Owner's Problem Description
The owner reports that the engine light is on, and the car jerks when accelerating, particularly at high speeds. The check engine light was confirmed with a code indicating 'No RPM Reference Pulse' when the engine was not running. The owner is unsure of any previous repairs since they just acquired the vehicle. Currently, the vehicle experiences a noticeable jerking sensation while driving, especially when going uphill.
Professional Technical Analysis
The 1992 Saturn SL1, which features a 1.9L SOHC engine and automatic transmission, has approximately 120,000 miles. The reported symptoms include a check engine light and a jerking sensation when accelerating, especially noticeable during high-speed driving or uphill climbs. The 'No RPM Reference Pulse' code is typically indicative of an issue with the crankshaft or camshaft position sensors or their circuits. However, this code often appears when the engine is not running and does not inherently indicate a fault by itself. This jerking behavior may suggest a misfire or fuel delivery issue, commonly associated with worn spark plugs or ignition components. Given the vehicle’s age and mileage, I would recommend checking the ignition system first, including spark plugs and wires, as these are common failure points leading to misfire symptoms. If these components are in good condition, the next step would be to investigate the sensor circuits for any wiring issues or faults. Additionally, considering the vehicle's history, the condition of the fuel system, including the fuel filter and pump, should also be evaluated, as restricted fuel flow can also lead to performance issues under load. Proper measurements of fuel pressure, spark quality, and sensor signals should be taken during diagnostic procedures to pinpoint the root cause of the problems.
Possible Causes
Most common causes (ordered by frequency):
- Crankshaft Position Sensor Failure: The crankshaft position sensor is critical for RPM reference; if it fails or has wiring issues, it can cause the PCM to not receive RPM signals, leading to misfires and jerking during acceleration. Typical resistance for this sensor should be 200-800 ohms, and any readings outside this range indicate potential failure. - Check engine light active with code indicating RPM reference issue, possible intermittent signals when tested.
- Ignition System Weakness: Worn spark plugs and faulty ignition wires can lead to weak spark generation, causing misfires and jerking during acceleration. Spark plugs should be replaced every 30,000 miles, and if they are not replaced recently, this could explain the symptoms. - Vehicle history indicates recent acquisition; possible lack of maintenance records for ignition components.
- Fuel Delivery Issues: A clogged fuel filter or failing fuel pump can restrict fuel flow under acceleration, leading to symptoms of jerking. Fuel pressure should be checked, which should normally be around 35-45 PSI. Any significant deviation from this could indicate a fuel system problem. - Symptoms worsen under load or during high-speed driving, indicating possible fuel delivery problem.
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 - OBD-II Scan: Begin by using an OBD-II scanner or the GeekOBD APP to retrieve stored codes and freeze frame data. Pay attention to the 'No RPM Reference Pulse' code and any other active codes. Document the conditions during the fault occurrence.
- STEP 2 - Live Data Monitoring: Use the GeekOBD APP to monitor live data from the engine sensors. Focus on RPM signals, fuel pressure (should be 35-45 PSI), and ignition timing. Any irregularities can point toward the root cause of the jerking sensation.
- STEP 3 - Component Testing: Conduct a thorough check of the ignition system, including spark plugs and wires. Measure resistance values of the crankshaft position sensor and inspect for any wiring issues. Use a multimeter to check for continuity and proper voltage.
- STEP 4 - Fuel Pressure Check: Install a fuel pressure gauge to monitor the system pressure during a running condition. Ensure it stays within the specified range of 35-45 PSI. If pressure is low, further investigation into the fuel pump and filter is required.
