Owner's Problem Description
The owner reports that the fuel pump in their 1991 Pontiac Firebird continues to run even when the ignition key is removed. They have already checked the relays located behind the master cylinder on the driver's side firewall, but pulling each relay did not turn off the fuel pump. The owner seeks advice on potential causes and solutions, including the location of the fuel relay.
Professional Technical Analysis
The 1991 Pontiac Firebird, equipped with a 3.1L V6 engine and an automatic transmission, is experiencing a condition where the fuel pump remains activated even after the ignition key has been removed. This situation often arises due to electrical faults in the fuel pump control circuit. The fuel pump is designed to operate only when the ignition is in the 'run' or 'start' position, typically controlled by the Engine Control Module (PCM) and components like the fuel pump relay and oil pressure switch. Given the vehicle's age, approximately 150,000 miles, potential wear and corrosion of electrical connectors and components must be considered. The owner has already identified three relays located behind the master cylinder, but the problem persists when these are disconnected, indicating that the issue may not solely lie within the relay. Furthermore, the oil pressure switch must be evaluated, as it can maintain power to the fuel pump if malfunctioning. When the relay and oil pressure switch are bypassed or faulty, the fuel pump may continue to receive voltage, causing it to run indefinitely. This problem is not only a nuisance but also poses a fire hazard if the vehicle is left unattended. Therefore, conducting a thorough diagnostic check of the circuit is crucial, focusing on testing the oil pressure switch and inspecting for any short circuits or damaged wiring. Additionally, using the GeekOBD APP can help monitor live data to pinpoint anomalies in the fuel system operation.
Possible Causes
Most common causes (ordered by frequency):
- Oil Pressure Switch Failure: The oil pressure switch is located on the lower left front of the engine. If this switch fails, it can send a continuous signal to the fuel pump, keeping it powered even when the ignition is off. This is common in older vehicles and can often be diagnosed by disconnecting the switch to see if the pump turns off. - Fuel pump shuts off when the oil pressure switch connector is unplugged, indicating it is likely faulty.
- Wiring Short Circuit: There may be a short circuit in the wiring harness that connects to the fuel pump relay or the oil pressure switch. This could result in unintentional power being supplied to the fuel pump, causing it to run continuously. Inspecting the wiring for chafing or corrosion is recommended. - Visual inspection of wiring shows signs of wear or damage, which may indicate a short circuit.
- Fuel Pump Relay Malfunction: The fuel pump relay itself may be faulty, causing it to stick in the 'on' position. However, as the owner has already checked and pulled each relay without effect, this may be less likely unless the relay is stuck internally. - Relay pulls out without affecting fuel pump operation, indicating no issue with relay function.
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 associated with the fuel pump relay and oil pressure switch. Look for any signs of wear, corrosion, or damage that could lead to short circuits or unintended connections. Pay close attention to the connectors and wiring insulation.
- STEP 2 - Oil Pressure Switch Testing: Disconnect the oil pressure switch connector and observe if the fuel pump turns off. If it does, the oil pressure switch is faulty and needs replacement. If the pump continues to run, further investigation into the wiring or relay may be necessary.
- STEP 3 - Relay Functionality Check: Although the owner has already pulled the relays, it may still be beneficial to test them with a multimeter. Check for continuity and proper operation when powered. A faulty relay may not always show visible signs of failure.
- STEP 4 - Circuit Testing: Using a multimeter, test the voltage at the fuel pump connector while the ignition is off. This can help identify if power is still being supplied to the pump due to a short circuit in the wiring. If voltage is present, further investigation into the wiring harness is warranted.
