Most Likely Culprits:
* Crank Position Sensor (CPS): This sensor tells the engine's computer the position of the crankshaft. A failing CPS can cause intermittent starting problems – the engine might start briefly, then shut down because the computer loses timing information. This is a very common cause of this symptom.
* Fuel Pump Relay: The relay might be failing intermittently. It's responsible for supplying power to the fuel pump. A failing relay might provide power for a short time, then cut off.
* Fuel Pump: The fuel pump itself could be weak or failing. It might provide enough pressure to start the engine, but then be unable to maintain sufficient pressure to keep it running.
* Ignition Switch: A failing ignition switch can provide power intermittently, leading to the same symptoms.
* Mass Airflow Sensor (MAF): A faulty MAF sensor can provide incorrect air/fuel mixture data to the engine computer, leading to stalling after a brief run.
* Throttle Position Sensor (TPS): Similar to the MAF sensor, a faulty TPS can lead to incorrect mixture and stalling.
* Distributor (if equipped): Older vehicles (though less likely on a '91) might have distributor issues, such as worn components, leading to intermittent spark.
Troubleshooting Steps (in order of ease and likelihood):
1. Check the fuel pressure: This requires a fuel pressure gauge. You need to see if the pressure drops significantly after the engine starts. Low or fluctuating pressure points to a fuel pump, fuel filter, or fuel pressure regulator problem.
2. Listen for the fuel pump: When you turn the key to the "on" position (but not cranking), you should hear the fuel pump prime for a few seconds. If you don't hear it, check the fuel pump relay.
3. Check the crank position sensor: This often requires a scan tool to check for codes or a multimeter to check the sensor's output voltage. It's usually relatively easy to access and replace.
4. Inspect the ignition system: Check all spark plug wires for cracks or damage, and consider replacing the spark plugs (they're inexpensive and a good preventative measure). A failing ignition coil is also possible, though less likely to produce this exact symptom.
5. Check the fuel filter: A clogged fuel filter can restrict fuel flow, leading to similar problems.
Important Notes:
* Safety First: Work on your vehicle in a safe and well-ventilated area. Disconnect the negative battery terminal before working on any electrical components.
* Professional Help: If you're not comfortable with automotive repair, take it to a qualified mechanic. Misdiagnosis can lead to unnecessary expense and further damage.
* Scan Tool: Using a scan tool (OBD-I for your 1991 model) will help greatly in diagnosing the problem. It can read diagnostic trouble codes (DTCs) that pinpoint potential issues.
Start with the easier and more likely causes (fuel pressure, fuel pump relay, CPS). If those don't solve the problem, you'll need to move on to more in-depth diagnostics. Remember to note exactly when the vehicle stalls (immediately after starting, after a few seconds, etc.) as this can help narrow down the possibilities.
