Owner's Problem Description
The owner reports that their 1991 Pontiac Sunbird convertible developed an 'almost start' condition. The vehicle cranks and starts but dies after about ten seconds. Background details include recent replacement of the fuel tank, sender, pump, filter, plugs, and wires. The owner can keep the engine running temporarily by spraying carb cleaner into the throttle body, suggesting a fuel delivery issue.
Professional Technical Analysis
This 1991 Pontiac Sunbird with a 3.1L engine, approximately 150,000 miles, is exhibiting a classic fuel delivery problem. The vehicle starts and runs for about ten seconds before stalling, indicating that while the ignition and mechanical components may be functioning correctly, the fuel system is not delivering the required fuel volume or pressure. New fuel components including the tank, sender, pump, and filter have been recently installed, suggesting that the issue may stem from injector failure or electrical problems. The owner mentions a fuel pressure of 41-45 PSI, which is within the acceptable range for this vehicle. However, the fact that the engine runs while carb cleaner is sprayed indicates that the injectors may not be firing correctly. The injectors for a 3.1L engine should have a resistance of 8-12 ohms; if one or more injectors have failed, it can prevent the engine from receiving the proper fuel mixture, leading to stalling. Diagnostic steps should include checking the injector pulse using a noid light and verifying the resistance across the injectors to ensure they are functioning. Additionally, inspecting the wiring harness and connections for signs of wear or damage is critical, as intermittent electrical connections can lead to erratic injector operation. The current condition of the vehicle raises safety concerns, as the stalling can occur unexpectedly while driving, posing a risk to both the driver and others on the road.
Possible Causes
Most common causes (ordered by frequency):
- Injector Failure: The injectors may be faulty, with one or more reading below the required 8-12 ohms resistance, indicating they are unable to open and deliver fuel. This is suggested by the engine running only with carb cleaner sprayed into the throttle body. Injector testing with a noid light will confirm if the PCM is sending the signal to the injectors. - The owner noted that injector #5 read 1.2 ohms, indicating a likely failure, and the engine only runs with carb cleaner.
- Electrical Connection Issues: Corrosion or damage to the wiring harness or connectors may prevent proper signal transmission to the injectors, causing them not to fire. This can be evaluated by inspecting the wiring for wear and testing the connections for continuity. - Intermittent injector operation can often be traced back to poor connections or damaged wiring, particularly in older vehicles like this Sunbird.
- Fuel Pump Issue: Although the pump is new, it is essential to ensure it is providing adequate fuel volume and pressure. Testing the fuel pressure at the fuel rail will confirm that the pump is functioning correctly under load. - Fuel pressure should remain steady around 41-45 PSI; fluctuations may indicate pump failure or blockage.
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 - Fuel Pressure Verification: Begin by confirming fuel pressure at the fuel rail with a gauge, ensuring it remains stable at 41-45 PSI. If pressure is inconsistent, further investigate the fuel pump and fuel delivery system.
- STEP 2 - Injector Testing: Use a noid light to check for injector pulse. If the light does not flash, the PCM may not be sending a signal or the injector may be faulty. Measure injector resistance to confirm functionality.
- STEP 3 - Electrical Inspection: Inspect the wiring harness and connectors for signs of damage or corrosion. Ensure all connections are secure and free of any debris that may cause intermittent issues.
- STEP 4 - Fuel Pump Load Test: Conduct a load test on the fuel pump to verify its performance under operating conditions. This helps to ensure that it can maintain adequate pressure when the engine is running.
