Chapter A: General Diagnosis
6
ioning of the extra gas pressure pushing on the pis-
ton
• A sharp knocking that stands out most at idle can
indicate a wrist pin that is loose in its bore. Ground-
ing the spark plug of the affected cylinder makes
the knock audible at top dead center as well as bot-
tom dead center. Retarding the spark decreases
wrist pin noise
• A rapid, steady dull pounding that increases with
load is typical of worn main bearings
Spark Knock
Spark knock, which is caused by uncontrolled combus-
tion, sounds like a metallic pinging noise. Spark knock
may be heard under a heavy load or on acceleration.
Detonation occurs when combustion of the air/fuel mix-
ture in the cylinder starts off correctly in response to ig-
nition by the spark plug, but one or more pockets of the
air/fuel mixture explode outside the envelope of the nor-
mal combustion. The collision of the two flame causes
a pinging noise. This can be caused by:
• Fuel with two low of octane rating
• Ignition timing that is to far advanced
• High engine operating temperature
• Excessive carbon build up in the combustion cham-
ber
Preignition occurs when the air/fuel mixture premature-
ly ignites before the spark plug fires. Then the spark plug
ignites the remaining mixture at the normal time. When
the two portions of burning mixture meet each other,
there is a sudden abnormal rise in cylinder pressure
causing engine vibration and a pinging noise. This can
be caused by:
• Hot spots in the combustion chamber
• Incorrect heat range spark plug
• Carbon deposits in the combustion chamber
Unusual Exhaust Color and Odor
Although a healthy catalytic converter can do a good job
of cleaning up the exhaust, you can tell something
about the internal engine condition by checking for un-
usual smoke or smells:
• Black exhaust smoke. This is caused by a rich
air/fuel mixture and is often accompanied by the
"rotten egg" smell of an overworked catalytic con-
verter
• Blue exhaust smoke indicates excessive oil burn-
ing, which gives off a pungent odor
• Cloudy white exhaust is often the result of engine
coolant leaking into the combustion chamber. Burn-
ing coolant also produces a distinctive chemical
odor. Check the temperature gauge for overheating
Keep in mind that oil vapor odors are not always the re-
sult of an internal engine problem. A clogged or mal-
functioning positive crankcase ventilation (PCV) system
can not only produce a burning oil smell, but can also
cause excessive crankcase vapor and increase oil con-
sumption. Always check all external sources before you
condemn the engine.
Internal Engine Diagnosis
Specific internal mechanical problems on a running en-
gine can be located by performing several basic tests.
To eliminate the possibility of internal engine problems,
performing the following tests:
• Intake manifold vacuum
• Cylinder compression
• Cylinder leakage
• Cylinder power balance
A brief description of standard test procedures and in-
terpreting results will be presented here.
Intake Manifold Vacuum Tests
Manifold vacuum tests are performed by connecting a
vacuum gauge to the intake manifold downstream of
the throttle plates. The gauge records the difference
between atmospheric pressure and manifold pressure.
Vacuum gauge readings can pinpoint manifold and
vacuum line leaks, valve and valve guide problems, in-
correct ignition and valve timing, exhaust restrictions,
and poor combustion chamber sealing.
A vacuum gauge is usually calibrated in inches of mer-
cury (in-Hg) or kilopascal (kPa). Normal vacuum at idle
is from 15 to 21 in-Hg (50 to 70 kPa) for most engines.
Gauge readings should be steady and decrease as the
throttle opens. Vacuum decreases as elevation in-
creases, and gauge readings must be corrected ac-
cordingly. Manufacturers provide specifications for
testing at sea level. To correct for altitude, subtract one
in-Hg (3.377 kPa) for every 1,000 feet (305 meters)
above sea level.
Some engines, especially if turbocharged or super-
charged, require a measure of manifold boost pressure
test. Boost pressure, also known as positive pressure,
specifications are provided by the manufacturer.
Gauges for reading manifold absolute pressure (MAP)
pressure may be calibrated differently than vacuum
gauges. Absolute pressure uses a reference point of
zero pressure, or total vacuum, regardless of atmos-
pheric pressure.
Refer to Book A1 "Engine Repair" in this series for more
detailed information on manifold vacuum test proce-
dures and results.
Summary of Contents for ASE-A8
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