Mityvac 06820 Diesel Compression Test Adapters - Manual (Page 9)

Page Number - 9
Form 822378
STICKING VALVE
A sticking valve will exhibit a rapid,
intermittent drop from the normal pointer
indication. This is unklike the regular drop
that characterizes a burned or leaking
valve.
A sticking valve condition may be pin-
pointed by directly applying a 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.
LEAKING PISTON RING
Vacuum at idle will be low but steady at
about 12” to 16” Hg. Open the throttle and
allow the engine to pick up speed to
about 2000 RPM. Then, close the throttle
quickly. The pointer should jump 2” to 5”
Hg above its low steady reading. A lesser
gain may indicate faulty rings, and a
complete cylinder leakage or compression
test should be done.
BLOWN CYLINDER HEAD
GASKET
At idle, the vacuum pump gauge pointer
will fluctuate between normal and a low
reading. The needle will drop sharply
about 10” Hg from a normal reading and
return each time the defective cylinder or
cylinders research firing position.
EXHAUST RESTRICTION
TEST
An exhaust restriction will cause normal or
near normal performance at engine idle but
cause very poor engine performance
under load, or at higher speeds.
1) Connect the pump hose to an intake
manifold vacuum fitting. Operate the
engine at idle and note the vacuum
reading and needle movement.
Compare readings and movements
against descriptions listed for burned
valves and late ignition or valve timing.
2) Watch the vacuum gauge as engine
speed is increased to approximately
2500 RPM.
3) An increase in vacuum over that
obtained at idle indicates an exhaust
system that is free of restrictions.
4) If the needle drops toward zero as
engine RPM is increased, either an
exhaust restriction or an over-active
Exhaust Gas Recirculation (EGR) valve
is causing the problem.
5) Test the EGR valve separately. If it is
found to be in good condition, the
problem is a restricted exhaust. Check
and replace if necessary.
INCORRECT IDLE AIR/FUEL
MIXTURE
When the needle on the gauge drifts
slowly back and forth at idle, over a range
of 4” to 5” Hg, the fuel mixture is too rich.
A lean mixture will cause an irregular drop
of the needle over about the same range.
INTAKE MANIFOLD OR AIR
INDUCTION LEAKS
If there are any air leaks in the air induction
system, the pump’s gauge needle will be
about 3” to 9” below normal but will remain
steady.
LATE IGNITION OR VALVE
TIMING
An extremely low but steady reading at
idle indicates late ignition or valve timing, or
a uniformly close setting of the valve lash.
Perform separate tests to determine which
of these problems, if any, have affected
the engine.