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Diagnosing mechanical engine conditions – Mityvac MV8500 silverline elite aUtoMotive test kit User Manual

Page 8

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LEAKING PISTON RING

Vacuum at idle will be low but steady at about 1 to

16” Hg . Open the throttle and allow the engine to

pick up speed to about 000 RPM, and then close

the throttle quickly . The pointer should jump 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 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

descriptions listed for burned valves and late

ignition or valve timing .
) Watch the vacuum gauge as engine speed is

increased to approximately 500 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 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 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 .

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 8433