Form 824426
Page Number - 7
the automotive vacuum sYstem
DiaGnosinG mechanicaL enGine conDitions
FIGURE 4: TYPICAL CARBURETOR VACUUM SUPPLY POINTS
Carburetor or Throttle Body
EGR Vacuum
• Zero at closed throttle
• Still zero when “S” vacuum is on
• Equal to manifold at greater
throttle opening
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
Spark Port Vacuum
• No vacuum at
closed throttle
• Equal to manifold vacuum off idle
• Originally used to control “spark” vacuum
to distributor advance diaphragm
VACUUM GAUGE CHECKS & DIAGNOSIS
The pump’s vacuum gauge readings give indications
of possible mechanical problems, but they are not
foolproof . Observe the gauge carefully and follow
the vacuum readings with further tests, where
possible, to confirm your diagnosis .
Do not look for the engine to produce specific
(numerical) amounts of vacuum . Much more
important than specific numbers, are the range
of the vacuum readings and the movement of the
needle (FIGURE 5) . Important things to notice about
the needle movement are HOW the needle moves
(in a smooth or jerky manner, erratic, etc .), what
direction it moves, whether movement is regular or
varying, and how far the needle moves .
The following gives some examples of what to
look for and the meanings of a variety of vacuum
gauge readings .
NORMAL ENGINE
Run engine at idle and connect the pump to an
intake manifold vacuum port . Watch the needle’s
movement on the gauge . At idle, the vacuum gauge
reading should be 16-22” Hg and steady .
BURNED OR LEAKING VALVE
At idle, burned or leaking valves will cause the
pointer on the gauge to drop to a low reading and
return to normal at a regular interval . The needle
will drop from 1 to 7” Hg at regular intervals
whenever the defective valve attempts to close .
STICKING VALVE
A sticking valve will exhibit a rapid, intermittent
drop from the normal pointer indication . This is
unlike the regular drop that characterizes a
burned or leaking valve .
A sticking valve condition may be pin-pointed by
directly applying lightweight oil to each valve guide .
When the sticking valve is reached, the situation will
be temporarily remedied .
WEAK OR BROKEN VALVE SPRING
Weak valve springs are indicated when the pointer
of the vacuum pump gauge fluctuates rapidly
between 10” and 21” Hg at idle . The fluctuations
will increase with engine speed . A broken valve
spring will cause the needle to fluctuate rapidly
at a regular interval . Again, this will occur every
time the valve attempts to close .
WORN VALVE GUIDES
Worn valve guides admit air which upsets the air/
fuel mixture . The vacuum gauge reading will
be lower than normal and will fluctuate rapidly
in a range of about 3” Hg . As the speed of the
engine is increased, the needle will steady .
