Diagnosing Mechanical Engine Conditions - Mityvac MI8000 Benutzerhandbuch

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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 4). 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 540-750 mbar (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 35-235 mbar (1 to 7" Hg) at regular
intervals whenever the defective valve attempts
to close.
With motor at idle speed,
vacuum pointer should hold
steady between 540 and 750
mbar.
FIGURE 4: VACUUM GAUGE READINGS
Form 824357
With motor at idle speed,
dropping back of vacuum
pointer indicates sticky
valves.
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 340 and 710 mbar (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 100 mbar (3" Hg). As the speed
of the engine is increased, the needle will steady.
LEAKING PISTON RING
Vacuum at idle will be low but steady at about 405
to 540 mbar (12 to 16" Hg). Open the throttle and
allow the engine to pick up speed to about 2000
RPM, and then close the throttle quick ly. The pointer
should jump 65 to 170 mbar (2 to 5" Hg) above
its low steady reading. A lesser gain may indicate
faulty rings, and a com plete cylinder leakage or
compression test should be done.
With motor at idle speed,
floating motion right and
left of vacuum pointer
indicates carburetor too rich
or too lean.
With motor at idle speed, low
reading of vacuum pointer
indicates late ignition timing
or intake manifold air leak.
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