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Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #1 ·
The first tool is diagnosis by the appearance of your spark plugs.
Down the page is animated vacuum gauges with explanations of what causes various readings and fluctuations.
Then it explains all of the ignition components and how they work. (Points & electronic)
Then it shows animated scope patterns with various issues and how to interpret what is going on.

Start a new thread if you have questions or comments.

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #2 ·
Spark plug visual and Vacuum gauge diagnosis.
The appearance of the electrodes and insulator of the spark plug - the " spark plug face " - provides information on the operating performance of the spark plug as well as on the mixture and combustion in the engine. The assessment of the face of the spark plug is, therefore an important part of engine diagnosis. By utilising the spark plug condition in conjunction with oscilloscope and emission analyser readings, diagnosis of particular engine faults are further enhanced and thus correct repairs can be carried out.

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NORMAL : The spark plug face shows
the insulator nose which has slight combustion
deposits. The deposits are not heavy enough
to cause any detrimental effect to engine
performance. The plug face colour can range
from brown to greyish white. There should be
minimal electrode erosion which indicates correct
heat range and thus the plug has been functioning
under good engine conditions.

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CARBON FOULED : The spark plug face shows
dry, soft, black sooty deposits on the insulator nose
and electrodes. An accumulation of this sooty carbon
causes the insulation between the center and ground
electrodes to deteriorate and an electrical leakage
path is formed by the carbon resulting in a misfire. An
overly rich fuel mixture, prolonged low speed idling or
driving, spark plug heat range too cold or an ignition
system fault are some causes for this plug condition.

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OVERHEATED : The spark plug face
shows the insulator nose which has a white
glazed or glossy appearance. Small black
deposits accumulate on the insulator nose and
the electrodes are prematurely worn. Lean air -
fuel mixture, over advanced ignition timing
or poor engine cooling efficiency are conditions
that cause an overheated spark plug appearance.

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MELTED : The spark plug face shows the
center and/or ground electrode surface is uneven
and cauliflower-like in appearance. The insulator
is blistered and possibly contains metallic deposits.
The melted firing end results from overheating.

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DEPOSITS : ( Ash fouling )
A build up of combustion deposits on the firing
end due to the burning of oil and or fuel additives
during normal combustion. In some circumstances
these deposits may reach temperatures which may
lead to pre-ignition. When the deposits become
heavier, ignition misfire may occur.

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OIL FOULED : The spark plug face shows
wet, black, oily carbon deposits on both the
insulator nose and electrodes. Oil entering
the combustion chamber due to mechanical
wear of piston rings, worn valve guides, or, a
build up of crankcase pressure can cause this
spark plug appearance.

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nose is cracked or split. This can be caused by
( other than mechanical impact ), thermal shock
( sudden heating or cooling ). Severe detonation
will often produce this type of breakage. A lean
air-fuel mixture, insufficient fuel octane rating or
over advanced ignition timing can can cause this
type of plug damage.

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LEAD FOULED : ( engines using leaded fuel )
Lead fouling usually appears as yellowish brown
deposits on the insulator nose. May cause misfire
during rapid acceleration or under heavy engine load
as the lead deposits become electrically conductive
at high temperatures which causes a reduction in
the insulation resistance between the center and
ground electrodes.

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NORMAL WEAR : The center and ground
electrodes are worn. The insulator nose and ground
electrode are covered with combustion deposits. The
plug has operated under normal conditions for a long
time and has reached the end of its life. Further
usage will cause ignition misfire, poor fuel economy,
and poor engine performance.

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #3 ·
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LEAD EROSION : ( engines using leaded fuel )
The ground electrode is badly worn and its appearance
is chipped. Lead additives in the fuel chemically react
with the nickel alloy electrodes at high temperatures
and separate the grain boundary of the nickel alloy.

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ABNORMAL EROSION : The center and/or
ground electrodes are badly worn relative to the
length of plug operation. Ignition misfire will occur
due to the high voltage required needed to bridge
the large gap. Harsh fuel and oil additives combined
with severe operating conditions such as high speed
driving cause abnormal erosion of the plug.

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and/or ground electrodes are very badly worn. The
surfaces are rough and oxidized in appearance.
Rapid abnormal erosion is produced by a more
aggressive operating environment than " abnormal
erosion ".

