DODGE NEON 1999 Service Repair Manual

The major difference between the two engines is
component location which affects the ignition system
service procedures. There are various sensors that
are in different locations due to a different cylinder
head and intake manifold.
The 2.0L engines use a fixed ignition timing sys-
tem. The distributorless electronic ignition system is
referred to as the Direct Ignition System (DIS).
Basic ignition timing is not adjustable.The
Powertrain Control Module (PCM) determines spark
advance. The system's three main components are
the coil pack, crankshaft position sensor, and cam-
shaft position sensor.
POWERTRAIN CONTROL MODULE
The Powertrain Control Module (PCM) controls the
ignition system (Fig. 1). The PCM supplies battery
voltage to the ignition coil through the Auto Shut-
down (ASD) Relay. The PCM also controls the ground
circuit for the ignition coil. By switching the ground
path for the coil on and off, the PCM adjusts ignition
timing to meet changing engine operating conditions.
During the crank-start period the PCM maintains
spark advance at 9É BTDC. During engine operation
the following inputs determine the amount of spark
advance provided by the PCM.
²Intake air temperature
²Coolant temperature
²Engine RPM
²Intake manifold vacuum
²Knock sensor
The PCM also regulates the fuel injection system.
Refer to the Fuel Injection sections of Group 14.
SPARK PLUGS
The 2.0L engines uses resistor spark plugs. For
spark plug identification and specifications, Refer to
the Specifications section at the end of this group.Remove the spark plugs and examine them for
burned electrodes and fouled, cracked or broken por-
celain insulators. Keep plugs arranged in the order
in which they were removed from the engine. An iso-
lated plug displaying an abnormal condition indicates
that a problem exists in the corresponding cylinder.
Replace spark plugs at the intervals recommended in
Group 0.
Spark plugs that have low mileage may be cleaned
and reused if not otherwise defective. Refer to the
Spark Plug Condition section of this group. After
cleaning, file the center electrode flat with a small
point file or jewelers file. Adjust the gap between the
electrodes (Fig. 2) to the dimensions specified in the
chart at the end of this section.
Always tighten spark plugs to the specified torque.
Over tightening can cause distortion and damage.
Tighten spark plugs to 28 N´m (20 ft. lbs.) torque.
SPARK PLUG CABLES
Spark plug cables are sometimes referred to as sec-
ondary ignition wires. The wires transfer electrical
current from the coil pack to individual spark plugs
at each cylinder. The resistor type, nonmetallic spark
plug cables provide suppression of radio frequency
emissions from the ignition system.
Check the spark plug cable connections for good
contact at the coil and spark plugs. Terminals should
be fully seated. The nipples and spark plug covers
should be in good condition. Nipples should fit tightly
on the coil. Spark plug boot should completely cover
the spark plug hole in the cylinder head cover. Install
the boot until the terminal snaps over the spark
plug. A snap must be felt to ensure the spark plug
cable terminal engaged the spark plug.
Loose cable connections will corrode, increase resis-
tance and permit water to enter the coil towers.
These conditions can cause ignition malfunction.
Fig. 1 Powertrain Control Module
Fig. 2 Setting Spark Plug Electrode Gap
8D - 2 IGNITION SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

Plastic clips in various locations protect the cables
from damage. When the cables are replaced the clips
must be used to prevent damage to the cables. The
#1 cable must be routed under the PCV hose and
clipped to the #2 cable.
ELECTRONIC IGNITION COILS
WARNING: THE DIRECT IGNITION SYSTEM GEN-
ERATES APPROXIMATELY 40,000 VOLTS. PER-
SONAL INJURY COULD RESULT FROM CONTACT
WITH THIS SYSTEM.
The coil pack consists of 2 coils molded together.
