1. Vacuum Leaks: These are a very common cause of rough idle.
* How to check: Carefully inspect all vacuum lines for cracks, holes, or loose connections. Pay close attention to the lines connected to the intake manifold, brake booster, and emission control components. Use a propane torch (with the engine running) to carefully check for changes in idle speed near potential leak points. A slight increase in RPMs near a leak confirms it.
* How to fix: Replace any damaged vacuum lines, tighten loose connections, and ensure all connections are sealed properly.
2. Dirty or Failing Throttle Body:
* How to check: Remove the throttle body and inspect it for buildup of carbon deposits, gum, and dirt. Look for a sticking throttle plate.
* How to fix: Clean the throttle body thoroughly with throttle body cleaner (following the instructions carefully). Use a throttle body cleaner specifically designed for this purpose. Make sure the throttle plate moves freely. If the throttle plate is sticking, you may need to replace it.
3. Dirty or Faulty Mass Airflow Sensor (MAF):
* How to check: Inspect the MAF sensor for dirt, oil, or other contaminants. Avoid touching the sensor element (the hot wire).
* How to fix: Clean the MAF sensor with MAF sensor cleaner (not throttle body cleaner!). If cleaning doesn't resolve the issue, the MAF sensor may be faulty and require replacement.
4. Ignition System Problems:
* How to check: This includes spark plugs, wires, distributor cap, and rotor. Check for worn, cracked, or fouled spark plugs. Inspect the spark plug wires for cracks or damage. Examine the distributor cap and rotor for corrosion, cracks, or wear.
* How to fix: Replace worn, damaged, or fouled spark plugs, wires, distributor cap, and rotor as needed.
5. PCV Valve:
* How to check: Inspect the PCV valve for proper operation. A clogged or faulty PCV valve can disrupt the engine's air-fuel mixture. You can often test this by removing the valve and seeing if it's stuck.
* How to fix: Replace the PCV valve if it's faulty.
6. Fuel Delivery Issues:
* How to check: Check the fuel filter for clogs. A clogged filter restricts fuel flow, leading to rough idle. Also, inspect fuel pressure using a fuel pressure gauge. You should consult your vehicle's service manual for the correct pressure specification.
* How to fix: Replace the fuel filter if it's clogged. If fuel pressure is low, address the underlying problem (e.g., fuel pump, fuel pressure regulator).
7. Idle Air Control (IAC) Valve:
* How to check: The IAC valve regulates idle speed. A dirty or faulty IAC valve can cause rough idle. This often requires specialized testing tools.
* How to fix: Clean the IAC valve with electronic parts cleaner. If cleaning doesn't help, replacement is often necessary.
8. Engine Sensors: Other sensors like the oxygen sensor (O2 sensor) can contribute to a rough idle, though it's less common as a primary cause. A faulty sensor will often trigger a check engine light.
Important Considerations:
* Check Engine Light: If the check engine light is on, get the codes read using an OBD-I scanner (for your '88 Astro). This will provide valuable clues to the problem.
* Service Manual: A factory service manual for your specific year and engine is invaluable for detailed troubleshooting steps, specifications, and diagrams.
* Safety: Always disconnect the negative battery terminal before working on any electrical components. Be cautious when working around hot engine parts.
Start with the simpler checks (vacuum leaks, throttle body) and progress to more complex issues. If you're not comfortable working on your vehicle, consult a qualified mechanic. Addressing these issues one by one will significantly increase your chances of finding the cause of the rough idle and fixing it.