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #4 ·

note : The vacuum gauge readings are measured in inches of mercury at sea level. In all 4-cylinder engines, a slight fluctuation will always be noticeable. This can be overcome by pinching the gauge hose slightly to enable a steadier reading.

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NORMAL MOTOR: steady reading of 17-21 when motor idling.

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NORMAL MOTOR: when throttle is opened and closed rapidly needle falls to 2 and swings back to 24 or 25, falling back to normal idle reading. Indicates rings and valves ok

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POOR RINGS: motor idling, hand reading steadily, but 2-4 points lower than normal. This may also indicate poor or contaminated engine oil.

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POOR RINGS/POOR OIL: when throttle is opened and closed rapidly needle falls to 0 and rises to only 24 or less.

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STICKING VALVE: needle drops occasionally about 4 points at idle speed.

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BURNT VALVE: needle drops regularly by several points at idle speed.

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LEAKY VALVE: needle drops 2-4 points when valve should close. short circuiting individual spark plugs will indicate cylinder in which the valve is defective, when engine idling.

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WORN VALVE GUIDES: rapid fluctuation of needle between 17 and 21, when engine idling.

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #5 ·
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WEAK VALVE SPRINGS: engine running fast, the needle moves between 10 and 21.Further increase in engine speed increases needle variations.

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LATE VALVE TIMING: with engine running at a higher idle speed needle reads from 8 to 15 and remains steady.

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RETARDED IGNITION: needle gives steady reading between 17 and 19 with engine running at a higher idle speed.

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DEFECTIVE IGNITION: if needle moves slowly between 14 and 16 at normal idle, check ignition components (plug gaps closed up/fouled contacts not synchronized).

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INTAKE SYSTEM LEAK: with engine running at idle or fast idle speeds needle reads between 3 and 5. Check induction manifold gaskets.

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HEAD GASKET LEAK: (leaking between cylinders): needle moves backwards and forwards regularly between 5 and 17.

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EXHAUST SYSTEM BLOCKAGE: high reading when engine first started, falling back to 1 or zero and gradually increasing to 15 or 17.

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INCORRECT CARBURETTOR ADJUSTMENT: needle moves slowly backwards and forwards between 18 and 21.Check carburetor idle mixture.

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #6 ·

OSCILLOSCOPES...the basics. (Skip intro if you like)
During the 1960's automotive workshops were introduced to the oscilloscope to help in the diagnosis of distributor breaker point ignition systems. With the introduction of electronic ignition systems the role of the oscilloscope to diagnose ignition system problems became more important. Distributorless ignition and on-board computer systems has caused a demand for a more sophisticated type of oscilliscope. The introduction of the lab scope function within the oscilloscope is now common place to cater for the increased demand by technicians to solve many problematic aspects found within the sophisticated electronic circuitry of the modern automobile.
However, although the increased use of on-board computer systems to control many assemblies of the automobile ( engine, brakes,transmission, and air-conditioning etc ) the modern vehicle still have "basic" problems that they have always had. Within the ignition system the spark plugs still wear, ignition leads still fail, distributor caps/rotors ( where fitted ) require service. Although on-board computers have very capable on-board-diagnostics there are areas where this type of diagnostics cannot address. Many a hour can be can be spent diagnosing the computer system when the problem was a faulty alternator diode or a partially failed ignition lead. The "basics" must be functioning correctly if the computer system is to function correctly.
In simple terms, an oscilloscope is a voltmeter that measures voltage change over a period of time, and this is displayed on a monitor as a wave form. Early oscilloscope design, which is still in widespread use today, uses "analog" oscilloscope technology. A cathode ray tube similar to that used on a television set consisting of two sets of deflector coils, an electron gun and a phosphorous coated screen is used in this type of oscilloscope. The vertical deflection coils move the electron path up and down on the screen in response to voltage input. The horizontal deflector coils create the "sweep" action, moving the electron gun left to right at a controlled speed, or in response to a "trigger" event. The phosphorous coating glows anywhere the electrons strike the surface and continues to glow for a short period of time after the electrons have hit it, which results in the continuous line or pattern being drawn on the screen.
Modern day oscilloscopes utilize digital storage scope technology. A digital storage oscilloscope takes samples of input signals at regular intervals and stores these in a computer's memory. The computer then "reconstructs" the wave form on a computer monitor. This type of system allows for more control over the display of wave forms including features like "record" and "playback". The record capability allows for the capture and playback of intermittent instantaneous anomalies which happen too fast for human eye detection. Once a capture has been effected, waveforms may be played back frame by frame to look for intermittent problems.

