* Ignition System: This is a very common cause. Even though the engine light isn't on, components are failing gradually. Consider these possibilities:
* Worn Spark Plugs/Wires: These are relatively inexpensive to replace and are a good starting point. Under load, the increased demand on the ignition system highlights any weaknesses. Look for cracked insulators, excessive wear, or corrosion.
* Ignition Coil(s): The coil(s) provide the high voltage needed for the spark plugs. A failing coil might only show problems under load. A visual inspection might reveal cracks or damage, but often they require testing with a multimeter or specialized ignition system tester. Series II engines are known to have coil problems.
* Distributor Cap and Rotor (if equipped): Older 3800 Series II engines (pre-1998) have a distributor. Cracks, carbon tracking, or worn contacts in the cap and rotor can cause misfires, especially under load.
* Crankshaft Position Sensor (CKP): This sensor tells the ECM when the engine is turning over. A failing CKP can lead to misfires, especially under acceleration, and often *won't* set a code.
* Fuel System:
* Fuel Injectors: A partially clogged or weak injector might provide enough fuel at idle, but not enough under load. This is hard to diagnose without specialized equipment. A fuel pressure test is necessary here.
* Fuel Pump: Similar to injectors, a weakening fuel pump might provide enough pressure at low demand, but struggle under load. A fuel pressure test is crucial.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to misfires under load. This is a cheap and easy check.
* Other Potential Issues:
* Mass Airflow Sensor (MAF): A faulty MAF sensor provides incorrect air-fuel mixture information to the ECM, leading to poor performance and misfires, sometimes without setting a code.
* Vacuum Leaks: A vacuum leak can disrupt the air-fuel mixture, leading to misfires. Listen carefully for hissing sounds near the intake manifold, vacuum lines, and PCV system.
* Engine Compression: Low compression in one or more cylinders will cause misfires, especially under load. A compression test is needed to diagnose this.
Troubleshooting Steps:
1. Start with the cheap and easy checks: Spark plugs, wires, fuel filter, visual inspection of the ignition components (coil, cap, rotor if applicable).
2. Check for vacuum leaks: Use a propane torch (carefully!) around suspected leak areas to see if the engine RPM changes.
3. Consider a professional diagnostic: An OBD-II scanner might not reveal the issue, but a mechanic with experience on these engines can perform more advanced diagnostic tests like checking fuel pressure, injector pulse width, and compression. They have the tools to test the ignition system components under load.
Without a diagnostic code, the process is more of an elimination game. Starting with the most likely and accessible components (ignition system, fuel filter) will save you time and money. If the problem persists, professional help will be needed. Remember to always prioritize safety when working on your vehicle.
