Here are some potential culprits:
* Throttle Position Sensor (TPS): This sensor tells the computer the position of the throttle plate. A faulty TPS can send incorrect signals, leading to erratic engine behavior, including unintended acceleration. This is a very likely suspect given your symptoms.
* Mass Airflow Sensor (MAF): This sensor measures the amount of air entering the engine. A dirty or malfunctioning MAF sensor can cause incorrect fuel mixtures, leading to poor performance and potentially unintended acceleration.
* Vacuum Leaks: A leak in the vacuum system can disrupt the operation of various components, including the throttle body and other engine controls. This can cause erratic idle and possibly unintended acceleration.
* Throttle Cable (if applicable): Older vehicles sometimes still use a throttle cable. A sticking or damaged cable could cause the throttle to remain partially open even when the accelerator pedal is released. (Less likely in a 1991, but worth considering.)
* Computer/PCM Issues: The Powertrain Control Module (PCM) or Engine Control Unit (ECU) is the car's "brain." A malfunctioning PCM can cause a wide range of problems, including the symptoms you're describing. This is a more serious and expensive issue to diagnose and repair.
* Transmission Issues: While less likely to be the primary cause of *unintended* acceleration, transmission problems can contribute to erratic behavior and should be considered if the acceleration feels like it's coming from the transmission itself.
* Wiring Problems: Damaged or corroded wiring in the engine bay can cause intermittent electrical faults affecting various engine control systems.
What you should do:
1. Get the diagnostic trouble codes (DTCs) read: A mechanic or auto parts store (often for free) can use an OBD-I scanner (you'll need one that works with your vehicle's year) to read the codes stored in the PCM. These codes will provide clues about the source of the problem.
2. Do not attempt to fix this yourself unless you have extensive automotive experience: Unintended acceleration is dangerous. Improper repairs can worsen the situation.
3. Have a qualified mechanic inspect the vehicle: Based on the DTCs and a visual inspection, they can diagnose the specific problem and recommend appropriate repairs. Focus on the TPS and MAF sensor as likely starting points based on your description.
Ignoring this problem could lead to a serious accident. Get your Lumina checked out immediately.