The function of the ignition primary circuit is to deliver current to the coil primary winding, allow collapsing of the coil primary and dissipation of all the excess voltage. A failure within the primary circuit can be the cause of a failure in the ignition secondary. Always perform a thorough check on the ignition primary before condemning any secondary components. Without a correct voltage source and proper switching, the secondary will not function correctly.
A typical ignition primary circuit consist of the following components : battery, ignition switch, ballast resistor, loom resistor ( some models ), ignition coil, a sensor to switch the primary on and off ( breaker points, pickup coil, hall effect sensor ), a condenser ( points ignition ), a noise suppressor, and a module which houses a switching transistor ( electronic ignition ), that controls the primary circuit to ground.
Battery : The available battery voltage determines the amount of energy in the ignition primary circuit .An undercharged battery lessens the available voltage in the ignition primary which in turn reduces the secondary output.
Ignition Switch : Power for the entire ignition system is via the ignition switch. Excessive resistance in the switch could cause a low primary circuit voltage and this in turn affects the efficiency of the entire ignition system.
Primary Resistor ( ballast or loom resistor ) : These resistors reduce the current within the primary circuit to prevent breaker point arcing and in some systems to protect other components within the ignition system. Because battery voltage is reduced when the starter is cranking the resistors are usually bypassed when the starter is engaged.
Ignition Coil : The ignition coil acts as a transformer. Current flowing through the coil primary windings creates a magnetic field which surround the secondary windings. This occurs when the breaker points ( points ignition ) are "closed" and while the transistor ( electronic ignition ) is "on". When the primary circuit is interrupted ( points "open" or transistor switches "off" ), the magnetic field collapses and a high voltage is induced in the secondary windings. The high voltage is then directed to the correct spark plug by the distributor cap and rotor.
Breaker points : Within the distributor is a cam mounted on the distributor shaft. The mechanical points are opened and closed by the rotation of the cam. The time the points are closed is termed "dwell". At this time primary current flows and coil saturation begins .A longer "dwell" creates better coil saturation which results in a higher secondary voltage.
Condenser : Used in breaker point ignition systems to prevent arcing across the contact faces as the points open and to dampen the high voltage spike induced in the ignition primary when it collapses. If the contacts were allowed to arc, premature failure of the contacts would occur resulting in poor vehicle performance or a no start condition.
Electronic Ignition Primary Components
Due to increased demand for better fuel economy and stringent emission standards electronic ignition systems are used. By utilising a better control over higher voltage output for a longer duration these standards are being met. Many electronic ignition systems also adjust timing and vary the dwell according to engine speed and primary source voltage.
Components that make up a typical electronic ignition systems include : Ignition module or igniter, armature wheel/magnetic pickup and a hall effect switch.
Armature/Magnetic pickup : The armature is fitted to the distributor shaft instead of a cam. The armature has one trigger arm per cylinder. A magnetic sensor, located next to the armature wheel generates a magnetic field. When the point of the trigger arm passes by, a voltage is induced in the pickup which then signals the ignition module to interrupt the current flow to the coil.
Hall Effect Switch : The Hall Effect will produce a voltage when the sensor is in a magnetic field. This type of distributor has a fixed permanent magnet and a sensor with a narrow air gap between them. If the shutter ( sometimes a part of the distributor rotor ) is between the sensor and the magnet, the magnetic field is interrupted and zero voltage occurs, signaling the ignition module. As the shutter moves away from the air gap, voltage is again produced.
Ignition Module : This is a solid state device that switches the primary current on and off based on the signals from the magnetic pickup. In most applications, the module varies dwell with engine speed and controls ignition timing.
Electronic ignition systems have been developed greatly over the years to increase ignition system performance and reliability. Types of systems include : Breaker Triggered Inductive Ignition, Pulse Generator Ignition, Hall Generator Ignition, Capacitor Discharge ignition, Electronic Spark Timing Ignition, Thick Film Integrated Ignition and the latest innovation is the Distributorless Ignition System.
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Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #7 ·