The coil pack is mounted on the valve cover (Fig. 3)
or (Fig. 4). High tension leads route to each cylinder
from the coil. The coil fires two spark plugs every
power stroke. One plug is the cylinder under com-
pression, the other cylinder fires on the exhaust
stroke. Coil number one fires cylinders 1 and 4. Coil
number two fires cylinders 2 and 3. The PCM deter-
mines which of the coils to charge and fire at the cor-
rect time.
The Auto Shutdown (ASD) relay provides battery
voltage to the ignition coil. The PCM provides a
ground contact (circuit) for energizing the coil. When
the PCM breaks the contact, the energy in the coil
primary transfers to the secondary causing the
spark. The PCM will de-energize the ASD relay if it
does not receive the crankshaft position sensor and
camshaft position sensor inputs. Refer to Auto Shut-
down (ASD) RelayÐPCM Output, in this section for
relay operation.
AUTOMATIC SHUTDOWN RELAY
The Automatic Shutdown (ASD) relay supplies bat-
tery voltage to the fuel injectors, electronic ignition
coil and the heating elements in the oxygen sensors.
A buss bar in the Power Distribution Center (PDC)
supplies voltage to the solenoid side and contact sideof the relay. The ASD relay power circuit contains a
20 amp fuse between the buss bar in the PDC and
the relay. The fuse also protects the power circuit for
the fuel pump relay and pump. The fuse is located in
the PDC. Refer to Group 8W, Wiring Diagrams for
circuit information.
The PCM controls the ASD relay by switching the
ground path for the solenoid side of the relay on and
off. The PCM turns the ground path off when the
ignition switch is in the Off position. When the igni-
tion switch is in On or Start, the PCM monitors the
crankshaft and camshaft position sensor signals to
determine engine speed and ignition timing (coil
dwell). If the PCM does not receive crankshaft and
camshaft position sensor signals when the ignition
switch is in the Run position, it will de-energize the
ASD relay.
The ASD relay is located in the PDC (Fig. 5). The
inside top of the PDC cover has label showing relay
and fuse identification.
Fig. 3 Ignition Coil PackÐSOHC
Fig. 4 Ignition Coil PackÐDOHC
Fig. 5 Power Distribution Center (PDC)
PLIGNITION SYSTEM 8D - 3
DESCRIPTION AND OPERATION (Continued)

CRANKSHAFT POSITION SENSOR
The PCM determines what cylinder to fire from the
crankshaft position sensor input and the camshaft
position sensor input. The second crankshaft counter-
weight has machined into it two sets of four timing
reference notches including a 60 degree signature
notch (Fig. 6). From the crankshaft position sensor
input the PCM determines engine speed and crank-
shaft angle (position).
The notches generate pulses from high to low in
the crankshaft position sensor output voltage. When
a metal portion of the counterweight aligns with the
crankshaft position sensor, the sensor output voltage
goes low (less than 0.5 volts). When a notch aligns
with the sensor, voltage goes high (5.0 volts). As a
group of notches pass under the sensor, the output
voltage switches from low (metal) to high (notch)
then back to low.
If available, an oscilloscope can display the square
wave patterns of each voltage pulse. From the fre-
quency of the output voltage pulses, the PCM calcu-
lates engine speed. The width of the pulses represent
the amount of time the output voltage stays high
before switching back to low. The period of time the
sensor output voltage stays high before switching
back to low is referred to as pulse-width. The faster
the engine is operating, the smaller the pulse-width
on the oscilloscope.
By counting the pulses and referencing the pulse
from the 60 degree signature notch, the PCM calcu-
lates crankshaft angle (position). In each group of
timing reference notches, the first notch represents
69 degrees before top dead center (BTDC). The sec-
ond notch represents 49 degrees BTDC. The third
notch represents 29 degrees. The last notch in eachset represents 9 degrees before top dead center
BTDC.
The timing reference notches are machined at 20É
increments. From the voltage pulse-width the PCM
tells the difference between the timing reference
notches and the 60 degree signature notch. The 60
degree signature notch produces a longer pulse-width
than the smaller timing reference notches. If the
camshaft position sensor input switches from high to
low when the 60 degree signature notch passes under
the crankshaft position sensor, the PCM knows cylin-
der number one is the next cylinder at TDC.
