Fuel System Issues:
* Fuel Pump: A weak or failing fuel pump can't deliver enough fuel at higher engine demands, leading to hesitation and stalling. Listen for a whirring sound from the fuel pump (usually near the fuel tank) when you turn the ignition key to the "on" position (before cranking). If it's weak or absent, that's a strong suspect. Fuel pressure testing is essential for confirmation.
* Fuel Filter: A clogged fuel filter restricts fuel flow, producing similar symptoms. Replacing this is relatively inexpensive and should be done regularly as part of routine maintenance.
* Fuel Injectors: Clogged or malfunctioning fuel injectors can cause a misfire and rough running. Testing injector operation (either with a fuel pressure gauge or specialized injector tester) is necessary to diagnose this. Cleaning or replacing injectors might be needed.
* Throttle Position Sensor (TPS): The TPS tells the computer the throttle position. A faulty TPS can lead to erratic fuel delivery, causing rough idling and hesitation. This can often be tested with a multimeter.
* Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty or faulty MAF sensor will provide inaccurate information to the computer, leading to improper fuel mixture and poor running. Cleaning it (carefully!) is a good first step; replacement might be necessary.
Ignition System Issues:
* Ignition Coil(s): A failing ignition coil can prevent spark to one or more cylinders, leading to misfires, rough running, and hesitation. Visual inspection for cracks or damage is a good start, but testing the coil's output is crucial. These often fail in sets, even if only one is visibly affected.
* Spark Plug Wires: Cracked, damaged, or poorly connected spark plug wires can lead to misfires. Inspect them carefully for any visible damage.
* Distributor Cap and Rotor (if equipped): On some 3.8L engines of this era, there's a distributor. Carbon tracking or wear in the cap and rotor can disrupt spark delivery. Inspect for cracks, burns, or excessive wear.
* Crankshaft Position Sensor (CKP): The CKP sensor tells the computer the engine's rotational position, crucial for precise spark timing. A failing CKP will result in rough running, misfires, and potentially stalling.
* Camshaft Position Sensor (CMP): Similar to the CKP, a failing CMP sensor will disrupt timing and fuel delivery.
Other Considerations:
* Vacuum Leaks: Small vacuum leaks can disrupt the engine's air/fuel mixture and cause rough running. Carefully inspect all vacuum hoses and connections.
* PCV Valve: A clogged Positive Crankcase Ventilation valve can increase crankcase pressure, affecting engine performance and potentially causing stalling.
* Throttle Body: A dirty or sticky throttle body can restrict airflow, resulting in rough idle and hesitation. Cleaning the throttle body is often a simple fix.
Troubleshooting Steps:
1. Start with the easy checks: Inspect spark plug wires, distributor cap and rotor (if applicable), vacuum hoses, and the PCV valve.
2. Check engine codes: Use an OBD-I scanner (a specific one for this year and model is needed) to retrieve any diagnostic trouble codes (DTCs) stored in the car's computer. These codes provide valuable clues about potential problems.
3. Fuel pressure test: This is crucial to rule out fuel pump and delivery issues.
4. Ignition system testing: Test the ignition coil(s), spark plug wires (using a spark tester), and the distributor cap and rotor (if applicable).
5. Sensor checks: Test the MAF, TPS, CKP, and CMP sensors using a multimeter (consult a repair manual for specific procedures and values).
Without diagnostic tools and a repair manual specific to your 1991 Buick Regal LTD 3.8L V6, pinpointing the exact cause will be difficult. A mechanic with experience on older vehicles will be best equipped to diagnose and repair this. Trying to fix things randomly could be expensive and not solve the root problem.
