How do you manually adjust idle speed 1990 Buick century?

Manually adjusting the idle speed on a 1990 Buick Century is not a straightforward process like turning a screw. It's controlled by the computer (ECM) and a variety of sensors, so there's no single adjustment screw like on older carburetted engines. Trying to directly adjust it without understanding the system could lead to drivability problems or even damage.

However, there are a few things that *might* affect idle speed and can be checked:

* Throttle Position Sensor (TPS): A faulty TPS can cause incorrect idle speed. You'll likely need a multimeter and a service manual to test the voltage output of the TPS. Cleaning the TPS with electronic contact cleaner is sometimes helpful. Replacing the TPS is more involved and should only be done if testing confirms it's bad.

* Idle Air Control (IAC) Valve: This valve controls the amount of air entering the engine at idle. It's often located on the throttle body. It can get dirty or stuck. Cleaning it with electronic contact cleaner is a possibility, but again, a service manual will be helpful. Replacing it is more likely to be necessary.

* Vacuum Leaks: Vacuum leaks anywhere in the intake system can affect idle speed. Carefully inspect all vacuum hoses for cracks or loose connections.

* Mass Airflow Sensor (MAF) Sensor: A dirty or faulty MAF sensor can affect the air/fuel mixture, which will in turn affect the idle. Cleaning the MAF sensor with MAF sensor cleaner (following the instructions carefully!) might help, but replacement may be necessary.

Important Considerations:

* Service Manual: You absolutely need a service manual specific to your 1990 Buick Century's engine (likely a 3.3L V6 or a 3.8L V6). It will have diagrams, specifications, and procedures for testing and adjusting components. A Haynes or Chilton manual will be useful.

* Diagnostic Codes: Check for any diagnostic trouble codes (DTCs) stored in the ECM. You'll need a code reader to do this. The codes will help pinpoint the problem area.

* Professional Mechanic: Unless you have significant experience working on fuel-injected cars, it's best to take your Buick to a qualified mechanic. Improper adjustments can cause more harm than good and lead to expensive repairs. The cost of a proper diagnosis and repair will likely be less than the potential damage you could cause.

In short, there's no simple "adjust the screw" solution. The idle speed is computer-controlled, and addressing the underlying issue (faulty sensor, vacuum leak, etc.) is crucial.