Fuel System:
* Fuel Pump: A weak or failing fuel pump might provide insufficient pressure, especially when the engine is under load (like when accelerating). Listen carefully near the fuel tank for a humming sound when the key is turned to the "on" position – the lack of a hum suggests a faulty fuel pump.
* Fuel Pressure Regulator: This regulates fuel pressure to the injectors. A faulty regulator can cause low pressure or leaks. A fuel pressure gauge test is necessary to diagnose this.
* Clogged Fuel Injectors: Despite a new fuel filter, injectors themselves can be clogged or malfunctioning. This requires specialized cleaning or replacement.
* Fuel lines/hoses: Check for cracks, leaks, or kinks in the fuel lines leading from the tank to the engine.
Ignition System (Beyond what's already been replaced):
* Ignition Coil: The coil provides the high voltage needed for sparking. A failing coil might produce weak or inconsistent sparks, especially when the engine is hot.
* Distributor: While you've replaced the cap and rotor, the distributor itself might have internal issues (worn gears, etc.). It's less likely but still possible.
* Pick-up coil (within the distributor): This component senses the position of the rotor and triggers the ignition. A faulty pick-up coil will lead to misfiring or no spark.
* Wiring Harness: Look for damaged, corroded, or loose connections within the ignition system's wiring harness. This includes the wires going to the coil and the crankshaft position sensor (if equipped).
* Crankshaft Position Sensor (CKP Sensor): This sensor tells the computer the engine's rotational position, vital for precise spark timing. A faulty sensor will lead to rough running or stalling.
Engine Problems:
* Vacuum Leaks: Vacuum leaks disrupt the air-fuel mixture, causing rough running and stalling. Inspect all vacuum lines and hoses for cracks or disconnections.
* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate readings to the computer, affecting fuel delivery and ignition timing.
* Mass Airflow Sensor (MAF Sensor): This sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor can cause running issues. (More likely on fuel-injected models.)
* EGR Valve: The Exhaust Gas Recirculation valve might be sticking, causing erratic engine operation.
Diagnostic Steps:
1. Listen for the fuel pump: Check if you hear the pump running when you turn the key on.
2. Check for vacuum leaks: Use a vacuum gauge or spray carburetor cleaner around potential leak points while the engine runs. A change in RPM indicates a leak.
3. Have the fuel pressure checked: A fuel pressure gauge test will determine if the fuel pump and regulator are working correctly.
4. Check for diagnostic trouble codes (DTCs): Use an OBD-I scanner (if your truck has the port) to read any stored codes in the computer's memory. These codes can help pinpoint the problem.
5. Consult a repair manual specific to your 1992 GMC Sierra: It will show you the location of components and provide troubleshooting guides.
It's difficult to diagnose remotely. The best approach is to systematically check each item on this list, using a process of elimination, or take it to a mechanic for proper diagnosis. The fuel system and ignition system are the most probable areas.
