Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 8)

Page Number - 8
Form 822378
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 diagno-
sis.
Do not look for the engine to produce
specific (numerical) amounts of vacuum.
Much more important than specific
numbers is 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 reading should
be 16-22” Hg and steady.
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 SPRING
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” Hg at regular intervals whenever
the defective valve attempts to close.