Owner's Problem Description
The owner describes that after the car sat idle for over two years, they replaced the timing belt, water pump, fuel and air filters, and added fresh fuel. Although the fuel pump runs when the ignition is turned on, the fuel cutoff light remains illuminated and the engine will not start. Previous checks indicated good spark and correct timing, and the inertia switch has been reset and verified for connection.
Professional Technical Analysis
The vehicle in question is a 1994 Ford Probe which has been inactive for over two years, leading to several potential issues affecting engine starting. The car has been serviced with a new timing belt, water pump, fuel filter, and air filter, along with fresh gasoline. The fuel pump is operational, as indicated by audible noise during ignition, and fuel flow was confirmed during previous maintenance. However, the fuel cutoff light remains illuminated, and the engine fails to start. With good spark and seemingly accurate timing verified statically, attention must shift to the fuel system and electrical components. The inertia switch, which is a common failure point in fuel delivery issues, has been checked and reset without resolution. Initial diagnostics show that disconnecting the inertia switch results in the fuel pump stopping, crucially indicating that power is not reaching the pump. The voltage reading of 5V across the inertia switch connector suggests inadequate power supply. A thorough examination of wiring harnesses and relay functionality is necessary. The relay click noted after a few seconds could indicate a fault in either the relay or the fuel pump circuit. It's crucial to check the fuel pressure using a gauge to confirm whether the new fuel pump is producing adequate pressure (typically 30-40 PSI for Ford Probe). This situation demands careful attention to the electrical schematics and potential faults in the wiring or relay system, which could hinder the pump's operation despite the audible indication of power.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Voltage Supply Issue: The fuel pump relay may be faulty or there may be an open circuit in the wiring leading to the pump. The reading of 5V at the inertia switch connector indicates that the pump is not receiving the required 12V, which is necessary for proper operation. Unplugging the inertia switch stops the pump's operation, indicating a possible fault in the relay or wiring. It's essential to verify relay function and inspect the integrity of the wiring harness leading to the fuel pump. - Voltage reading of 5V at the inertia switch connector indicates insufficient power supply to fuel pump; relay click heard suggesting potential relay fault.
- Faulty Fuel Pump: Despite hearing the fuel pump running, it may not be producing the required pressure. The pump could have internal wear or damage, especially if it has been inactive for an extended period. A fuel pressure test should be conducted to confirm if the pump is functioning correctly, as it should maintain approximately 30-40 PSI under normal operating conditions. - Fuel pump operational sound does not guarantee pressure output; pressure gauge needed to verify fuel delivery performance.
- Inertia Switch Malfunction: The inertia switch, designed to cut off fuel flow in the event of an accident, might be faulty or malfunctioning, preventing the fuel pump from receiving power. Resetting the switch did not resolve the issue, indicating that the switch itself may need replacement. Testing continuity through the switch while the ignition is on can confirm its status. - Resetting the inertia switch did not turn off the fuel cutoff light; continuity test required to check switch integrity.
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 - Voltage Verification: Start by measuring the voltage at the inertia switch connector with the ignition in the 'run' position. You should expect to see approximately 12V. If the reading is significantly lower (like the 5V observed), you may have a power supply issue affecting the fuel pump relay. This step is crucial as it helps to confirm whether the pump is receiving adequate voltage.
- STEP 2 - Relay Functionality Check: Inspect the fuel pump relay for proper operation. You can do this by swapping it with a similar relay in the fuse box or testing it with a multimeter for continuity. A faulty relay will prevent the fuel pump from receiving power, leading to the symptoms observed. If the relay checks out, ensure that wiring leading to the relay is intact.
- STEP 3 - Fuel Pressure Testing: Connect a fuel pressure gauge to the fuel rail or test port, and turn the ignition to the 'on' position without starting the engine. The pressure should stabilize between 30-40 PSI. If the pressure is below specification, the pump may not be functioning correctly. If it is adequate, check for fuel delivery at the injectors.
- STEP 4 - Inertia Switch Continuity Test: With the ignition in the 'on' position, remove the inertia switch and test for continuity across the terminals. If there is no continuity, the switch is faulty and should be replaced. This will ensure that the fuel pump can receive power when needed.
