1. Diagnosis is Key: Before adjusting anything, diagnose *why* the idle is incorrect. A rough idle or incorrect idle speed could be due to many factors, including:
* Vacuum Leaks: Check all vacuum hoses for cracks, leaks, or loose connections. A small leak can significantly affect idle.
* Dirty Air Filter: A clogged air filter restricts airflow and can affect the idle. Replace if necessary.
* Faulty Throttle Position Sensor (TPS): This sensor tells the computer the throttle position. A faulty TPS will cause erratic idle and other driveability problems. You'll likely need a multimeter to test it.
* Dirty or Faulty IAC (Idle Air Control) Valve: This valve regulates air bypassing the throttle plate to control idle speed. It can get dirty or fail. Cleaning it is often possible, but replacement might be necessary.
* Spark Plugs and Wires: Worn spark plugs or faulty wires can cause a rough idle.
* Fuel Delivery Issues: Problems with fuel injectors, fuel pressure regulator, or fuel pump can also impact idle.
2. Locate the Idle Adjustment Screw (if accessible): On some 1990 350s, there might be a small screw accessible on the throttle body that allows for minor idle speed adjustment. However, this screw is often limited in its range and should only be used for very minor fine-tuning AFTER addressing other potential issues. Turning it too much can disrupt the computer's control and cause problems. Its location varies slightly depending on the specific throttle body.
3. Using a Scan Tool (Recommended): A scan tool can read the computer's data, including idle air control values and other relevant parameters. This allows for much more precise diagnosis and adjustment. A scan tool will show if the IAC is functioning correctly and help isolate other potential problems.
4. Procedure (if minor adjustment is needed):
* Warm Up the Engine: Let the engine run until it reaches normal operating temperature.
* Locate the Idle Speed Adjustment Screw (if accessible and after diagnosing other issues): It's usually on the throttle body.
* Very Small Adjustments Only: Turn the screw very slightly (1/8th of a turn at most) in either direction. Wait a few seconds to see the effect.
* Check the Idle Speed: Use a tachometer to verify the idle speed. The ideal idle speed is typically around 650-800 RPM, but consult your owner's manual for the precise specification.
* Repeat as Necessary: Make small adjustments until the idle speed is correct.
5. Professional Help: If you're uncomfortable working on your vehicle's fuel system or computer controls, take it to a qualified mechanic. Improper adjustments can damage the engine or its control systems. Improperly adjusting the idle screw can cause driveability issues or even damage the throttle position sensor or computer.
In summary: Don't jump straight to adjusting the idle screw. Diagnosing the underlying problem is crucial. Use a scan tool if possible for better diagnostics. If the idle is consistently off even after addressing other possible issues, consider professional help. A minor idle adjustment is usually the last step, not the first.
