* Dirty Idle Air Control (IAC) Valve: This valve regulates the amount of air entering the engine at idle. If it's dirty or malfunctioning, it can't maintain a consistent idle speed, causing the RPM to fluctuate. This is the most common culprit.
* Vacuum Leaks: Leaks in the vacuum lines connected to the intake manifold or other components can cause erratic air intake, disrupting the idle. These leaks can be subtle and difficult to find.
* Faulty Throttle Position Sensor (TPS): This sensor tells the computer the position of the throttle. A faulty TPS can send incorrect signals, leading to RPM fluctuations.
* Dirty Throttle Body: A buildup of carbon deposits in the throttle body can interfere with smooth airflow, impacting idle.
* Mass Air Flow (MAF) Sensor Issues (less likely, but possible): While less directly involved in idle, a faulty MAF sensor can provide inaccurate readings to the engine computer, potentially affecting idle control.
* Failing Engine Control Unit (ECU): Though less common, a failing ECU could malfunction and cause unpredictable idle behavior. This is generally diagnosed after checking the simpler components first.
* Weak or dying battery: While seemingly unrelated, a weak battery can sometimes cause inconsistent power supply that indirectly affects the engine's electrical systems and may contribute to erratic idle.
Troubleshooting Steps (in order of likelihood and ease):
1. Clean the IAC Valve: This is the easiest and often most effective fix. You can usually access it relatively easily with a few tools. Consult a repair manual for your specific model. Spraying it with throttle body cleaner is a common method.
2. Inspect Vacuum Lines: Carefully check all vacuum lines for cracks, holes, or loose connections. Replace any damaged lines.
3. Clean the Throttle Body: Thoroughly clean the throttle body with throttle body cleaner, paying attention to removing any carbon buildup.
4. Check for other vacuum leaks: Use a vacuum gauge and carefully listen around the intake and vacuum lines for hissing sounds indicative of a leak.
5. Diagnose TPS & MAF Sensor (requires a multimeter or specialized scanner): These sensors are more difficult to test without the right tools and knowledge. You might need a mechanic or a more advanced diagnostic tool.
Before attempting any repairs, consult a repair manual specific to your 1990 Honda Civic. These manuals provide diagrams and detailed instructions to avoid damaging your car. If you're not comfortable working on your car yourself, take it to a qualified mechanic.
