Owner's Problem Description
The owner of a 2003 Ford Focus reports that the vehicle will not start despite turning over fine. Previous repairs include replaced spark plugs and cap, a new head, new timing belt, and a new fuel pump. The owner suspects a fuel issue but is unsure. There are no diagnostic codes, but there are occasional sounds of trying to backfire through the exhaust, which were present before the head change.
Professional Technical Analysis
This case involves a 2003 Ford Focus with a 2.0L I4 engine and an automatic transmission, which has been subjected to significant repairs, including a new head and timing belt, alongside a recently replaced fuel pump. The vehicle has approximately 120,000 miles on the odometer, indicating a history of maintenance but also potential component wear. The symptom pattern includes the engine turning over without firing, and occasional backfire sounds through the exhaust, suggesting a misfire or fuel delivery issue. Given the replacement history, we must consider the fuel system's integrity, including the fuel pump control unit, located under the rear seat, which is known for failure. It's essential to check the fuel pressure, which should be around 35-45 PSI for proper injector operation, and to examine any potential issues with the fuel pump relay and the inertia switch. Additionally, the lack of fault codes may indicate that the PCM (Powertrain Control Module) is not receiving the necessary input from the fuel system or that there may be an intermittent issue affecting injector operation. A systematic approach should involve verifying fuel pressure, testing injector pulse width with an oscilloscope, and checking for any air intake blockages or vacuum leaks that could impact engine operation. This vehicle's current condition poses a safety concern due to the potential for unexpected backfiring during troubleshooting.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Control Module Failure: The fuel pump control module located under the rear seat is known for its potential failure. This module controls the fuel pump's operation and may not be delivering the required voltage or signal to the pump, leading to insufficient fuel pressure for the injectors. Normal operation should show a steady voltage around 12V at the pump during operation. - The fuel pump runs, but there are inconsistent pressure readings. The control module should be tested for proper voltage output during operation.
- Fuel Pressure Insufficient: Even with a new fuel pump, insufficient pressure can occur due to a clogged fuel filter or a malfunctioning fuel pressure regulator. Fuel pressure should consistently measure between 35-45 PSI; any significant drop may indicate a blockage or a failed regulator. - Fuel pressure gauge readings show less than 35 PSI, indicating potential blockage or regulator issues.
- Injector Pulse Failure: If the PCM is not sending the correct pulse width to the fuel injectors, they won't operate correctly. This can be caused by a faulty crankshaft position sensor or wiring issues affecting the signal. Injector pulse width should be tested with an oscilloscope to ensure proper operation. - No injector pulse is detected during cranking, which may indicate a sensor or wiring issue.
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 Test: Use a fuel pressure gauge to measure the pressure at the fuel rail. The pressure should read between 35-45 PSI. If the pressure is lower than this range, inspect the fuel pump control module and fuel filter for blockages or failures.
- STEP 2 - Injector Pulse Width Verification: Connect an oscilloscope to the injector harness to check for pulse width during cranking. The pulse should be in the range of 2-6 ms for proper operation. No pulse may indicate a PCM issue or a sensor fault.
- STEP 3 - Crankshaft Position Sensor Inspection: Inspect the crankshaft position sensor and wiring for any signs of damage or wear. This sensor is critical for PCM operation and should provide a clean signal for injector timing.
- STEP 4 - Visual Inspection of Fuel System: Conduct a thorough visual inspection of the fuel system components, including fuel lines, connections, and the fuel filter to ensure there are no leaks or blockages. Verify that the fuel pump is securely mounted and operational.
