Vacuum Gauge Readings: What's Normal at Idle?

Sea level warm idle: roughly 17 to 22 inHg.
Altitude adjustment: expected vacuum drops about 1 inHg per 1,000 feet.
Healthy snap test: quick drop, then rebound above idle before settling.

Interpret Your Vacuum Test

Compare cylinder compression readings and flag weak cylinders.

Check charging voltage, ripple, and cable voltage drop readings.

Estimate how long a key-off current draw can drain the battery.

Find common battery group sizes, dimensions, and terminal layouts.

Estimate cold cranking amp needs from engine size and climate.