The crankshaft position sensor mounts to the
engine block behind the generator, just above the oil
filter (Fig. 7).
CAMSHAFT POSITION SENSOR
The PCM determines fuel injection synchronization
and cylinder identification from inputs provided by
Fig. 6 Timing Reference Notches
Fig. 7 Crankshaft Position Sensor
8D - 4 IGNITION SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

the camshaft position sensor (Fig. 8) or (Fig. 9) and
crankshaft position sensor. From the two inputs, the
PCM determines crankshaft position.
The camshaft position sensor attaches to the rear
of the cylinder head (Fig. 10). A target magnet
attaches to the rear of the camshaft and indexes to
the correct position. The target magnet has four dif-
ferent poles arranged in an asymmetrical pattern. As
the target magnet rotates, the camshaft position sen-
sor senses the change in polarity (Fig. 11). The sen-
sor input switches from high (5 volts) to low (0.30
volts) as the target magnet rotates. When the north
pole of the target magnet passes under the sensor,
the output switches high. The sensor output switches
low when the south pole of the target magnet passes
underneath.
The camshaft position sensor is mounted to the
rear of the cylinder head. The sensor also acts as a
thrust plate to control camshaft endplay on SOHC
engines.
COMBINATION ENGINE COOLANT TEMPERATURE
SENSOR
The coolant temperature sensor provides an input
voltage to the PCM and a separate input voltage to
the temperature gauge on the instrument panel. The
PCM determines engine coolant temperature from
the coolant temperature sensor. As coolant tempera-
ture varies, the coolant temperature sensor resis-
tance changes resulting in a different input voltage
to the PCM.
When the engine is cold, the PCM will demand
slightly richer air-fuel mixtures and higher idle
speeds until normal operating temperatures are
reached.
Fig. 8 Camshaft Position SensorÐSOHC
Fig. 9 Camshaft Position SensorÐDOHC
Fig. 10 Target Magnet ÐTypical
Fig. 11 Target Magnet Polarity
PLIGNITION SYSTEM 8D - 5
DESCRIPTION AND OPERATION (Continued)

SOHC
The coolant sensor threads into the end of the cyl-
inder head, next to the camshaft position sensor (Fig.
12). New sensors have sealant applied to the threads.
DOHC
The coolant sensor threads into the intake mani-
fold next to the thermostat housing (Fig. 13). New
sensors have sealant applied to the threads.
INTAKE AIR TEMPERATURE SENSOR
The intake air temperature sensor measures the
temperature of the air as it enters the engine. The
sensor supplies one of the inputs the PCM uses to
determine injector pulse-width.
The MAP/Intake Air Temperature (IAT) sensor,
located on the intake manifold, combines the MAP
and Intake Air Temperature (IAT) functions into one
sensor (Fig. 14) or (Fig. 15).
KNOCK SENSOR
The knock sensor threads into the side of the cyl-
inder block in front of the starter motor. When the
knock sensor detects a knock in one of the cylinders,
it sends an input signal to the PCM. In response, the
PCM retards ignition timing for all cylinders by a
scheduled amount.
Knock sensors contain a piezoelectric material
which constantly vibrates and sends an input voltage
(signal) to the PCM while the engine operates. As the
intensity of the crystal's vibration increase, the knock
sensor output voltage also increases.
NOTE: Over or under tightening effects knock sen-
sor performance, possibly causing improper spark
control.
MANIFOLD ABSOLUTE PRESSURE SENSOR (MAP)
The PCM supplies 5 volts to the MAP sensor. The
MAP sensor function converts intake manifold pres-
sure into voltage. The PCM monitors the MAP sensor
output voltage. As vacuum increases, MAP sensor
voltage decreases proportionately. Also, as vacuum
decreases, MAP sensor voltage increases proportion-
ately.
