Owner's Problem Description
The owner reports that their 1990 Nissan Sentra cranks but does not start. They confirmed that the fuel pump activates when the key is in the 'run' position. However, the fuel injector does not spray fuel, even though the injector works fine in a friend's car. All battery grounds were checked and are good, indicating that the issue lies elsewhere in the fuel delivery system.
Professional Technical Analysis
The subject vehicle is a 1990 Nissan Sentra, likely equipped with a 1.6L engine and an estimated mileage of approximately 150,000 miles. The primary symptom is a no-start condition despite cranking, with the fuel pump operational as confirmed by auditory cues. Diagnostics reveal that the fuel injector does not spray fuel when energized, indicating potential issues upstream in the fuel delivery system. The connection between the fuel pump, the main relay, and the injector must be examined. As the injector functions correctly in another vehicle, the focus shifts to the electrical supply to the injector, specifically the relay and associated wiring. The vehicle's electrical schematic indicates that the injector circuit includes a relay that activates based on the engine control module (ECM) signals. The absence of power at the injector suggests a fault in this relay or its connections. Safety considerations include verifying that the battery is fully charged and that all grounds are secure to prevent intermittent power loss. Additionally, any previous repair history, especially regarding the fuel system, should be reviewed for insight into possible recurring issues. This scenario typically points to relay failure or wiring issues, common in vehicles of this age and mileage.
Possible Causes
Most common causes (ordered by frequency):
- Main Relay Failure: The main relay is responsible for supplying power to the fuel injector circuit. If the relay is faulty, it may not provide the necessary voltage to the injector. Testing the relay for continuity and operation is essential. The relay should engage when the ignition is in the 'run' position, and a voltage check at the injector connector should read approximately 12V when activated. If the relay fails, replacement with an OEM part (P/N 25230-50F00) is advisable, typically costing around $30-$50. - No voltage at injector connector during cranking, relay clicks but does not engage fully, visual inspection shows signs of wear in relay contacts.
- Wiring Issues: Corroded or damaged wiring between the main relay and the injector can prevent proper power delivery. Inspect the wiring harness for any exposed wires, corrosion, or loose connections. Voltage drop tests should be performed to ensure that the wiring is intact and providing the necessary voltage to the injector. This wiring failure is common in older vehicles, especially those exposed to moisture. - Visual inspection reveals frayed wires in the harness, voltage drop exceeding 1V under load conditions, intermittent connection observed.
- Ground Connection Issues: A poor ground connection can affect the operation of the fuel injector. Check the ground wires associated with the injector and relay for continuity and secure connection to the chassis. A resistance check should read below 5 ohms for good ground. Corroded or loose connections can lead to voltage drops that inhibit injector operation. - Resistance reading above 10 ohms at the ground connection points, visible corrosion on ground terminals.
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 - Initial OBD-II Scan: Utilize a professional OBD-II scanner or the GeekOBD APP to retrieve any trouble codes and analyze freeze frame data. This may help identify if there are any stored codes related to the fuel injection system. Live data monitoring can also provide additional insights into the injector's operation during cranking.
- STEP 2 - Relay and Wiring Inspection: Inspect the main relay for proper operation and continuity. Use a multimeter to test for voltage at the injector connector while cranking the engine. If no voltage is present, further investigate the relay and wiring integrity. Check the relay pinouts against the vehicle's service manual for accuracy.
- STEP 3 - Ground and Power Testing: Verify the ground connections serving the injector and relay. Perform a resistance test to ensure less than 5 ohms for good grounds. Additionally, check for continuity in the power supply wire leading to the injector. Any resistance above 1 ohm may indicate a fault in the wiring.
- STEP 4 - Injector Testing: If power and ground are confirmed, test the injector directly with a dedicated injector tester or by applying battery voltage directly. This will confirm if the injector is functional. If it sprays correctly during this test, the issue lies upstream in the electrical supply.
