Checks
| Causes
|
| • | Misfiring is a general term that applies to a poor running engine. |
| • | With misfiring, the ignition spark is not igniting the fuel/air
mixture at the proper time. While other possible causes must be investigated,
the spark plugs should be inspected first. |
| • | 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 or dirty spark plugs may operate well at idle speeds, but
frequently fail at higher RPMs. Bad spark plugs are often responsible for
the following conditions: |
| - | Poor engine performance |
| • | Spark plugs with less than the recommended amount of service can
sometimes be cleaned and regapped, then returned to service. If there is any
doubt about the serviceability of a spark plug, replace it. |
|
Normal Spark Plug Operation
|
| • | Results in brown to grayish tan deposits 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 insulator tip around the center electrode. |
| • | These deposits are normal combustion by-product of fuels and
lubricating oils which contain additives. |
|
Pre-ignition Misfiring
| A pre-ignition
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
|
| • | Indicated by dry carbon deposits on the portion of the spark plug
inside of the cylinder. |
| • | Excessive idling and driving at slower speeds under light engine
loads can keep the spark plug temperatures so low that these 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 spark plug insulator tip may become conductive
and cause the high voltage arc to track along the tip to some point where
the high voltage arcs to join the spark plug shell. This arc ignites the
air fuel mixture later than normal which, in effect, retards the ignition
timing. |
| • | Heavy carbon deposits may be conductive to the extent that the
arc path now becomes a shunt path to the spark plug shell, which prevents
the spark from igniting the air/fuel mixture. |
| • | A power and fuel loss results. |
| • | The spark plug may be permanently damaged by arc tracking and
must be replaced. |
|
Oil Fouling
| Oil fouling appears as wet oily deposits on
the portion of the spark plug inside of the cylinder. This may be caused
by the following conditions:
| • | Oil getting past worn piston rings |
| • | Breaking in a new or recently overhauled engine |
|
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 shiny glaze on
the insulator around the center electrode. |
| • | If the fouling is found only in one or two of the cylinders,
valve stem clearances or the intake valve seals may be allowing excess lubricating
oil to enter the cylinder, particularly if the deposits are heavier on
the intake valve side of the spark plug. |
|
Excessive Gap
|
| • | The air space between the center and side electrodes at the bottom
of the spark plug is too wide for consistent firing. This may be due to improper
gap adjustment or to 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. |
| • | Electrode wear may also be the result of combustion gasses leaking
past the spark plug threads. |
| • | Excessively lean fuel may also cause electrode wear. |
| • | Excessive electrode wear can prevent the high voltage from arcing
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 cause a spark plug to run hot,
eventually leading to excessive gap wear. |
| • | In extreme cases, an overtightened or undertightened spark plug
can cause exhaust blowby. The cylinder head seats must make good contact
for sufficient heat transfer and spark plug cooling. |
| • | Dirty or damaged threads in the head or on the spark plug can
keep the spark plug from seating even though the proper torque is applied. |
| • | Once the spark plugs are properly seated, tighten the spark plugs
properly. |
|
Cracked 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 insulator tip temperature
which causes the insulator material to crack. |
| • | The upper insulators can be broken when a poorly-fitting tool
is used during servicing, or when the spark plug is hit from the outside. |
| • | Cracks in the upper insulator may be inside the shell or invisible. |
| • | The breakage may not cause problems until oil or water penetrates
the crack later. |
| • | A crack in the insulator causes the high voltage to ground out.
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 during gapping when the tool is pushed against
the center electrode or the surrounding insulator, causing the insulator to
crack. |
| • | Bend only the side electrode when gapping a spark plug. |
|
Heat Shock Breakage
|
| • | Usually, heat shock breakage occurs during severe engine operating
conditions such as higher RPMs or heavy loading. |
| • | 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
|
| • | Fouling deposits between the electrodes ground out the high voltage
necessary to fire the spark plug. |
| • | The arc between electrode 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 boot and grounds on
the spark plug shell or the engine. |
|
Corona Discharge
|
| • | Corona discharge is a steady blue light appearing around the insulator
just above the shell crimp. |
| • | Corona discharge is the visible evidence of a high tension field
and does not affect on ignition system performance. |
| • | The discharge may repel dust particles and leave a clear ring
on the insulator just above the shell. |
| • | This ring is sometimes mistaken for evidence that combustion gasses
have blown between the shell and the insulator. |
| • | Do not mistake corona discharge for flashover or for a shorted
insulator. |
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