Cars Simplified: Everything Automotive Explained

Engine Vacuum Testing

Vacuum testing an engine involves connecting a vacuum gauge to the vacuum side of the engine's intake. Usually this involves connecting to a non-vital intake line coming from the intake manifold, after the throttle body.

Why Test Vacuum?

Internal combustion piston engines produce vacuum (negative pressure) because they draw in air to use in the combustion. Through timing of the intake valves, each piston draws in air for a given amount of time. In good condition, an engine will have all of its pistons pull in the same amount of air in a constant and uniform manner. If a piston pulls in air in a way dissimilar to the others, it will leave a signature on the overall vacuum pulled by the engine, and measuring that can help determine the type of problem in that piston.

How to Test Engine Vacuum

With a vacuum gauge, find a vacuum line that can be disconnected from a component that it non-vital during testing, such as a brake booster. With the vacuum gauge connected with a good seal, start up the engine safely and read the gauge.

What Readings Indicate

Reading on Gauge:Indicates:
17" to 21", steady needle at idle.Good reading on most engines.
Needle bounces back and forth from 4" to 8".Burned or constantly leaking valve.
Low, steady reading.Likely late valve or ignition timing.
Snap acceleration needle drops to 0", then reads 20" to 23" of vacuum instead of 27".Likely worn piston rings.
Good reading at idle, but as engine speeds up to 2500 rpm, the needle slowly drops.Possible exhaust restriction.