Chapter A: General Diagnosis
5
Avoid the following actions unless required by the diag-
nostic procedure:
• Jumpering or grounding of the components or con-
nectors
• Connecting test equipment probes to components
or connectors
• Connect the ground lead first when using test
probes
• Ground the protective packaging of any component
before opening
• Resting solid-state components on metal work-
benches, or on top of TVs, radios, or other electri-
cal devices
Ignition OFF When Disconnecting Battery
Always turn the ignition OFF when connecting or dis-
connecting battery cables, battery chargers, or jumper
cables. Failing to do so may damage the Powertrain
Control Module (PCM) or other electronic components.
Electric Coolant Fan Caution
An electric fan under the hood can start up even when
the engine is not running and can injure you. Keep
hands, clothing, and tools away from any underhood
electric fan.
COMPREHENSIVE ENGINE TESTING
It is important to follow instructions from both the vehi-
cle and the test equipment manufacturers when per-
forming engine tests. Some tests require the ignition to
be disabled while the starter motor cranks the engine.
Others may require bypassing the fuel pump relay, idle
speed controller, or some other electrical or electronic
component. The proper test equipment, and the knowl-
edge to use it correctly, are essential.
The following are general guidelines for performing
comprehensive engine tests. For some tests, more de-
tailed procedures can be found in other books in this
series. The tests here are not presented in any specific
order. The sequence in which they are performed will
vary.
Unusual Engine Noises
Engine noises can be divided into two general cater-
gories: those that originate in the top end of the engine,
and those that originate in the bottom end of the engine.
Begin engine noise diagnosis by determining where in
the engine the noise is coming from. Bottom end, or
crankcase, noises occur at crankshaft speed, so they
tend to produce a high-frequency knock or rumble. Top
end, or valve train, noises occur at a lower frequency
because these parts operate at one-half crankshaft
speed.
A stethoscope is a handy tool for isolating noises. You
can also use a timing light to determine whether a noise
is from the top or bottom end of the engine. Connect the
timing light and listen . If the engine noise cycles in time
with the flashing light, the sound is coming from the bot-
tom end. Sounds that are audible with every other flash
of the timing light originate in the top end of the engine.
Top-End Noises
The top end of a healthy engine produces a high
pitched, whirring noise with a very rapid and much
fainter sewing machine like clicking coming from the
valves. The more valves the engine has and the higher
the idle speed, the more the individual clicks will blend
into a consistent drone. Any deviation is abnormal and
indicates a problem. Listen for:
• An irregular clacking or knocking noise caused by
excessive camshaft endplay
• An irregular slapping or thumping at the front of the
engine caused by a loose timing belt. A tight belt
makes a whirring, whining, hum that rises and falls
in pitch with RPM
• A single, clear clack whenever a particular valve
opens can be a collapsed lifter or a broken valve
spring.
• A loud, cycling, valve rattle that you can hear over
the normal valve noise can indicate either worn
valve guides or rocker arm pivots
Low pressure or restricted oil flow will produce an ex-
cessively loud, rhythmic clatter
Bottom-End Noises
Healthy engines produce an evenly pitched, rapid,
whirring sound and nothing else. Knocking or thumping
noises are signs that something is wrong. In general,
bottom-end noise can be caused and indicated by:
• An irregular knock at idle that can be made louder
or fainter by playing with the clutch pedal indicates
too much crankshaft endplay
• A sharp clattering knock that may be continuous at
idle or only appear when the throttle closes sud-
denly, can indicate a bad connecting rod bearing.
The noise will diminish if the spark plug for the of-
fending cylinder is grounded
• A hollow metallic clatter that is loudest when the en-
gine is cold may be piston slap caused by too much
piston to cylinder wall clearance. Grounding the
spark plug of the affected cylinder will often make
piston slap louder because it eliminates the cush-
Summary of Contents for ASE-A8
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