Key on, before the engine starts running, the PCM
determines atmospheric air pressure from the MAP
sensor voltage. While the engine operates, the PCM
determines intake manifold pressure from the MAP
sensor voltage. Based on MAP sensor voltage and
inputs from other sensors, the PCM adjusts spark
advance and the air/fuel mixture.
The MAP/IAT sensor mounts to the intake mani-
fold (Fig. 14) or (Fig. 15).
THROTTLE POSITION SENSOR (TPS)
The TPS mounts to the side of the throttle body.
The TPS connects to the throttle blade shaft. The
TPS is a variable resistor that provides the Power-
Fig. 12 Engine Coolant Temperature SensorÐSOHC
Fig. 13 Engine Coolant Temperature SensorÐDOHC
Fig. 14 MAP/IAT sensorÐSOHC
8D - 6 IGNITION SYSTEMPL
DESCRIPTION AND OPERATION (Continued)

train Control Module (PCM) with an input signal
(voltage). The signal represents throttle blade posi-
tion. As the position of the throttle blade changes,
the resistance of the TPS changes.
The PCM supplies approximately 5 volts to the
TPS. The TPS output voltage (input signal to the
powertrain control module) represents throttle blade
position. The TPS output voltage to the PCM varies
from approximately 0.38 volts to 1.2 volts at mini-
mum throttle opening (idle) to a maximum of 3.1
volts to 4.4 volts at wide open throttle.
Along with inputs from other sensors, the PCM
uses the TPS input to determine current engine oper-
ating conditions. The PCM also adjusts fuel injector
pulse width and ignition timing based on these
inputs.
IGNITION SWITCH
In the RUN position, the ignition switch connects
power from the Power Distribution Center (PDC) to a
30 amp fuse in the fuse block, back to a bus bar in
the PDC. The bus bar feeds circuits for the Power-
train Control Module (PCM), duty cycle purge sole-
noid, EGR solenoid, and ABS system. The bus bar in
the PDC feeds the coil side of the radiator fan relay,
A/C compressor clutch relay, and the fuel pump relay.
It also feeds the Airbag Control Module (ACM)
LOCK KEY CYLINDER
The lock cylinder is inserted in the end of the
housing opposite the ignition switch. The ignition key
rotates the cylinder to 5 different detents (Fig. 16):
²Accessory
²Off (lock)
²Unlock
²On/Run
²Start
IGNITION INTERLOCK
All vehicles equipped with automatic transaxles
have an interlock system. The system prevents shift-
ing the vehicle out of Park unless the ignition lock
cylinder is in the Off, Run or Start position. In addi-
tion, the operator cannot rotate the key to the lock
position unless the shifter is in the park position. On
vehicles equipped with floor shift refer to Group 21 -
Transaxle for Automatic Transmission Shifter/Igni-
tion Interlock.
DIAGNOSIS AND TESTING
TESTING FOR SPARK AT COILÐ2.0/2.4L
WARNING: THE DIRECT IGNITION SYSTEMS GEN-
ERATES APPROXIMATELY 40,000 VOLTS. PER-
SONAL INJURY COULD RESULT FROM CONTACT
WITH THIS SYSTEM.
The coil pack contains independent coils. Each coil
must be checked individually.
CAUTION: Spark plug wire damage may occur if
the spark plug is moved more than 1/4 inch away
from the engine ground.
CAUTION: Do not leave any one spark plug cable
disconnected any longer than 30 seconds or possi-
ble heat damage to catalytic converter will occur.
CAUTION: Test must be performed at idle and in
park only with the parking brake on.
NOTE: New isolated engine valve cover may not
provide adequate ground. Use engine block as
engine ground.
Fig. 15 MAP/IAT sensorÐDOHCFig. 16 Ignition Lock Cylinder Detents
PLIGNITION SYSTEM 8D - 7
DESCRIPTION AND OPERATION (Continued)

Use a new spark plug and spark plug cable
for the following test.
