Owner's Problem Description
The owner of a 1997 Pontiac Sunfire reported an intermittent starting issue where the vehicle would stall after starting. The problem appeared after returning from a family event. Previous attempts to resolve the issue included replacing the fuel pump. The vehicle exhibits good ignition spark during cranking, but the owner expressed frustration over the unresolved stalling and starting problems.
Professional Technical Analysis
The 1997 Pontiac Sunfire in question, equipped with a 2.2L 4-cylinder engine and a manual transmission, has approximately 150,000 miles on the odometer. The owner reported intermittent starting issues where the vehicle would stall shortly after starting. The vehicle's ignition system appears functional, as confirmed by good spark during cranking. However, fuel delivery issues must be investigated, particularly as the owner has already replaced the fuel pump. Upon further investigation, it was discovered that a dark green/white wire from the Powertrain Control Module (PCM) to the fuel pump relay had an open circuit due to a break in the wire. This intermittent connection caused the fuel pump to operate inconsistently; the vehicle would start when the broken wire touched, but vibrations from driving would cause the connection to fail, leading to stalling. This scenario underscores the importance of inspecting wiring for continuity and damage, especially in high-vibration areas. Additionally, the fuel pressure should be monitored, ideally maintaining 35-45 PSI during operation. A proper diagnostic procedure utilizing tools such as a multimeter and visual inspection of wiring diagrams is critical. The current condition of the vehicle poses a safety concern as the stalling could occur unexpectedly while driving, necessitating immediate attention to prevent potential accidents.
Possible Causes
Most common causes (ordered by frequency):
- Open Circuit in Fuel Pump Relay Wiring: The dark green/white wire from the PCM to the fuel pump relay has an open circuit due to a break, causing intermittent fuel pump operation. This issue results in the vehicle starting when the wire makes contact, but vibration causes it to lose connection, leading to stalling. Proper wire continuity testing should confirm the integrity of the circuit. - Intermittent starting issues, stalling when driving, confirmed wire break under inspection.
- Fuel Pump Relay Malfunction: The fuel pump relay may be faulty, causing inconsistent power delivery to the fuel pump. This can lead to the pump not activating when the ignition is turned on, even if the pump itself is new. Testing the relay for proper operation and replacing it if necessary is crucial. - Fuel pump does not activate consistently, relay testing shows intermittent connectivity.
- Low Fuel Pressure: Although the fuel pump has been replaced, it is essential to check the fuel pressure. Low fuel pressure (below 35 PSI) can cause stalling or failure to start. A fuel pressure gauge should be used to verify the pump's output under cranking conditions. - Fuel pressure gauge readings below specifications during testing.
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 - Visual Inspection: Begin with a thorough visual inspection of the wiring harness between the PCM and fuel pump relay. Look for signs of wear, fraying, or breaks in the insulation that could indicate potential failure points. Pay particular attention to areas near the radiator where vibrations may cause wire fatigue.
- STEP 2 - Continuity Testing: Use a multimeter to perform continuity tests on the dark green/white wire to ensure there are no breaks in the circuit. A reading of 0 ohms should be expected if the wire is intact; any higher resistance indicates a problem that needs to be addressed.
- STEP 3 - Fuel Pressure Check: Connect a fuel pressure gauge to the test port on the fuel rail. Start the vehicle and check the fuel pressure, which should be within the 35-45 PSI range. If the pressure is low, further investigation into the fuel pump operation and relay functionality is required.
- STEP 4 - Relay Functionality Test: Test the fuel pump relay by applying voltage directly to it while observing if the fuel pump activates. If the pump runs when the relay is bypassed, the relay is likely faulty and should be replaced.
