1. Fuel Pump Relay: Even though the pump runs when directly wired, the relay that controls it may be faulty. It might be failing to energize the pump when the ignition is on, preventing the pump from priming and delivering fuel. Replace the relay as a first step (relatively inexpensive).
2. Crankshaft Position Sensor (CKP Sensor): This sensor tells the ECM the engine's rotational position. Without a reliable signal from the CKP, the ECM won't activate the fuel injectors. A faulty CKP sensor will prevent the injectors from firing, even if the fuel pump is working.
3. Cam Position Sensor (CMP Sensor) (if equipped): Some engines from that era used a CMP sensor. Similar to the CKP sensor, a faulty CMP sensor will prevent the ECM from properly timing fuel injection.
4. ECM/PCM Issues: The engine control module could be faulty. This is less likely if the fuel pump works (even if only with direct power), but a malfunctioning ECM could prevent injector activation. This is usually the most expensive fix to diagnose and remedy.
5. Injector Wiring/Connectors: Check the wiring and connectors to the injectors for corrosion, loose connections, or broken wires. A simple wiring fault is easier to fix than a faulty ECM.
6. Injector Driver Module (if separate): Some systems use a separate module to control the injectors. This module could be malfunctioning.
7. Fuel Pressure Regulator: While the pump runs, it might not be building sufficient pressure. A faulty fuel pressure regulator could cause low fuel pressure, preventing sufficient fuel delivery to the injectors, even if they are otherwise operational. Check fuel pressure using a fuel pressure gauge. Low pressure indicates a problem here, the pump, or clogged fuel lines.
8. Ignition System Problems: A complete failure of the ignition system (e.g., distributor, coil) may *indirectly* affect fuel injection. The ECM might not trigger the injectors if it doesn't detect proper engine cranking or running.
Troubleshooting Steps:
1. Check the fuel pressure: This is crucial. Use a fuel pressure gauge to determine if the pump is building sufficient pressure.
2. Test the fuel pump relay: Swap it with a known good relay of the same type.
3. Inspect all wiring and connectors: Look for corrosion, damage, or loose connections in the fuel pump, injector, and engine sensor circuits.
4. Check the CKP and CMP sensors (if applicable): These can be tested with a multimeter or using a scan tool to check for signal output. The values will vary with engine type. Look up the specifications for your engine.
5. Use a diagnostic scan tool: A scan tool will read the ECM's diagnostic trouble codes (DTCs), potentially pinpointing the problem. This is the most effective way to pinpoint many electrical issues.
The fact that the fuel pump works when directly wired strongly suggests the problem lies *after* the pump, and most likely within the engine's control systems (ECM, sensors, relays). Start with the cheaper components (relay, fuse, sensor inspection) before considering the more expensive options (ECM replacement).