(1) Insert a new spark plug into the new spark
plug boot. Ground the plug to the engine (Fig. 17).
Do not hold with your hand.
(2) Starting with coil insulator #1, remove it from
the DIS coil.
(3) Plug the test spark plug cable onto #1 coil
tower. Make sure a good connection is made; there
should be a click sound.
(4) Crank the engine and look for spark across the
electrodes of the spark plug.
CAUTION: Always install the cable back on the coil
tower after testing to avoid damage to the coil and
catalytic converter.
(5) Repeat the above test for the remaining coils. If
there is no spark during all cylinder tests, proceed to
the Failure To Start Test.
(6) If one or more tests indicate irregular, weak, or
no spark, proceed to Check Coil Test.
CHECK COIL TEST
NOTE: Coil one fires cylinders 1 and 4, coil two
fires cylinders 2 and 3. Each coil tower is labeled
with the number of the corresponding cylinder.
(1) Remove the ignition cables and measure the
resistance of the cables. Resistance must be between
ranges shown in chart in specifcation section in this
group. Replace any cable not within tolerance.
(2) Disconnect the electrical connector from the
coil pack.
(3) Measure the primary resistance of each coil. At
the coil, connect an ohmmeter between the B+ pin
and the pin corresponding to the cylinders in ques-
tion (Fig. 18). Resistance on the primary side of eachcoil should be 0.45 - 0.65 ohm. Replace the coil if
resistance is not within tolerance.
(4) Remove ignition cables from the secondary tow-
ers of the coil. Measure the secondary resistance of
the coil between the towers of each individual coil
(Fig. 19). Secondary resistance should be 11,000 to
14,000 ohms. Replace the coil if resistance is not
within tolerance.
FAILURE TO START TESTÐ2.0/2.4L
This no-start test checks the camshaft position sen-
sor and crankshaft position sensor.
Use the DRB scan tool to test the camshaft posi-
tion sensor and the sensor circuits. Refer to the
appropriate Powertrain Diagnostics Procedure Man-
ual. Refer to the wiring diagrams section for circuit
information.
The Powertrain Control Module (PCM) supplies 8
volts to the camshaft position sensor and crankshaft
position sensor through one circuit. If the 8 volt sup-
Fig. 17 Testing For Spark
Fig. 18 Terminal Identification
Fig. 19 Checking Ignition Coil Secondary
Resistance
8D - 8 IGNITION SYSTEMPL
DIAGNOSIS AND TESTING (Continued)

ply circuit shorts to ground, neither sensor will pro-
duce a signal (output voltage to the PCM).
When the ignition key is turned and left in the On
position, the PCM automatically energizes the Auto
Shutdown (ASD) relay. However, the controller de-en-
ergizes the relay within one second because it has
not received a camshaft position sensor signal indi-
cating engine rotation.
During cranking, the ASD relay will not energize
until the PCM receives a camshaft position sensor
signal. Secondly, the ASD relay remains energized
only if the controller senses a crankshaft position
sensor signal immediately after detecting the cam-
shaft position sensor signal.
(1) Check battery voltage. Voltage should approxi-
mately 12.66 volts or higher to perform failure to
start test.
(2) Disconnect the harness connector from the coil
pack (Fig. 20).
(3) Connect a test light to the B+ (battery voltage)
terminal of the coil electrical connector and ground.
The B+ wire for the DIS coil is the center terminal.
Do not spread the terminal with the test light
probe.
(4) Turn the ignition key to theON position.The
test light should flash On and then Off.Do not turn
the Key to off position, leave it in the On posi-
tion.
(a) If the test light flashes momentarily, the
PCM grounded the ASD relay. Proceed to step 5.
(b) If the test light did not flash, the ASD relay
did not energize. The cause is either the relay or
one of the relay circuits. Use the DRB scan tool to
test the ASD relay and circuits. Refer to the appro-
priate Powertrain Diagnostics Procedure Manual.
