Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 7)

Page Number - 7
Form 822378
mixture will be leaned out by the extra air
entering the system through the leaks,
thus causing problems such as burned
exhaust valves, uneven idle, stalling, pre-
ignition, burned spark plugs, etc.
Additionally, any vacuum operated device
affected by the vacuum leak will not
function properly.
A normal gasoline engine should develop
16-22” Hg of intake manifold vacuum at
idle. This is an indication that the engine is
breathing properly. If the vacuum is
lower, the engine is running less
efficiently. The lower the manifold
vacuum, the less efficiently the engine is
running and the lower the gas mileage
will be.
The vacuum distribution system supplies
vacuum to vacuum motors (servos) in the
air conditioning, power brake booster,
speed control servo, emission controls,
manifold absolute pressure (MAP)
sensor, and automatic transmission
control systems. In older vehicles,
vacuum is also supplied to the distributor
vacuum advance or retard mechanism.
These devices can be connected directly
to manifold vacuum, or can be controlled
through electric solenoids, thermostatic
switches, or other vacuum controls.
TROUBLESHOOTING THE
VACUUM SYSTEM
Most vacuum problems can be traced to
leaks, which occur in hoses, connectors,
motor diaphragms or valves. Pinched
lines or clogged valves will also not
allow vacuum flow. Problems can also be
traced to improper mechanical operation
of devices driven by vacuum motors.
The vacuum pump can be used to
measure the amount of vacuum in a
hose. The vacuum gauge feature is very
useful for detecting a fluctuating vacuum
supply or a leaky enables you to check
all types of vacuum operated devices.
On a vacuum motor, for example, the
pump is used to evacuate the diaphragm
chamber, which allows you to check the
mechanical operation of the device as
well as the amount of vacuum, required
to actuate it. Test for leaking diaphragm
by applying 10” Hg vacuum to the device
(FIGURE 4). Observe the gauge to see if
the needle drops after the actuator stops
moving. If the needle continues to drop, a
leaking diaphragm is indicated. If the
diaphragm is okay, the vacuum should
hold for one minute with the needle
steady.