Which issue is likely if an engine's vacuum gauge shows fluctuating readings at higher RPMs?

When an engine's vacuum gauge shows fluctuating readings at higher RPMs, this is indicative of a weak or broken valve spring. The valve springs are crucial in maintaining proper valve closure and timing. At higher RPMs, if a valve spring is weak or broken, the valve may not close fully or may float, causing inconsistent engine vacuum readings due to the valves not sealing properly.

In contrast, a vacuum leak typically results in a steady drop in vacuum rather than fluctuations and would not be the most likely cause at higher RPMs. A blown head gasket often presents consistent symptoms such as loss of power or coolant, rather than fluctuating vacuum. Similarly, a defective fuel injector may cause an engine to run rough or experience poor performance but wouldn’t directly produce fluctuating vacuum readings as the primary symptom. Therefore, the behavior of the vacuum gauge at higher RPMs aligns best with issues involving valve spring integrity.