During a compression test, what should you do with the throttle plate?

During a compression test, it is essential to have the throttle plate fully open. This allows the engine to draw in the maximum amount of air, which ensures that the compression readings are accurate. When the throttle is open, it prevents any restriction on airflow that might occur if the throttle plate is closed or only partially open. This unrestricted airflow enables the engine cylinders to fill properly, leading to an accurate measurement of the cylinder compression pressure.

Having the throttle fully open also minimizes the chance of false readings that can occur from incomplete combustion or insufficient air supply, which could lead to misleading results regarding the engine's health. Thus, for a reliable and valid compression test, keeping the throttle plate blocked fully open is the appropriate approach.