What does fluctuating readings between 10 and 20 inches of vacuum on a vacuum gauge suggest?

Fluctuating readings between 10 and 20 inches of vacuum on a vacuum gauge can indicate issues related to engine dynamics. Specifically, a weak or broken valve spring is a plausible cause for such fluctuations. When a valve spring is compromised, it may not be able to keep the valve seated properly during the engine cycle. This malfunction can lead to inconsistent air intake, causing variations in the vacuum reading as the engine struggles to maintain a steady airflow.

In an operating engine, stable vacuum readings typically indicate proper valve operation and efficient air movement through the engine. When fluctuations occur, it suggests that the timing of the valves opening and closing is being negatively impacted, often due to the springs’ inability to do their job effectively.

Other potential causes of fluctuating vacuum, such as a leaking intake manifold or a restrictive exhaust, would also affect engine performance, but the specific fluctuating nature of the readings points more towards issues with valve control — with a weak or broken valve spring being a likely culprit in this case.