Checks
| Causes
|
| • | Misfiring is a general term that applies to a poor running engine. |
| • | With misfiring, the ignition spark is not igniting the air fuel
mixture at the proper time. Before investigating other possible causes, inspect
the spark plugs. |
| • | Spark voltage should not reach ground before arcing across the
gap at the tip of the spark plug. This leaves the air fuel mixture unburned,
causing misfiring. |
| • | Worn spark plugs or dirty spark plugs may operate well at idle
speeds, but often fail at higher RPMs. Bad spark plugs are often responsible
for the following conditions: |
| • | Poor engine performance |
| • | You sometimes can clean, regap, and reuse spark plugs with less
than the recommended amount of service. If there is any doubt about the serviceability
of a spark plug, replace the spark plug. |
|
Normal Spark Plug Operation
|
| • | Brown deposits or grayish-tan deposits are present on the area
of the spark plug that enters the cylinder. |
| • | A small amount of reddish brown, yellow, and white powdery residue
may also be present on the tip of the insulator around the center electrode. |
| • | These deposits are a normal combustion by-product of fuels and
of lubricating oils which contain additives. |
|
Preignition Misfiring
| A misfire occurs if the spark plug tip overheats and ignites
the mixture before the spark arcs across the gap.
|
Flashover
| Flashover
occurs when a damaged spark plug boot permits high voltage to short over the
insulator to the spark plug shell or to the engine. Dirt and moisture may
contribute to spark plug boot damage and to the high voltage charge loss.
|
Carbon Fouling
|
| • | Dry carbon deposits on the portion of the spark plug on the inside
of the cylinder indicate carbon fouling. |
| • | Excessive idling and driving at slower speeds under light engine
loads can keep the spark plug temperatures so low that deposits are not burned
off. |
| • | Rich fuels or poor ignition system output may cause carbon fouling. |
| • | A spark plug of the incorrect heat range may cause carbon fouling. |
| • | Carbon deposits on the tip of the spark plug insulator can conduct
the spark. This may cause the high voltage arc to track along the tip to
some point where the high voltage arcs to the spark plug shell. This arc ignites
the air fuel mixture late, which, in effect, retards the ignition timing. |
| • | Heavy carbon deposits can allow conductivity to the point that
the arc path now becomes a shunt path to the spark plug shell. This prevents
the spark from igniting the air fuel mixture. |
| • | Carbon deposits can result in a loss of power and of fuel economy. |
| • | Arc tracking may permanently damage the spark plug. The spark
plug requires replacement. |
|
Oil Fouling
| Wet oily deposits appear on the portion of the spark plug
on the inside of the cylinder. This may be caused by the following conditions:
| • | Oil that gets past worn piston rings |
| • | A new engine or a recently overhauled engine that is not broken
in |
|
Deposit Fouling
|
| • | Deposit fouling occurs when the normal reddish brown, yellow,
or white deposits of combustion by-products become sufficient enough to cause
misfiring. |
| • | In some cases, these deposits melt and form a glaze on the insulator
around the center electrode. |
| • | If the fouling is found only in one or two of the cylinders, excess
oil may be entering the cylinder. Inspect for excessive valve stem clearances
or for leaking intake valve seals. This is indicated by heavier deposits on
the intake valve side of the spark plug. |
|
Excessive Gap
|
| • | The air space between the center and the side electrodes at the
bottom of the spark plug is too wide for consistent firing. This may be from
incorrect gap adjustment or by excessive wear of the electrodes during use. |
| • | Excessive electrode wear on low mileage spark plugs may indicate
vehicle operation at speeds higher than the speeds for which the engine was
designed. |
| • | Excessive electrode wear on low mileage spark plugs may indicate
that the spark plug's heat range is too high. |
| • | Wear to the electrode may be the result of combustion gas that
leaks past the spark plug threads. |
| • | Excessively lean fuel may also cause electrode wear. |
| • | Excessive electrode wear can prevent the arcing of the high voltage
across the electrodes. |
| • | Fuel remains unburned, and a power loss results. |
| • | A gap that is too small may cause idling instability. |
|
Improper Torque or Seating
|
| • | Incorrect torque or seating can increase the temperature of the
operation of a spark plug. The heat can wear the gap. |
| • | In extreme cases, an overtightened spark plug, or an undertightened
spark plug can cause exhaust blowby. The cylinder head seats must make good
contact for sufficient heat transfer and spark plug cooling. |
| • | Dirty threads or damaged threads in the head or on the spark plug
can keep the spark plug from seating correctly. Incorrect seating may occur
even though the correct torque is applied. |
| • | Correct seating of the spark plugs is necessary prior to correct
tightening of the spark plugs. |
|
Cracked Insulators or Broken Insulators
|
| • | Usually, cracked insulators or broken insulators are the result
of incorrect installation or of heat shock. |
| • | Heat shock is a rapid increase in the temperature at the tip of
the insulator that causes the insulator material to crack. |
| • | A poorly fitting tool may break the upper insulators. |
| • | A shock or a hit from the outside may break the upper insulators. |
| • | Cracks in the upper insulator may be on the inside of the shell
or invisible. |
| • | The cracks may not cause problems until oil, or water penetrates
the crack at a later time. |
| • | A crack in the insulator causes the high voltage to short to ground.
The spark does not jump the electrode gap, and the air fuel mixture is not
ignited. |
| • | Fuel is unburned, and power is lost. |
| • | Damage may result from the tool for setting the gap pushes against
the center electrode or against the insulator. The tool may cause the insulator
to crack. |
| • | The tool should bend only the side electrode in order to set the
gap of a spark plug. |
|
Heat Shock Breakage
|
| • | Usually, heat shock breakage occurs during severe engine operating
conditions, at high RPMs, or with heavy loads. |
| • | Overadvanced timing may cause heat shock breakage. |
| • | Low grade fuels may cause heat shock breakage. |
| • | Spark plugs with broken insulators or with cracked insulators
require replacement. |
|
Bridged Electrodes
|
| • | Deposits between the electrodes ground the high voltage necessary
to fire the spark plug. |
| • | The arc between the electrodes does not occur, and the air fuel
mixture is not ignited. |
| • | Fuel remains unburned, and a power loss results. |
|
Damaged Secondary Wiring
|
| • | Damage to secondary wiring causes a similar condition to a cracked
insulator. |
| • | The high voltage arcs through the wire or through the boot. The
high voltage grounds on the spark plug shell or on the engine. |
|
Corona Discharge
|
| • | Corona discharge is a steady blue light that appears around the
insulator just above the shell crimp. |
| • | Corona discharge is the visible evidence of a high tension field
and does not affect ignition system performance. |
| • | The discharge may repel dust particles and leave a clear ring
on the insulator just above the shell. |
| • | Sometimes, this ring is mistaken for evidence of combustion gas
that blows between the shell and the insulator. |
| • | Corona discharge must not be mistaken for flashover or for a shorted
insulator. |
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