The oscilloscope is an excellent tool to use to diagnose ignition primary patterns. The following graphical diagrams are representative of normal and abnormal primary scope patterns.
Note : The primary wave form shown is typical and does not represent all ignition systems.
Normal Primary Superimposed Pattern for Contact Breaker Ignition System

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A Condenser Oscillations : The dissipating energy from the collapsing magnetic field of the coil is discharged to the condenser. This energy is stored briefly in the condenser, then pushes it back to the coil. Each time the condenser gives up the stored energy, the energy weakens. This repeats until all of the energy is used up.
B Coil Oscillations : The energy in the primary winding is decaying due to the collapse of the magnetic field. These oscillations show the energy dissipating.
C Points Close : The contact breaker distributor points close, dwell begins and current flows through the coil. There should be a sharp, clean 90 degree drop in the wave form at this point. "Dwell" is the time the points remain closed.
D Points Open : At this point the contacts have opened and this causes the magnetic field in the primary to collapse, inducing a high voltage in the secondary winding. The current flow has stopped and spark begins. The upward spike is a representation of the inductive "kick" in the primary winding when the coil current is interrupted.

Normal Primary Parade Pattern for Contact Breaker Ignition System
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Oscilloscope set at 200 volt scale.
4 cylinder engine with firing order of 1 3 4 2 shown.

primary pattern showing arcing at points open section of pattern.
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Point arcing may be caused by :
.......defective condenser or bad condenser circuit.
......misaligned, pitted, or burnt points.
......too high a charging voltage.
......defective ballast resistor.
......worn breaker plate assembly.

primary pattern showing condenser oscillation fault
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Condenser series resistance appears as reduced oscillations in the condenser portion of the pattern.
This may be caused by :
.......defective condenser.
.......poor connection at condenser pig tail or ground clamp.
Condenser leakage, either internal or external, also appears as reduced oscillations in the condenser pattern.
An intermittently open condenser may appear as varying large to small oscillations in the condenser part of the primary ignition pattern.

primary pattern showing point misalignment
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point bounce is caused by :
.........weak point spring.
.........dry cam lobe or hardened deposits on cam lobes.
.........pitted or misaligned points.
.........loose points.

Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #8 ·

The Ignition Secondary Circuit is termed the high voltage circuit. Depending on the manufacturer of the ignition system, this circuit may be required to handle voltages as high as 35,000 volts. Components that make up the secondary circuit are : Secondary coil windings, distributor cap and rotor ( non - Distributorless Ignition System equipped vehicles ), spark plug leads and spark plugs.


When the ignition coil primary current is interrupted by points opening ( contact distributor ignition ) or module/transistor ( electronic ignition ) turns off, the magnetic field in the coil collapses and induces a very high voltage into the secondary coil windings. The available voltage is then distributed from the coil tower through the coil high tension lead to the center terminal of the distributor cap
( non DIS equipped vehicles ).The voltage then passes to the distributor rotor and is in turn directed in appropriate order to each outer terminal of the distributor cap as the distributor shaft rotates. Each outer terminal has a high tension lead and each lead is connected to a spark plug. The arranged order of the terminals and leads determines the firing sequence of the engine. The spark plugs provide a pre - determined gap within the combustion chamber so that each time a high voltage is delivered from the distributor, a spark will occur. If the correct air/fuel ratio is present in the cylinder at the instant the high voltage bridges the plug gap, ignition and combustion will take place.
Some Distributorless Ignition Systems use only plug leads to direct the high voltage to the spark plug, while others either mount a coil directly on the spark plug, or use printed circuits such as the GM Quad 4.


The oscilloscope is an excellent tool that provides a convenient way to observe the secondary ignition circuit operation. The secondary wave form supplies information about voltage changes in all phases of an ignition cycle as they occur. Discrepancies in the operation of the secondary ignition components ( high tension leads, spark plugs, distributor cap/rotor etc. ) are readily displayed as are mechanical and emission related faults. Recent oscilloscope designs have the ability to capture and store the screen wave forms for future reference when recalled from a saved data storage area. By comparing a known good secondary ignition wave form to a pattern with a possible fault, correct diagnosis of the component failure and diagnosis time is minimized. Recalling both patterns from the data base becomes another tool to use in the diagnosis and engine tuning areas of todays complex automobile.

