* Throttle Position Sensor (TPS): A faulty TPS provides inaccurate readings to the engine control unit (ECU). Incorrect readings can lead to erratic fuel delivery and ignition timing, resulting in surging. This is a very common cause of this type of problem.
* Mass Air Flow Sensor (MAF) or Air Flow Meter (AFM): (Depending on whether it's a fuel-injected or carbureted model; the '86 likely has a MAF) A dirty or malfunctioning MAF sensor provides inaccurate readings of the air entering the engine. This throws off the air/fuel mixture, causing surging. Clean it first (carefully, following instructions specific to your sensor), and if that doesn't help, replacement may be necessary.
* Vacuum Leaks: A significant vacuum leak in the intake manifold, vacuum lines, or PCV system can disrupt the engine's air/fuel mixture, causing erratic idle and surging. Inspect all vacuum lines carefully for cracks or loose connections. Use a vacuum gauge to pinpoint leaks.
* Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. A dirty or malfunctioning IAC valve can cause a rough idle and surging. Cleaning it is often a worthwhile first step.
* Fuel Pressure Regulator: A faulty fuel pressure regulator can cause fluctuating fuel pressure, resulting in inconsistent fuel delivery and surging.
* Ignition System: Problems with the distributor cap, rotor, spark plug wires, or spark plugs can cause misfires that lead to surging. Check for worn or damaged components, and consider replacing them as a set. Also, ensure proper timing.
* Computer (ECU): In rare cases, a faulty ECU can contribute to surging. This is usually diagnosed through diagnostic tests.
Troubleshooting Steps:
1. Check for Vacuum Leaks: This should be your first step. Start by visually inspecting all vacuum lines. Use a propane torch (carefully!) to help locate leaks by observing the change in engine RPM near a leak. A vacuum gauge is a more precise method.
2. Clean the MAF/AFM Sensor (or TPS): Use a sensor cleaner specifically designed for this purpose. Follow the instructions carefully.
3. Check the IAC Valve: Clean it thoroughly using appropriate cleaner. Sometimes removing and reinstalling it can help restore proper functioning.
4. Check Fuel Pressure: A fuel pressure gauge is needed here to confirm proper fuel pressure.
5. Inspect the Ignition System: Replace worn or damaged parts (cap, rotor, wires, plugs) as needed.
6. Check Engine Codes: If the Fiero has a diagnostic system (check your owner's manual), retrieve any stored diagnostic trouble codes (DTCs). These codes can provide valuable clues about the problem.
If you're not comfortable working on your car yourself, it's best to take it to a qualified mechanic familiar with older Fiero models. The 2.8L engine, while capable, has some quirks and requires some specific knowledge to diagnose properly. Remember to replace parts with genuine or high-quality equivalent parts to avoid further issues.
