper issue potentially related to the ignition module or PCM (Powertrain Control Module). The vehicle also shows no stored trouble codes, which complicates the diagnosis. Current safety concerns involve ensuring the vehicle will not stall unexpectedly once it is operational. The overall condition of the vehicle suggests a thorough examination of the ignition system, including connections and components related to spark generation, as well as potential testing of the PCM.
Possible Causes
Most common causes (ordered by frequency):
- Ignition System Failure: The lack of spark during cranking typically indicates a failure in the ignition system, which could be due to a faulty ignition control module or damaged wiring. The ignition module's resistance should be checked, as it often fails due to heat exposure. Typical resistance values for the module are around 800-1200 ohms. If the module is functioning, inspect the wiring harness for any signs of wear or damage, especially around the starter area where heat can cause insulation breakdown. - Confirmed lack of spark during cranking, resistance values for the new crank sensor at 850 ohms indicate it is functioning properly, but no ignition signal detected.
- PCM Malfunction: A faulty PCM can lead to issues with signal generation for the ignition system and fuel injectors. Given the symptoms and no stored trouble codes, the PCM may not be sending the proper signals. Testing the PCM with a specialized scanner can determine if it is functioning correctly. If found faulty, replacement with an OEM PCM is recommended, typically costing between $400-$800. - Absence of stored codes, symptoms consistent with PCM failure, and lack of control over ignition timing and fuel delivery.
- Faulty Ignition Coils: If the ignition coils are failing, they may not produce the necessary spark for ignition. Testing the coils for primary and secondary resistance values is critical. The primary resistance should be around 0.5-1.5 ohms, and secondary resistance should be 6,000-12,000 ohms. Additionally, check for arcing or damage to the coil terminals. - No spark detected from the ignition coils when cranked, indicating potential failure.
- Fuel System Issues: Although the fuel pump is operational, there could be issues with the fuel injectors or fuel pressure regulator. The fuel pressure should be tested to ensure it meets specifications (35-45 PSI). If the fuel injectors are clogged or the regulator is faulty, this could prevent proper fuel delivery. - Fuel pump operational but pressure measurements needed to confirm system integrity.
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: Begin with a comprehensive scan using GeekOBD APP to retrieve any stored codes and monitor live data. Specifically, check for any codes related to the ignition system or PCM. This initial step can provide valuable insights into the vehicle's operational status and any fault patterns.
- STEP 2 - Spark Testing: With the ignition coils accessible, remove the spark plug wires and use a spark tester to check for spark while cranking the engine. If no spark is detected, this indicates a potential issue with the ignition system components. Document the resistance values of the ignition coils.
- STEP 3 - Fuel Pressure Testing: Connect a fuel pressure gauge to the fuel rail to check the fuel pressure. It should read between 35-45 PSI at the fuel rail. If the pressure is low, inspect the fuel pump and fuel filter for blockages.
- STEP 4 - PCM Testing: If no spark is present and fuel pressure is adequate, consider testing the PCM. Use a specialized scanner to check the PCM functionality and inspect wiring for any signs of damage. If the PCM is suspected to be faulty, it may require replacement.
