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, and then close
the throttle quick ly . The pointer should jump 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 .
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 reach firing position .
EXHAUST RESTRICTION TEST
An exhaust restriction will cause normal or near nor-
mal 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
descrip tions 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
If the gauge needle drifts slow ly 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 gauge needle will be about 3 to 9” Hg
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 .
DiaGnosinG mechanicaL enGine conDitions
FIGURE 5: VACUUM GAUGE READINGS
With motor at idle speed,
vacuum pointer should hold
steady between 16 and 22 .
With motor at idle speed,
dropping back of vacuum
pointer indicates sticky
valves .
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 .
Page Number - 8
Form 824426
