Diagnosing Mechanical Engine Conditions - Craftsman 20930 Operator's Manual

THE AUTOMOTIVE VACUUM SYSTEM
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 4: TYPICAL CARBURETOR VACUUM SUPPLY POINTS

DIAGNOSING MECHANICAL ENGINE CONDITIONS

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
Venturi Vacuum
• Weak or zero at cruise or idle
• Strong at wide open throttle
Manifold Vacuum
• Available with engine running
• Strongest at closed throttle
• Gradually weakens as throttle opens
• But stays strong if the choke is closed
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.
6
Position when "S" and "E"
vacuum are "on"
Throttle plate (closed position)