The oscilloscope is an excellent tool to use to diagnose ignition secondary patterns. The following graphical diagrams are representative of normal and abnormal secondary scope patterns.
Note : The secondary wave form shown is typical and does not represent all ignition systems.

Normal Secondary Superimposed Pattern for Contact Breaker Ignition System
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Normal Secondary Superimposed Pattern for Electronic Ignition System.
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A The Firing Section is represented by the Spark Line and Firing Line. The Spark Line represents the time/voltage necessary to maintain the spark. The length of this line is the amount of time ( in milliseconds ) that the spark plug is firing. The height of the Spark Line is the voltage ( Burn Voltage ) that is required to maintain the spark after the initial Firing KV.
B The pattern section from B to C is termed the Intermediate Section.This reflects the dissipation of the remaining energy in the ignition coil after the plug stops firing. The amount of oscillations is an indication of coil winding and insulation condition. A shorted winding will show limited oscillations.
C Section C to D is termed the Dwell Section. C represents the point "close" ( contact breaker ignition ) and module "on"
( electronic ignition ) systems.
D Represents the point "open" ( contact breaker ignition ) and module "off" ( electronic ignition ) systems.
E Represents the coil current limiting section of the wave form.
F Firing KV. This is the voltage required to start a spark across the plug gap. This voltage is dependent on several factors including : mixture, compression, plug gap, rotor gap, ignition timing and resistance of high tension ignition cables.

Secondary Wave Forms
Summary of Firing KV : This is the voltage required to ionize or initially develop the arc across the spark plug gap.

Variables that raise KV include :
high compression
lean mixture
high resistance in high tension cable
wide rotor gap
worn spark plug
retarded ignition timing
sudden engine loading

Variables that lower KV include:
low compression
rich mixture
small spark plug gap
grounded or fouled spark plug
advanced ignition timing
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Summary of Burn Time : This is the measurement of how long the spark is maintained in the combustion chamber when the spark plug fires. Most of the available energy in the ignition coil is used maintaining the spark during this period.
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Factors that affect Burn Time include :
coil output
secondary circuit resistance
air/fuel charge quality and flow
secondary short circuits that cause energy to be shunted elsewhere
low primary current ( high primary resistance )

Summary of Burn KV : The KV measurement taken from the center of the spark line. This is the KV required to maintain the spark after the initial Firing KV.
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There is a close relationship between Firing KV and Burn KV. If Firing KV is high, Burn KV will be high and the inverse is true. Higher compression will result in a high Firing KV and because the high pressure is present for the duration of the spark, the voltage needed to continue the spark, Burn KV, will be also proportionately high.
At the moment of coil firing, there is a finite amount of energy that can be delivered. If the engine variables require a high Firing KV and high Burn KV, most of the energy will have been used. Consequently, the duration ( or Burn Time ) of the spark will be short.
If the Firing KV is low and the Burn KV is low, Burn Time will lengthen due to the longer time required to dissipate the coil energy.

Burn KV Slope

This measurement is the difference in Burn KV from the start of the Spark Line to the end of the Spark Line. The wave form may show a positive slope or a negative slope. If the slope is from high to low it is termed negative slope and if the slope goes from low to high it is termed positive slope.

The ideal Spark/Burn Line would be straight and have a Burn KV of zero. However, a straight Spark/Burn Line with a low Firing KV could be a fouled spark plug on some engines. The normal KV measurement should be from 0 to .9 KV Positive or Negative.
The Burn KV slope and Firing KV can be used to identify whether a problem exists in the fuel/combustion/mechanical or electrical/ignition systems.

