Where is the air control valve on a 1991 4.3 s10?

The 1991 S10 4.3L (specifically the TBI version, as that's the most common for that year) doesn't have a single component called an "air control valve" in the same way some later fuel-injected systems do. The air/fuel mixture is controlled by a few interacting components. What you might be looking for depends on the specific issue you're trying to troubleshoot.

Here are some possibilities and where to find them:

* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position and is crucial for controlling the air/fuel ratio. It's located on the throttle body. The throttle body itself is on the intake manifold, usually near the firewall.

* Air Flow Meter (AFM) / Mass Air Flow Sensor (MAF): 1991 4.3L TBI systems use an AFM. This measures the amount of air entering the engine. It's typically located in the air intake tube, between the air filter and the throttle body.

* MAP (Manifold Absolute Pressure) Sensor: This sensor measures the pressure in the intake manifold, which helps the computer determine the engine load. Its location varies slightly depending on the exact setup, but it's usually on the intake manifold itself or nearby.

* Idle Air Control (IAC) Valve: This valve controls the amount of air bypassing the throttle plate at idle to maintain a consistent idle speed. It's usually mounted on the throttle body.

To determine which component might be the source of your problem, describe the symptom you're experiencing. For example, rough idle, stalling, poor acceleration, etc. Knowing the symptom will help pinpoint the likely culprit.