1. Powertrain Control Module (PCM) / ECM Issues:
* PCM Failure: The PCM is the "brain" of the system. A faulty PCM is the most likely culprit if other components check out. This requires professional diagnostics or replacement.
* PCM Programming Issues: While less common, a corrupted or improperly flashed PCM can disrupt injector pulses.
* Internal PCM Wiring: A short or open circuit *within* the PCM itself can prevent signal output.
2. Cranking/Ignition System Problems:
* Crankshaft Position Sensor (CKP) or Cam Position Sensor (CMP) Failure: The PCM needs these sensors to know the engine's rotational position to time the fuel injection. A faulty sensor will prevent injector pulses.
* Ignition Switch Issues: A problem with the ignition switch may not be providing proper power or ground to the PCM, preventing it from functioning correctly.
* Low Battery Voltage: Insufficient battery voltage can prevent the PCM from operating correctly.
3. Wiring and Connectors:
* Open or Short Circuits in Wiring: Check the wiring harness between the PCM, injectors, and related sensors for breaks, chafing, or corrosion. Pay close attention to connectors for loose connections or damage.
* Poor Ground Connections: A bad ground connection can prevent the PCM from sending signals.
* Damaged Wiring Harness Connectors: Inspect all connectors for corrosion, broken pins, or loose connections.
4. Fuel Pump Relay or Circuit Issues:
* Relay Failure: Though not directly related to the *pulse*, a faulty fuel pump relay will prevent the fuel pump from running, ultimately leading to no fuel at the injectors, even if a pulse *is* present. Check the relay itself and its connections.
* Wiring to the Fuel Pump: Verify that the fuel pump receives power and ground.
5. Fuel Injector Circuit Issues:
* Damaged Injectors: While less likely to cause a complete lack of pulse, a shorted injector could cause the PCM to shut down the entire system as a protective measure.
* Injector Wiring: Check the wiring leading to the fuel injectors.
6. Other Sensors:
* Throttle Position Sensor (TPS): A faulty TPS can interfere with the PCM's calculations and prevent proper injector operation.
* Mass Airflow Sensor (MAF): A malfunctioning MAF sensor can cause the PCM to make incorrect fuel delivery decisions, but usually leads to a different symptom rather than no pulse.
Troubleshooting Steps:
1. Check for codes: Use an OBD-I scanner (appropriate for 1991) to retrieve any diagnostic trouble codes (DTCs) stored in the PCM. These codes will give valuable clues.
2. Visual Inspection: Carefully examine all wiring, connectors, and components for any obvious damage.
3. Voltage and Ground Checks: Use a multimeter to verify voltage and ground at the PCM, injectors, sensors, and relay.
4. Sensor Testing: Test the CKP, CMP, TPS, and MAF sensors to ensure they're providing accurate signals.
5. Fuel Pressure Test: Check the fuel pressure to see if the fuel pump is working.
6. Injector Pulse Test: Use a noid light or multimeter to check for injector pulse at each injector. This is crucial to confirm the lack of pulse.
Important Note: Troubleshooting fuel injection systems requires some mechanical knowledge and electrical testing skills. If you're not comfortable working with automotive electrical systems, it's best to take the vehicle to a qualified mechanic. Improperly diagnosing and repairing electrical systems can lead to further damage.