Refer to the wiring diagrams section for circuit
information.
(5) Crank the engine. (If the key was placed in the
off position after step 4, place the key in the On posi-tion before cranking. Wait for the test light to flash
once, then crank the engine.)
(6) If the test light momentarily flashes during
cranking, the PCM is not receiving a crankshaft posi-
tion sensor signal.
(7) If the test light did not flash during cranking,
unplug the crankshaft position sensor connector.
Turn the ignition key to the off position. Turn the
key to the On position, wait for the test light to
momentarily flash once, then crank the engine. If the
test light momentarily flashes, the crankshaft posi-
tion sensor is shorted and must be replaced. If the
light did not flash, the cause of the no-start is in
either the crankshaft position sensor/camshaft posi-
tion sensor 8 volt supply circuit, or the camshaft
position sensor output or ground circuits.
IGNITION TIMING PROCEDURE
The engines for this vehicle, use a fixed ignition
system. The PCM regulates ignition timing. Basic
ignition timing is not adjustable.
CAMSHAFT POSITION SENSOR AND CRANKSHAFT
POSITION SENSOR
The output voltage of a properly operating cam-
shaft position sensor or crankshaft position sensor
switches from high (5.0 volts) to low (0.3 volts). By
connecting an Moper Diagonostic System (MDS) and
engine analyzer to the vehicle, technicians can view
the square wave pattern.
ENGINE COOLANT TEMPERATURE SENSOR
Refer to Group 14, Fuel System for Diagnosis and
Testing.
INTAKE AIR TEMPERATURE SENSOR
Refer to Group 14, Fuel System, for Diagnosis and
Testing.
MANIFOLD ABSOLUTE PRESSURE (MAP) SENSOR
TEST
Refer to Group 14, Fuel System for Diagnosis and
Testing.
THROTTLE POSITION SENSOR
To perform a complete test of the this sensor and
its circuitry, refer to the DRB scan tool and appropri-
ate Powertrain Diagnostics Procedures manual. To
test the throttle position sensor only, refer to the fol-
lowing:
The Throttle Position Sensor (TPS) can be tested
with a digital voltmeter (DVM). The center terminal
of the sensor is the output terminal. One of the other
terminals is a 5 volt supply and the remaining ter-
minal is ground.
Fig. 20 Ignition Coil Engine Harness Connector
PLIGNITION SYSTEM 8D - 9
DIAGNOSIS AND TESTING (Continued)

Connect the DVM between the center and sensor
ground terminal. Refer to Group 8W - Wiring Dia-
grams for correct pinout.
With the ignition switch in the ON position, check
the output voltage at the center terminal wire of the
connector. Check the output voltage at idle and at
Wide-Open-Throttle (WOT). At idle, TPS output volt-
age should be approximately 0.38 volts to 1.2 volts.
At wide open throttle, TPS output voltage should be
approximately 3.1 volts to 4.4 volts. The output volt-
age should gradually increase as the throttle plate
moves slowly from idle to WOT.
Check for spread terminals at the sensor and PCM
connections before replacing the TPS.
SPARK PLUG CONDITION
NORMAL OPERATING CONDITIONS
The few deposits present will be probably light tan
or slightly gray in color with most grades of commer-
cial gasoline (Fig. 21). There will not be evidence of
electrode burning. Gap growth will not average more
than approximately 0.025 mm (.001 in) per 1600 km
(1000 miles) of operation for non platinum spark
plugs. Non-platnium spark plugs that have normal
wear can usually be cleaned, have the electrodes filed
and regapped, and then reinstalled.
CAUTION: Never attempt to file the electrodes or
use a wire brush for cleaning platinum spark plugs.
