Mityvac MI8000 Manual Del Usuario página 6

the automotive vacuum sYstem
VACUUM DISTRIBUTION
All modern automobiles have a vacuum distribution
system (FIGURE 2), consisting of lines, hoses,
fittings and vacuum devices. This system must be
leak proof. If it is not, the engine air/fuel 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 540-750
mbar (16-22" Hg) of intake manifold vacuum at
idle. This is an indication that the engine is breath-
ing 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 sys tems. In older
vehicles, vacuum is also supplied to the distributor
vacuum advance or retard mechanism. These
devices can be connected directly to man ifold
vacuum, or can be controlled through electric
solenoids, thermostatic switches, or other
vacuum controls.
Carburetor or Throttle Body
EGR Vacuum
• Zero at closed throttle
• Still zero when "S" vacuum is on
• Equal to manifold at greater
throttle opening
Spark Port Vacuum
• No vacuum at
closed throttle
• Equal to manifold vacuum off idle
• Originally used to control "spark" vacuum
to distributor advance diaphragm
FIGURE 3: TYPICAL CARBURETOR VACUUM SUPPLY POINTS
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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 hose. The vacuum pump
feature 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 a leaking diaphragm by applying
340 mbar (10" Hg) vacuum to the device (FIGURE
3). Observe the gauge to see if the needle drops
after the actuator stops moving. If the needle con-
tinues to drop, a leaking diaphragm is indicated. If
the diaphragm is okay, the vacuum should hold for
one minute with the needle steady.
Venturi Vacuum
• Weak or zero at cruise or idle
• Strong at wide open throttle
Position when "S" and "E"
vacuum are "on"
Throttle plate (closed position)
Manifold Vacuum
• Available with engine running
• Strongest at closed throttle
• Gradually weakens as throttle opens
• But stays strong if the choke is closed
Form 824357