A vacuum gauge fluctuates rapidly between about 12 and 17 in. Hg. This could be an indication of:

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Quizplus · Accepted Answer

Fluctuations in vacuum pressure indicate a problem with the engine's compression. In this case, burnt or sticking valves could cause the gauge to fluctuate between 12 and 17 in. Hg. Worn compression rings or a restricted exhaust system would typically produce a steady, low reading on the gauge, while a leaking head gasket would cause a steady, high reading.

A Vacuum Gauge Fluctuates Rapidly Between About 12 and 17