Piston And Connecting Rod - Hummer H1 Service Manual

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ROAD TESTING
The purpose of road testing is basically to confirm or deny the
existence of an engine fault. Procedure is as follows:
1. Note condition of wheels/tires before test. Worn, damaged
tires or wheels can cause vibration or noise.
2. Check and adjust lube levels in engine and transmission
and top off engine coolant if necessary.
3. Turn off A/C system.
4. Start and run engine at idle speed with transmission in
neutral. Record any noise or vibration that may be present.
ÒRevÓ engine and note any noise/vibration plus any
change in magnitude or frequency.
5. Drive vehicle at legal speed and note any evidence of
noise, vibration, low power, miss, etc.
a. If vibration was noted, drive vehicle above vibration
speed. Then shift into neutral and coast down through
problem range. If vibration still occurs, problem is
with chassis component. However, if vibration ceases,
problem is with driveline component.
b. If noise is noted, shift into neutral at about 35 mph
and note if noise changes in pitch, frequency, or stops
altogether.
6. Operate A/C system and note effect. If noise or vibration
begins, problem is related to A/C compressor, clutch,
accessory mounting bracket, or belt. However, if noise/
vibration is absent, note this and proceed to next step.
7. Connect compound gauge to manifold air intake tube
fitting. Then press accelerator pedal to wide open position
and record boost reading which should be 2 psi or greater.
ENGINE NOISE DIAGNOSIS
Engine noises are usually sensitive to changes in rpm and load.
For example, a collapsed lifter will make an audible ÒtickingÓ
noise that increases in frequency as rpm goes up.
Engine noises can also be determined according to speed of the
related rotating component. For instance, valvetrain noises will
be at a frequency that is one half crankshaft speed.
Accessories
Noise caused by engine accessories such as the vacuum pump
alternator, power steering pump, A/C clutch, or drive pulleys is
speed sensitive. An increase or decrease in engine rpm will
generally change pitch and frequency, or even cause it to stop.
A mechanics stethoscope will generally help pinpoint a noise
source. Removing the accessory drivebelt will also help verify
an accessory generated noise.
Bearings
Bearing noise can be differentiated by pitch. A damaged con-
necting rod bearing makes a higher pitched, metallic knocking
sound. This as opposed to the lower pitch thump of a worn, or
spun main bearing. A failed rod bearing can be confusing as it
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seems to make a greater frequency noise. This is because the
bearing may knock at both ends of piston travel.

Piston and Connecting Rod

Piston slap is usually caused by severely worn cylinder bores
and pistons, partially collapsed piston skirts, worn pin bores,
severely worn rings, or an undersized piston. An incorrectly as-
sembled, or installed connecting rod and piston assembly will
also produce slap.
Slap always occurs at crankshaft speeds. In severe cases, slap
may occur in both directions of piston travel. Although fairly
common on high mileage engines, slap can be difficult to hear
on low mileage engines. As the term implies, piston slap is ap-
propriate for the sound generated. It occurs when a piston be-
gins to rock within the cylinder as it travels up and down. This
action causes the skirt to slap the cylinder wall as it straightens,
then rocks away from the direction of thrust.
Valvetrain and Camshaft
Rocker arm and hydraulic lifter noise is probably the most eas-
ily identifiable. Both make tapping (or clicking) noises that
only differ in pitch and volume. Although damaged valve
springs or pushrods are more difficult to hear, they usually
cause a power loss or rough engine condition and require in-
spection to locate.
A worn, missing, or incorrect thickness camshaft thrust washer
will cause excessive cam end play. The most frequent result is
an audible knocking sound localized at the rear of the block.
Timing gear and chain noise is not always audible, even when
the chain and gears are severely worn. These components,
when failed, are usually discovered during timing checks
prompted by a low power or no-start condition.
NOTE: Valvetrain noise can also be the result (or first indica-
tor) of low oil pressure.
Starter Noise
The starter bendix can hang and prevent quick disengagement
after startup. The resulting noise can be misdiagnosed as en-
gine related. A mechanics stethoscope is fairly effective at dif-
ferentiating engine and starter noise.
Knocking Noise At Idle
Knocking noises can be from connecting rod bearings, a
cracked flywheel, converter touching the housing or cover, A/
C compressor, or loose exhaust component.
Look for exhaust components grounding against the body,
frame, or driveline component. Remove the converter access
cover and visually inspect the flywheel and converter. Check
the cover and converter housing for signs of contact. A fly-
wheel cracked at the hub will allow the converter to wobble
slightly. Test the A/C system for incorrect charge levels which
can produce compressor knock.
Engine 2-5