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8-4
DIAGNOSIS GUIDE
ENGINE STARTS, BUT
FAILS TO KEEP RUNNING
ENGINE RUNS, BUT
MISSES
GROUP 8-
ENGINE
FUEL SYSTEM
Idle fuel mixture needle(s) not
vropE>rly adjusted.
Engine idle speed set too low.
The choke not operating properly.
Float setting' incorrect.
Fuel inlet system not operating
properly.
Dirt or water in the fuel lines or
in the fuel filter.
Carburettor
icing.
Fuel pump
defective.
Determine if the miss is steady
Ol'
·erratic and
at
what speed the miss
occurs by operating the engine at
various speeds under load.
MISSES STEADILY AT
ALL SPEEDS
Isolate the miss by operating the
engine with one cylinder not firing.
This is done by operating the en-
gine with the ignition wire removed
from one spark plug at a time, until
.all cylinders have been checked.
Ground the spark
plug
wire re-
moved.
If the engine speed changes when
.a particular cylinder is shorted out,
that cylinder
was
delivering power
before
being
sho1·ted
out.
If
no
~hange
in the engine operation is
evident, the miss was caused by
that cylinder not delivering power
befo1·e being shorted out. In this
case, check the:
IGNITION
SYSTEM
If
the miss is isolated in a par-
ticular cylinder, perform a spark
test on the ignition lead of that cy-
linder.
If
a good spark does not occur,
the trouble is in
the
seconduy cir-
cuit
of
the
system.
Check the spark
plug wire
and
the distributor cap.
If
a good spark occurs, check the
spark plug.
If
the spark plug is not
at
fault, a mechanical component of
the engine is probably at fault.
ENGINE
Intake
manifold
gasket leak.
Perform
a manifold vacuum
or
compression
test
to
determine
which
mechanical component of the
engine
is
at fault.
MISSES ERRATICALLY
AT ALL SPEEDS
EXHAUST SYSTEM
Exhaust system restricted.
IGNITION
SYSTEM
Breaker points
not
properly ad-
justed.
Defective breaker
points,
conden-
ser, secondary wiring, coil or spark
plugs.
Check for dirt in the carburettor
not allowing fuel to enter or be dis-
charged from the idle system.
IGNITION SYSTEM
Defective spark plugs.
Leakage in the high tension wir-
ing.
Open circuit in primary resis-
tance wire.
Breaker points not properly ad·
jus ted.
High Tension leakage across the
coil, rotor or distributor cap.
Defective ignition switch.
FUEL SYSTEM
Float setting incorrect.
Fuel inlet system not operating
properly.
Dirt or water in the fuel lines or
carburettor.
Restricted fuel filter.
Loose booster venturi (V-8).
COOLING SYSTEM
Check the cooling system for in-
ternal leakage and/ or for a condi-
tion that prevents the engine from
reaching normal operating temper-
ature.
ENGINE
Perform a manifold vacuum or
compression
test
to
determine
which mechanical component of the
engine is at
fault.
MISSES AT
IDLE ONLY
FUEL SYSTEM
Idle fuel mixture needle(s) not
properly adjusted.
Restriction in idle fuel
system.
IGNITION SYSTf:M
Excessive play in the distributor
shaft.
Worn distributor
cam.
Defective coil, rotor, condenser,
Breaker points, ignition wiring or
<>park plugs.
EN~INE
Valve clearance
set too
tight.
Worn tamshaft lobe(s)
.
Perform a manifold vacuum or
compression test to determine which
mechanical component of the engine
is at fault.
MISSES
AT HIGH SPEED
ONLY
FUEL SYSTEM
Power valve or passages clogged
or damaged.
Low or erratic fu el pump pres-
sure.
Fuel inlet system not operating
properly.
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