How come the engine only revs up when gas is applied in a 98 Chevy blazer?

In a 1998 Chevy Blazer, the engine only revving up when the gas pedal is applied is the expected and normal behavior. The gas pedal controls the throttle, which regulates the amount of air entering the engine. More air means more fuel is injected (in a fuel-injected engine like the Blazer's), leading to a faster engine speed (RPM).

If the engine is revving *without* the gas pedal being pressed, that's a problem indicating a malfunction somewhere in the throttle system or possibly the engine control unit (ECU). Potential causes of a problem (opposite of your description) could include:

* Faulty Throttle Position Sensor (TPS): This sensor tells the ECU the throttle's position. A faulty TPS could send incorrect signals, causing the engine to rev unexpectedly.

* Stuck Throttle Plate: The throttle plate itself might be sticking slightly open, allowing more air in than intended.

* Vacuum Leak: A leak in the vacuum system can affect the throttle control.

* Faulty Idle Air Control (IAC) Valve: This valve regulates air flow at idle. A malfunction could lead to high idle.

* Faulty ECU: While less common, a problem with the engine's computer could cause erratic behavior.

Since your question describes the *normal* operation of the engine, there's no problem to diagnose. If you were experiencing the opposite—the engine revving without gas—then investigating the above would be necessary.