Owner's Problem Description
The owner of a 1992 Ford Mustang with a V8 engine reports that the fuel injectors are not functioning. Despite having fuel pressure at the rail and a functioning fuel pump, the injectors do not spray fuel. Previous attempts included testing the injector wires with a wire tester, which temporarily caused them to spray. The vehicle has spark, but the owner is uncertain if the pickup coil is faulty or if there are wiring issues.
Professional Technical Analysis
The 1992 Ford Mustang is equipped with a V8 engine and an automatic transmission, with an estimated mileage of around 150,000 miles. The symptoms reported include non-functioning fuel injectors, despite confirming fuel pressure at the rail and spark at the plugs. A systematic approach is essential for diagnosing injector issues, especially when previous tests yielded intermittent results. Initial checks should include verifying the injector fuse and ensuring no shorts in the wiring harness. The fuel pressure should be maintained between 35-45 PSI, and any voltage at the injector terminals should be measured with a multimeter; it should be around 12V during cranking. The pickup coil’s functionality is crucial as it provides the necessary signal for injector operation. If both the fuel pump and spark are confirmed to be operational, the next step is to perform a continuity test on the injector harness, ensuring the resistance is within the acceptable range of 12-14 ohms for each injector. Additionally, the potential for a failing Engine Control Module (ECM) must be considered, as it could impact the injector pulse width modulation. Monitoring the vehicle's electrical system behavior during cranking and while attempting to start can yield further insights into the root cause of the injector failure.
Possible Causes
Most common causes (ordered by frequency):
- Injector Wiring Issue: The injector wiring may have a short or open circuit, preventing voltage from reaching the injectors. This can be confirmed through continuity testing which should show resistance between 12-14 ohms for each injector circuit. If there is no continuity or resistance outside this range, the harness may be damaged and needs replacement or repair. - No voltage at injector terminals during cranking, continuity test shows open circuit or high resistance.
- Faulty Pickup Coil: A malfunctioning pickup coil can lead to insufficient signal for the ECM to operate the injectors. If the pickup coil fails, the engine may not start or run poorly. Testing the pickup coil resistance should reveal a value typically between 500-1500 ohms. - Intermittent no-start condition, resistance testing shows values outside specified range.
- Engine Control Module (ECM) Failure: If the ECM is not functioning correctly, it may fail to send the proper signal to the injectors. Symptoms may include erratic injector operation or no operation at all. A diagnostic scan can reveal error codes related to the fuel system or injector circuit. - No injector pulse on oscilloscope, error codes indicating fuel system issues.
- Fuel Injector Failure: Although less common, individual fuel injectors can fail due to internal blockages or electrical issues. Testing each injector's resistance and operation can confirm their functionality. The acceptable resistance range is typically 12-15 ohms per injector. - Resistance testing shows values outside the normal range, visual inspection reveals blockages.
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: Start with a thorough visual inspection of the injector wiring harness for any signs of wear, damage, or corrosion. Check the connectors for secure fit and any corrosion that may impede electrical flow. Ensure the injector fuse is intact and replace if necessary.
- STEP 2 - Voltage Testing: Use a multimeter to check for voltage at the injector terminals while cranking the engine. You should expect to see around 12V at the terminals. If there is no voltage, perform a continuity test on the injector harness to determine if there is an open or short circuit.
- STEP 3 - Injector Resistance Testing: With the engine off, check the resistance of each injector using the multimeter. The resistance should be between 12-15 ohms. If any injectors show significantly higher or lower readings, they may be faulty and require replacement.
- STEP 4 - Pickup Coil Testing: If the voltage and resistance tests are inconclusive, check the pickup coil. Measure its resistance to ensure it is within 500-1500 ohms. If outside this range, the pickup coil should be replaced.
