Ignition System:
* Worn spark plugs or wires: This is a very common cause. Old, worn, or fouled plugs prevent efficient combustion, leading to a rough idle. Worn wires can cause misfires as well.
* Distributor cap and rotor: Cracks or corrosion in the distributor cap and worn rotor can disrupt spark delivery.
* Ignition coil: A failing ignition coil can weaken the spark, resulting in misfires and a rough idle.
* Ignition control module (ICM): This module controls the ignition system. A faulty ICM can lead to various ignition problems, including a rough idle.
Fuel System:
* Dirty fuel injectors: Clogged injectors prevent the proper amount of fuel from reaching the cylinders. Cleaning or replacing them might solve the problem.
* Fuel filter: A clogged fuel filter restricts fuel flow, leading to a lean fuel mixture and a rough idle.
* Fuel pressure regulator: A malfunctioning regulator can cause inconsistent fuel pressure, affecting the air/fuel mixture.
* Vacuum leaks: Leaks in the vacuum lines can disrupt the air/fuel mixture, causing a rough idle.
Other Potential Causes:
* Mass airflow sensor (MAF) sensor: A faulty MAF sensor provides inaccurate readings to the engine control unit (ECU), leading to improper fuel delivery.
* Throttle position sensor (TPS): Similar to the MAF sensor, a faulty TPS provides incorrect information about the throttle position, affecting fuel mixture.
* Engine control module (ECU): While less common, a failing ECU can cause a variety of problems, including a rough idle.
* PCV valve: A clogged positive crankcase ventilation (PCV) valve can cause excessive pressure buildup in the crankcase, leading to a rough idle.
* Vacuum leaks: Leaks in the intake manifold gasket or other vacuum lines can disrupt the air/fuel mixture.
* Low engine compression: Worn piston rings or valve problems can lead to low compression in one or more cylinders, causing a rough idle.
Troubleshooting Steps:
1. Check the basics: Start by inspecting the spark plugs and wires for visible signs of wear or damage. Replace them if necessary.
2. Listen for unusual noises: A clicking, ticking, or hissing sound might indicate a problem with the ignition system, fuel system, or vacuum lines.
3. Check for vacuum leaks: Visually inspect all vacuum lines and connections for cracks or loose fittings. Use a propane torch (carefully and with the engine running) to check for vacuum leaks. If the idle changes when propane is near a leak, you've found your problem.
4. Check engine codes: Use an OBD-II scanner (though 1990 might not have OBD-II, it may have a different diagnostic system) to read any diagnostic trouble codes (DTCs) stored in the ECU. These codes can provide valuable clues about the cause of the rough idle.
5. Consult a repair manual: A repair manual specific to your 1990 Chevy Lumina will provide detailed information on troubleshooting and repair procedures.
It's important to systematically check these potential causes. Starting with the simpler and more common issues is often the most efficient approach. If you're not comfortable working on your car yourself, it's best to take it to a qualified mechanic for diagnosis and repair.
