1. Verify Power and Ground to the Injectors:
* Power: Use a test light or multimeter to check for power at the injector connectors. There should be power only when the engine is cranking or running (depending on your specific TBI system). Power should be a switched 12V, meaning it's only present when the ignition is on.
* Ground: Ensure a good ground connection at the injector's mounting location and the engine block itself. Corrosion can significantly hinder a ground's effectiveness.
2. Check the Injector Wiring Harness:
* Continuity: Use a multimeter to check for continuity between the injector connector and the ECM (Engine Control Module) connector. Any breaks or shorts in the wiring will prevent pulses from reaching the injectors. Consult a wiring diagram for your specific year and model to identify the correct wires.
* Harness Condition: Visually inspect the harness for any damaged or frayed wires, especially near the engine where it's subjected to heat and movement.
3. Test the Fuel Pressure Regulator (FPR):
* Vacuum Leak: A malfunctioning FPR can affect fuel pressure and injector operation, even with a new fuel pump. Check for vacuum leaks around the FPR and its vacuum line.
* Pressure Test: Use a fuel pressure gauge to measure the fuel pressure at the fuel rail. Your vehicle's repair manual will specify the correct pressure. Low pressure means the problem isn't solely injector related.
4. Confirm ECM Operation:
* Codes: Although you have a new ECM, it's possible it's faulty or not properly programmed for your vehicle. Check for diagnostic trouble codes (DTCs) using an OBD1 scanner. Even without a scanner, some TBI systems have a "check engine" light that can offer clues.
* ECM Power and Ground: Verify adequate power and ground to the ECM itself.
5. Check the Crankshaft Position Sensor (CKP Sensor):
* The CKP sensor signals the ECM when the engine is cranking or running. A faulty CKP sensor prevents the ECM from sending the signal to activate the injectors. A failed CKP would usually result in a no-start condition, but it's worth checking.
6. Ignition System:
* While less likely if the engine cranks, a weak ignition system can interfere with the ECM's ability to control the injectors. Verify proper battery voltage and check the ignition coil and related components.
7. Throttle Position Sensor (TPS):
* Although not directly causing injector non-pulsing, a faulty TPS can prevent the ECM from properly controlling fuel delivery. Consider testing its operation.
Important Notes:
* Wiring Diagram: Obtain a wiring diagram for your specific 1991 GMC S15 TBI. This is crucial for tracing wires and checking for continuity.
* Repair Manual: A factory service manual will provide valuable troubleshooting information and specifications for your vehicle.
* Professional Help: If you're not comfortable working with automotive electrical systems, it's best to seek professional help from a qualified mechanic.
Working systematically through these steps will help you pinpoint the cause of the problem. Remember safety first – always disconnect the battery negative terminal before working on electrical components.
