Owner's Problem Description
The owner reports that their 1990 Jaguar XJ6 is experiencing a no-start condition. Fuel pressure is confirmed to be reaching the injectors, and the fuel pump has been replaced, functioning properly. The crank angle sensor was also replaced, but the injectors are not firing. There is power to the ignition coil, and spark is present. The owner is frustrated with the ongoing issues and is seeking assistance to resolve this problem quickly.
Professional Technical Analysis
The 1990 Jaguar XJ6 features an inline 6-cylinder engine with an approximate mileage of 120,000 miles. The reported no-start condition raises several diagnostic considerations. Although fuel pressure to the injectors is confirmed, further investigation is warranted to determine the injector pulse signal's presence. Given that the crank angle sensor (CAS) has been replaced, it's crucial to confirm that the new sensor functions correctly and is aligned properly. The CAS is essential for the PCM to determine when to pulse the injectors. A lack of injector pulse could stem from a faulty PCM, poor connections, or a failed fuel pump relay. It is noted that there is power to the ignition coil, indicating that the ignition system is functional, but the injector circuit must be examined closely. The fuel pump relay's location is often challenging to find in these models and can be a common failure point, especially with age and high mileage. A thorough check of the relay, wiring, and any related fuses is necessary to ensure proper operation. Safety concerns include ensuring all electrical connections are secure and avoiding any accidental shorts during diagnosis. The current condition of the vehicle suggests a systematic approach to diagnosing the injector circuit and relay functionality, as this can greatly impact the engine's ability to start.
Possible Causes
Most common causes (ordered by frequency):
- Fuel Pump Relay Failure: The fuel pump relay can fail, preventing power from reaching the fuel pump during cranking, despite the pump being new. Testing the relay's operation and continuity is critical. The relay is often located in the engine compartment fuse box, and a visual inspection for corrosion or damage should be performed. - No voltage at the fuel pump connector during cranking, audible relay clicks when activated.
- Injector Pulse Signal Absence: The injectors may not be receiving the pulse signal due to a malfunctioning PCM or wiring issues. Utilizing a noid light to check for injector pulses is essential. If no pulse is detected, further investigation into the wiring harness and PCM functionality is needed. - Noid light shows no activity, indicating no injector pulse; wiring harness inspection shows signs of wear.
- Crank Angle Sensor Misalignment: Even though a new crank angle sensor has been installed, if it is misaligned or defective, it will fail to provide the correct signal to the PCM, affecting injector operation. The sensor should be verified for correct installation and functionality. - Engine cranking but not starting, confirmed spark at the coil but no injector operation.
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 - OBD-II Scan and Injector Pulse Check: Start by connecting a professional OBD-II scanner or the GeekOBD APP to check for any stored trouble codes. Once codes are retrieved, focus on the injector circuit by using a noid light to verify the presence of injector pulses. If no pulses are detected, this indicates a potential PCM issue or wiring problem.
- STEP 2 - Relay Functionality Testing: Locate the fuel pump relay, typically found in the engine compartment. Test the relay for continuity using a multimeter; it should show continuity when activated. Also, verify that power is reaching the relay when the ignition is in the 'on' position. If the relay fails, replace it with an OEM part (P/N C2S29883).
- STEP 3 - Wiring Inspection: Examine the wiring harness connected to the fuel pump and injectors for any signs of damage or corrosion. Ensure all connectors are secure and free from corrosion. Use the GeekOBD APP to monitor voltage at the fuel pump connector during cranking to confirm the relay and wiring integrity.
- STEP 4 - Crank Angle Sensor Validation: Confirm the proper installation of the crank angle sensor. Measure the voltage output at the sensor connector while cranking; it should provide a stable signal. If the signal is erratic or absent, replace the sensor with an OEM part (P/N C2S28560) and ensure correct alignment.
