Owner's Problem Description
The owner of a 1991 Honda Civic LX reports intermittent stalling while driving. The tachometer drops to zero suddenly, followed by a clicking sound from under the dash. After a few minutes, all warning lights illuminate and the vehicle dies. Restarting requires a jump, and the issue recurs shortly after. The vehicle has a new battery, and the alternator was tested and deemed functional.
Professional Technical Analysis
The 1991 Honda Civic LX, equipped with a 1.5L 4-cylinder engine and automatic transmission, is experiencing intermittent stalling at approximately 180,000 miles. The symptoms include a sudden drop in tachometer reading to zero, warning lights activating, and eventual engine shutdown. This behavior can indicate potential issues with the electrical system or engine management components. Given that the alternator has been tested and confirmed operational, attention should focus on the crankshaft position sensor (CPS), which is known to fail intermittently in older Honda models. A failing CPS can cause erratic tachometer readings, loss of spark, and ultimately engine stalling. Additionally, voltage fluctuations in the electrical system could lead to erratic PCM behavior, exacerbated by poor connections or wiring issues. When the car dies, the clicking sound from under the dash may indicate a relay or module failure, possibly linked to the ignition system. Current safety concerns include the reliability of the vehicle while in motion, as stalling could lead to dangerous situations on the road. Critical inspection of the CPS, electrical connections, and potential short circuits should be prioritized, alongside a thorough check of the vehicle's ground connections and battery terminals for corrosion. Understanding these interconnected systems is vital for accurate diagnosis and repair.
Possible Causes
Most common causes (ordered by frequency):
- Crankshaft Position Sensor (CPS) failure: The CPS may be experiencing thermal failure or deterioration, leading to intermittent loss of signal to the PCM. This could cause the tachometer to drop to zero and result in the engine stalling. Common symptoms include no-start conditions when hot, with normal operation when cool. It's critical to test the CPS resistance (should be within 200-800 ohms) and check for proper signal output under cranking conditions. - Intermittent stalling, tachometer dropping to zero, clicking from under the dash when the car dies.
- Faulty electrical connections: Corrosion or loose connections at the battery terminals or ground points may lead to voltage drops, causing the PCM to lose power intermittently. This may explain the clicking sound from the relay or module when the vehicle stalls and the warning lights illuminate. A thorough inspection of all electrical connections for integrity and tightness is essential. - Clicking sound from under the dash during stall, potential corrosion on battery terminals or ground connections.
- PCM malfunction: The Powertrain Control Module may be experiencing internal faults or software glitches, which can disrupt its ability to manage engine functions. This may lead to erratic tachometer readings, stalling, and warning lights illuminating. Diagnostic trouble codes (DTCs) should be retrieved to evaluate any stored codes that might indicate PCM issues. - Intermittent stalling, possible DTCs related to PCM performance, erratic gauge behavior.
- Ignition system failure: Components such as the ignition coil, ignition control module, or distributor may be failing under heat or load conditions, causing a loss of spark and resulting in engine stalling. Testing these components for resistance values (coil primary: 0.5-2 ohms, secondary: 6,000-15,000 ohms) can help identify faults. - Loss of spark, intermittent starting issues, potential heat-related failures in ignition components.
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 Diagnostic Scan: Use GeekOBD APP to scan for any stored DTCs and monitor live data. Pay special attention to any codes related to the CPS or PCM. This will provide insight into the potential failure points and help narrow down the diagnostic process. Check for freeze frame data to evaluate conditions at the time of the failure.
- STEP 2 - Electrical System Inspection: Inspect battery terminals, ground connections, and wiring harnesses for corrosion, looseness, or damage. Clean any corroded terminals and ensure tight connections. Measure battery voltage under load to ensure proper output (should be above 12.4V).
- STEP 3 - Crankshaft Position Sensor Testing: Disconnect the CPS connector and measure its resistance. It should be within 200-800 ohms. If resistance is out of specification, replace the CPS. Additionally, check for proper signal output using an oscilloscope while cranking the engine.
- STEP 4 - Ignition System Evaluation: Inspect and test ignition components (coil, module, distributor) for proper operation. Measure resistance values and look for any signs of wear or overheating. Ensure that all connectors are secure and free from corrosion.
