Owner's Problem Description
The owner reports that their 1990 Subaru Legacy was running with a misfire. After using a scope to listen to the injectors, it was found that two of the four injectors were not making the usual sound. A noid light test confirmed that these two injectors were not receiving power, indicating two dead cylinders. The owner is seeking to understand why only two injectors are firing while the others are not, especially after a recent engine replacement.
Professional Technical Analysis
This 1990 Subaru Legacy, equipped with a 2.2L engine and approximately 150,000 miles, is experiencing a significant injector power issue resulting in a misfire condition. The owner has identified that injectors for cylinders 1 and 2 are not firing, which indicates a potential electrical problem affecting only these two injectors. The vehicle's wiring, particularly around the injector circuits, must be inspected thoroughly. Given that the engine was recently replaced, it is crucial to check for proper connections at the injector harness and any potential shorts or opens in the wiring harness. The injector circuit should be tested for battery voltage; ideally, it should show around 12V when the ignition is on. If voltage is absent, a fault in the wiring or the Engine Control Module (ECM) could be to blame. Additionally, the noid light testing showed power only for a brief moment, indicating potential issues with the ECM's signal output or ground integrity. It is essential to assess the wiring integrity and connections, especially given that the vehicle was stationary for nearly a year prior to the engine swap, which could have affected the wiring's condition. This situation requires a thorough examination to ensure all connections meet specifications and that there are no underlying issues causing the injector circuit malfunction.
Possible Causes
Most common causes (ordered by frequency):
- Wiring Issue: A break or corrosion in the wiring harness leading to the injectors for cylinders 1 and 2 can result in a lack of power supply. This is especially relevant considering that the vehicle had been sitting for an extended period prior to the engine swap, potentially causing deterioration in the wiring's insulation or connections. - Visual inspection reveals frayed wires or corrosion at the connectors, specifically in the injector harness.
- ECM Malfunction: The Engine Control Module may not be supplying the necessary signal to activate the injectors for cylinders 1 and 2. This could be due to an internal failure or a software issue that prevents proper injector operation. - Stored fault codes related to injector control may indicate an issue with the ECM, requiring further diagnostics.
- Faulty Injector Driver Circuit: The driver circuit within the ECM that controls the injectors could be malfunctioning, leading to a lack of signal for injectors in specific cylinders. This could potentially be tested using oscilloscope diagnostics. - Oscilloscope tests show no signal on the injector driver circuit for the affected cylinders, indicating a potential internal ECM issue.
- Grounding Issues: Insufficient grounding for the injector circuit can result in erratic operation or failure to engage. Ensuring that the ground connections are clean and secure is critical. - Testing ground connections reveals high resistance readings, indicating poor contact that can disrupt 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 - Visual Inspection: Begin with a thorough visual inspection of the wiring harness and connectors associated with the injectors for cylinders 1 and 2. Look for any signs of wear, corrosion, or disconnections, especially around the injector connectors and near the ECM.
- STEP 2 - Voltage Testing: With the ignition on, use a multimeter to test for battery voltage at the injector connectors. Expect to see approximately 12V. If voltage is absent, further investigation into the wiring is required.
- STEP 3 - Ground Testing: Check the ground connections for the injector circuit to ensure they are secure and clean. Use a multimeter to check for resistance; ideally, resistance should be less than 5 ohms.
- STEP 4 - Signal Testing: Use an oscilloscope to check the signal integrity at the injector driver circuit within the ECM for cylinders 1 and 2. This will confirm whether the ECM is outputting the necessary signals to activate the injectors.
