Owner's Problem Description
The owner reports that their 1991 Suzuki Mehran experiences jerking when changing gears, especially during acceleration from low to high speed and vice versa. The issue persists even when driving in neutral, where the vehicle jerks intermittently. Previous attempts to clean the spark plugs provided temporary relief but the jerking returned after a few days. The owner has since replaced the spark plugs and air filter, which improved the situation temporarily, but they still seek a permanent solution.
Professional Technical Analysis
This analysis focuses on a 1991 Suzuki Mehran with a 1.0L engine and an estimated mileage of 150,000 miles, experiencing significant jerking during gear shifts. The symptoms include intermittent jerking while accelerating, with a notable decline in performance during gear changes. Notably, the vehicle exhibits a jerking sensation when accelerating in both first gear and while in neutral, suggesting potential issues within the ignition system, fuel delivery components, or mechanical linkages such as the throttle cable. A thorough examination of warning signs, including engine misfires and rough idling, indicates possible ignition system failures. Previous repair attempts, including spark plug cleaning, provided only temporary relief, hinting at deeper underlying issues. This vehicle's age and mileage should be considered, as wear on components such as spark plugs and ignition coils can lead to performance issues. The electrical system's behavior, including irregular voltage supply to the ignition system, must also be assessed. In addition, it is essential to verify the integrity of fuel delivery, examining the fuel filter and pump pressure to ensure proper fuel metering. Current vehicle conditions suggest possible safety concerns due to unpredictable jerking, which can affect driver control.
Possible Causes
Most common causes (ordered by frequency):
- Ignition System Malfunction: The ignition system may be experiencing intermittent failures due to worn spark plugs or a failing ignition coil. On the 1991 Suzuki Mehran, the ignition coil should produce a primary voltage of approximately 12V and secondary voltage upwards of 20kV. A drop in spark plug performance can lead to misfires, especially under load during gear changes, causing the jerking sensation when accelerating. - Check engine light activation, rough idle, and misfire codes (P0300-P0304) stored in the PCM indicate ignition issues.
- Fuel Delivery Issues: A clogged fuel filter or failing fuel pump can restrict fuel flow, causing the engine to starve during acceleration. The fuel pressure should be maintained between 35-45 PSI. If the fuel pressure drops below this range, especially under load, it can lead to jerking and lack of power. - Low fuel pressure readings during acceleration tests and potential fuel pump noise indicate delivery issues.
- Mechanical Linkage Issues: The throttle cable may be sticking or improperly adjusted, causing erratic throttle response. This can lead to inconsistent power delivery during gear changes, contributing to the jerking sensation. Verify that the throttle operates smoothly without binding. - Visual inspection of throttle cable and linkage shows signs of wear or binding, affecting throttle operation.
- ECU Software Glitch: An outdated or faulty ECU may not be properly managing fuel and ignition timing, especially during transitions between gears. Reprogramming or updating the ECU software can resolve erratic behavior. - Diagnostic tool shows outdated software version, and recent ECU resets indicate potential software-related issues.
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 Scan and Data Retrieval: Use an OBD-II scanner or GeekOBD APP to retrieve Diagnostic Trouble Codes (DTCs) from the ECM. Check for any stored misfire codes and monitor live data for fuel trim and ignition timing during idle and acceleration. This step will help identify if the issue lies within the ignition system or fuel delivery.
- STEP 2 - Fuel System Pressure Testing: Connect a fuel pressure gauge to the fuel line and check the fuel pressure during idle and acceleration. Ensure it maintains a pressure of 35-45 PSI. If the pressure is below specifications, check the fuel filter and pump for blockages or mechanical failure.
- STEP 3 - Ignition System Component Inspection: Inspect spark plugs and ignition coils for wear. Measure the resistance of the ignition coil (should be 0.5-1.5 ohms for primary and 10-15k ohms for secondary). Replace any components showing signs of wear or failure. Use the GeekOBD APP to monitor ignition timing during tests.
- STEP 4 - Mechanical Linkage Inspection: Visually inspect the throttle cable and linkage for proper operation. Ensure that the throttle opens smoothly without binding and is properly adjusted. Any issues found should be corrected to ensure consistent throttle response.
