Spark Plug Visual Diagnosis
Worn or dirty plugs give
satisfactory operation at idling speed, but under operating conditions they
frequently fail. Faulty plugs are indicated in a number of ways poor fuel economy, power loss, loss of speed, hard starting and
general poor engine performance. Spark plugs may also fail due to carbon
fouling, excessive plug gap, or broken insulators. Fouled plugs are identified
by black carbon deposits. The black deposits are usually the result of
slow-speed driving and short runs where sufficient engine temperature is seldom
reached. Worn pistons, rings, faulty ignition, over-rich air/fuel mixture and
spark plugs which are too cold will also result in carbon deposits. Excessive
gap wear, on plugs of low mileage usually indicated the engine is operating at
high speeds or loads that are consistently greater than normal or that a plug
which is too hot is being used. Electrode wear may also be the result of plug
overheating due to insufficient spark plug torque or too much spark plug torque.
An excessively lean air/fuel mixture will also result in excessive electrode
wear.
Broken insulator are usually the result of improper installation or
carelessness when regapping the plug. Broken upper
insulators usually result from a poor fitting wrench or an outside impact. The
cracked insulator may not show up right away, but will as soon as oil or
moisture penetrates the crack. The crack may not be visible. Broken lower
insulators usually result from carelessness when regapping
and generally are visible. This type of break may result from the plug
operating too hot, which may happen in periods of high-speed operation or under
heavy loads. When regapping a spark plug, always make
the gap adjustment by bending the ground (side) electrode. Spark plugs with
broken insulators should always by replaced.
Resistor type spark
plugs with tapered seats are used. No gasket is needed on these plugs. These
spark plugs are a High Efficiency (HE) spark plug that has a fine wire
electrode,
- Normal service is a mixture of
idling, low speed, and high speed operation.
- Occasional high speed driving is
essential to good spark plug performance.
- Increased combustion heat burns
away excess carbon deposits, which accumulate from frequent idling or
stop-and-go driving.
- The heat resistant, insulating boot
covers the spark plug terminal.
- The boot extends over a portion of
the spark plug insulator.
- These boots prevent engine
misfiring.
- Corona discharge is a steady blue
light appearing (in darkness) around the insulator, just above the shell
crimp.
- Do not mistake this for a shorted
insulator. This is visible evidence of a high-tension field.
- This has no effect on ignition
system performance.
- This discharge repels dust
particles.
- This leaves a clear ring on the
insulator just above the shell.
- This is not evidence that
combustion gases are escaping.
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Step |
Action |
Value(s) |
Yes |
No |
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DEFINITION:
A visual inspection of the spark plug firing end. |
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Do
you see a brown to grayish tan deposit with a slight electrode wear? |
-- |
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Is
the repair complete? |
1.524 mm (0.060 in) |
System OK |
-- |
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Do
you see dry, fluffy, black carbon deposits? |
-- |
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This
indicates poor ignition output. Is the ignition coil(s) to the spark plug
connections OK? |
-- |
-- |
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Repair
or install new parts as necessary. Is the repair complete? |
-- |
System OK |
-- |
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Do
you see wet, oily deposits with very little electrode wear? |
-- |
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Is
the repair complete? |
1.524 mm 0.060 in |
System OK |
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Do
you see red, brown, yellow and/or white colored coatings on the insulator? |
-- |
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These
are the by-products of combustion. Is the insulator heavily coated? |
-- |
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Is
the repair complete? |
1.524 mm (0.060 in) |
System OK |
-- |
|
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Are
there heavy deposits of dark colored coatings, especially on the side of the
plug facing the intake valve? |
-- |
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This
is the result of failed intake valve seats or from excessive valve stem to
valve guide clearances. This may not occur in all cylinders. Repair or
install new parts as necessary. Is the repair complets?
|
-- |
System OK |
-- |
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Is
there a shiny, yellow glaze coating on the spark plug insulator? |
-- |
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These
are the melted by-products of combustion.
Is
the repair complete? |
1.524 mm (0.060 in) |
System OK |
-- |
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Are
the insulator tips blistered or burned? Are the electrodes badly eroded? |
-- |
-- |
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This
is the result of overheating. The spark plug heat range may be too hot. The
fuel octane rating may be to low for the application, refer the customer to
the Owner Manual for correct fuel rating. Is the plug heat range correct? |
-- |
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Check
the torque value to ensure good spark plug seat contact. Are the spark plugs
tightened according to specifications? Refer to Fastener
Tightening Specifications |
-- |
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Check
for a misfiring engine, refer to DTC
P0230 Fuel Pump Relay Control Circuit for V6 or DTC
P0300 Engine Misfire Detected for V8 in Engine Controls. Is the vehicle
operating properly?. |
-- |
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Check
for a lean/rich fuel mixture, refer to Fuel
System Diagnosis for V6 or Fuel
System Diagnosis for V8 in Engine Controls. Repair or install new parts
as necessary. Is the repair complete? |
-- |
System OK |
-- |
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Gap
and install the correct spark plug(s). Is the repair complete? |
-- |
System OK |
-- |
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