Lesson 3/Learning Event 2
INTAKE MANIFOLD PRESSURE (VACUUM) TEST
Knowing the compression of engine cylinders is very useful in determining engine condition, but this is not the
only test you can make for this purpose. You can also measure the amount of suction or vacuum that the
cylinders have on the intake stroke. On compression-ignition engines, this is not your job, so we will look only
at the spark-ignition engine.
Actually, a vacuum measurement is still a measurement of pressure, but the pressure is less than the normal air
pressure that is all around us. Instead of being measured in pounds of pressure per square inch, as compression
is, the vacuum is measured in inches of mercury.
The term "inches of mercury" refers to the distance that mercury in a U-shaped tube moves when a vacuum is
applied to one end of a tube. For example, if the mercury rises 10 inches, the value of the vacuum is 10 inches
The vacuum gage that you will be using has a scale and a needle pointer like the compression gage. Each mark
on the scale stands for 1 inch of mercury. Most gages will also read a small amount of pressure for checking
fuel pump pressure. (This test is covered in another lesson.) The vacuum gage is issued to your unit as a kit
and comes with adapters and hoses so it can be attached to any engine.
Connecting the Gage
The vacuum reading is obtained by connecting the vacuum gage to the intake manifold and then operating the
engine. Recall that the intake manifold is a common pipe that connects to all of the cylinders. Almost all
engines contain some sort of a plug or connection in the intake manifold. Remove the plug or fitting and
replace it with a suitable adapter from the vacuum gage kit and connect the vacuum gage.