This would damage the platinum pads which would
shorten spark plug life.Some fuel refiners in several areas of the United
States have introduced a manganese additive (MMT)
for unleaded fuel. During combustion, fuel with MMT
may coat the entire tip of the spark plug with a rust
colored deposit. The rust color deposits can be misdi-
agnosed as being caused by coolant in the combustion
chamber. Spark plug performance is not affected by
MMT deposits.
COLD FOULING (CARBON FOULING)
Cold fouling is sometimes referred to as carbon
fouling because the deposits that cause cold fouling
are basically carbon (Fig. 21). A dry, black deposit on
one or two plugs in a set may be caused by sticking
valves or misfire conditions. Cold (carbon) fouling of
the entire set may be caused by a clogged air cleaner.
Cold fouling is normal after short operating peri-
ods. The spark plugs do not reach a high enough
operating temperature during short operating peri-
ods.Replace carbon fouled plugs with new
spark plugs.
FUEL FOULING
A spark plug that is coated with excessive wet fuel
is called fuel fouled. This condition is normally
observed during hard start periods.Clean fuel
fouled spark plugs with compressed air and
reinstall them in the engine.
OIL FOULING
A spark plug that is coated with excessive wet oil
is oil fouled. In older engines, wet fouling can be
caused by worn rings or excessive cylinder wear.
Break-in fouling of new engines may occur before
normal oil control is achieved.Replace oil fouled
spark plugs with new ones.
OIL OR ASH ENCRUSTED
If one or more plugs are oil or ash encrusted, eval-
uate the engine for the cause of oil entering the com-
bustion chambers (Fig. 22). Sometimes fuel additives
can cause ash encrustation on an entire set of spark
plugs.Ash encrusted spark plugs can be cleaned
and reused.
HIGH SPEED MISS
When replacing spark plugs because of a high
speed miss condition;wide open throttle opera-
tion should be avoided for approximately 80 km
(50 miles) after installation of new plugs.This
will allow deposit shifting in the combustion chamber
to take place gradually and avoid plug destroying
splash fouling shortly after the plug change.
Fig. 21 Normal Operation and Cold (Carbon) Fouling
8D - 10 IGNITION SYSTEMPL
DIAGNOSIS AND TESTING (Continued)

ELECTRODE GAP BRIDGING
Loose deposits in the combustion chamber can
cause electrode gap bridging. The deposits accumu-
late on the spark plugs during continuous stop-
and-go driving. When the engine is suddenly
subjected to a high torque load, the deposits partially
liquefy and bridge the gap between the electrodes
(Fig. 23). This short circuits the electrodes.Spark
plugs with electrode gap bridging can be
cleaned and reused.
SCAVENGER DEPOSITS
Fuel scavenger deposits may be either white or yel-
low (Fig. 24). They may appear to be harmful, but
are a normal condition caused by chemical additives
in certain fuels. These additives are designed tochange the chemical nature of deposits and decrease
spark plug misfire tendencies. Notice that accumula-
tion on the ground electrode and shell area may be
heavy but the deposits are easily removed.Spark
plugs with scavenger deposits can be consid-
ered normal in condition, cleaned and reused.
CHIPPED ELECTRODE INSULATOR
A chipped electrode insulator usually results from
bending the center electrode while adjusting the
spark plug electrode gap. Under certain conditions,
severe detonation also can separate the insulator
from the center electrode (Fig. 25).Spark plugs
with chipped electrode insulators must be
replaced.
PREIGNITION DAMAGE
Excessive combustion chamber temperature can
cause preignition damage. First, the center electrode
dissolves and the ground electrode dissolves some-
what later (Fig. 26). Insulators appear relatively
deposit free. Determine if the spark plugs are the
correct type, as specified on the VECI label, or if
Fig. 22 Oil or Ash Encrusted
Fig. 23 Electrode Gap Bridging
Fig. 24 Scavenger Deposits
Fig. 25 Chipped Electrode Insulator
PLIGNITION SYSTEM 8D - 11
DIAGNOSIS AND TESTING (Continued)