It is one of the easiest and cheapest ways to check an engine for serious issues is to use a plain ole vacuum gauge. A vacuum gauge can tell you a lot about an engine's condition.

How to use vacuum gauge

There are some guidelines that you can follow when using a vacuum gauge on gasoline engines.

1) At idle, the more vacuum the better.

2) The highest manifold vacuum will be on deceleration.

3) A closed throttle will create more vacuum than an open throttle.

Engine Vacuum Test



1. Normal Engine: On a normal engine, accelerate to around 2000 rpm and then quickly release the throttle.

The engine should snap right back to a steady 17- 21 "hg vacuum.

2. Steady low between 5-10 "hg vacuum: This indicates that the engine has a leak in the intake manifold or the intake gasket.

3. Steady low between 10-15 "hg vacuum:

This reading indicates late valve timing. There's a good chance the vehicle has jumped timing.

4. Steady low between 15-18 "hg vacuum: This just low reading indicates retarded ignition timing.

Advance the timing on the distributor to correct this problem.

5. Fluctuating Needle: A fluctuating needle indicates there's a problem with a valve or a there's an engine misfire.

6. Needle drops during acceleration: If the needle drops steady during acceleration there's a restriction in the exhaust or intake.