Which of the following is NOT an indicator of a restricted exhaust system?

A momentary increase in vacuum when the throttle is closed can indicate that the exhaust system is operating properly rather than being restricted. When the throttle is closed, a well-functioning exhaust allows for a temporary drop in backpressure, which can cause a brief spike in vacuum readings. This behavior suggests that there is enough exhaust flow and that the exhaust system is not obstructed.

On the other hand, consistent low vacuum readings, lack of increase in vacuum when the throttle is closed, and significant drops in vacuum at higher RPMs generally indicate that there is a restriction present in the exhaust system. These other options reflect signs of backpressure that can negatively impact engine performance, such as reduced efficiency, power loss, and potential engine damage over time. Hence, option C stands out as an indicator that does not align with symptoms of a restricted exhaust system.