If the slope is Negative and Firing KV is high the problem is usually electrical/ignition related.
If the slope is Positive and the Firing KV is high the problem is usually fuel, mechanical, or combustion related.
If the slope is fairly flat with little Positive or Negative slope and the Burn Time is over 2.5 ms ( milliseconds ) and Firing KV is low, check for a voltage leak to ground or a possible fouled spark plug.
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Super Moderator
8 / 71 240Z, HLS30-40031, L24-052899, Sunshine Yellow
38 Posts
Discussion Starter · #9 ·
Spark Plug Firing Voltage
The spark plug firing voltage is the voltage required to overcome distributor ignition rotor gap and spark plug gaps and to establish a spark across the spark plug electrodes. The condition of the spark plugs and/or the secondary circuit, engine temperature, fuel mixture and compression pressures may affect the required firing voltages.
The initial high voltage to ionize or bridge the gap of the spark plug is called the Firing KV ( Kilovolts ) and can be observed on an oscilloscope by using the Firing KV Parade function. By observing the Firing KV Parade pattern, Average KV ( Kilovolts ) and Delta KV ( Kilovolts ) can be measured to ascertain if problems exist in a particular cylinder, several cylinders or is common to all cylinders during combustion.
When using the Firing KV Parade pattern in conjunction with measurements from the Secondary Raster Pattern, confirmation of particular problems are readily determined.

Average Firing KV Summary.
The normal measurement range ( at the manufacturers recommended engine idle speed and operating temperature ) is : 8KV to 15KV with no more than 3KV difference between cylinder to cylinder .

Variables that Raise Average KV :
High compression
Lean Fuel Mixture
Spark Plug Lead Resistance
Wide Rotor Gap
Worn Spark Plug
Retarded Ignition Timing

Variables that Lower Average KV :
Low Compression
Rich Fuel Mixture
Small Plug Gap
Advanced Ignition Timing
Shorted Spark Plug Lead

Delta KV Summary.
The Delta KV is established by expressing the difference between the highest and lowest Firing KV readings during a set sampling period when using a digital oscilloscope. The normal measurement range of Delta KV is 2 - 4 KV.
A low Delta KV would indicate that Firing KV's remained stable during the sampling period ( i.e. fuel mixture, turbulence, plug gap, compression remained at a constant value ).
A high fluctuation, or high Delta KV, would indicate Firing KV's varied greatly during the sampling period.

Conditions that would result in high Delta KV readings include :
Lean Mixtures
Intermittently Fouled Spark Plug
Intermittent Compression Loss
Vacuum Leaks
Exhaust Gas Re Circulation
Fuel Injector Spray Pattern
Intermittent Loss of Secondary Voltage

Secondary Ignition Firing KV Pattern
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Oscilloscope set to 20KVscale.Wave form for 1994 Nissan Pulsar GA16 ( DE ) 1597cc engine
with Nissan Electronic Ignition ( firing order 1342 ) shown.

Secondary Ignition Firing KV Variation Pattern
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Snap Kilovolts ( Snap KV )
Snap KV measures the voltage required to fire the spark plugs under a loaded condition. This is accomplished by snapping the accelerator open very quickly. During this sudden opening of the accelerator, combustion pressure ( compression ) in the combustion chamber rises greatly, fuel mixture momentarily leans out and ignition timing advances.
Snap KV is therefore a Firing KV reading during acceleration. As the cylinder pressures are higher, ignition timing is more advanced and the fuel mixture is leaner during the sudden opening of the throttle, the required voltage to fire the spark plugs is much higher than that at an idle condition.

The importance of the Snap KV measurement is shown by comparing it to the maximum or total voltage the coil is able to provide. If a coil produces 30 kilovolts maximum voltage and an engine requires 28 kilovolts to fire the spark plugs under a snap throttle condition, a reserve of only 2 kilovolts is left. With this scenario it is probable that the ignition system will fail and an ignition misfire will occur during heavy acceleration uphill.

note : The Snap KV test will not express the same amount of voltage required to fire the spark plugs at 80 KPH, accelerating hard up a hill. Only actual driving conditions can simulate this of required firing voltage, i.e., road testing with special equipment attached to the ignition system or the use of a dynamometer whilst monitoring the oscilloscope readings.

To ensure that a misfire under load is not attributed to demand being more than is able to be provided, Snap KV should be no higher than three quarters ( 3/4 ) of the total coil output. ( i.e., if the maximum coil output is 40 KV, maximum allowable Snap KV would be 30 KV. A reserve voltage, or margin of 10 KV. )

Determining the cause of high Snap KV can be difficult. Under most conditions Snap KV is generally proportional to Average KV. A Cylinder with a high Average KV will tend to produce a higher Snap KV than other cylinders. In general, a high Snap KV is related to spark plug electrode condition. When a spark plug is new, the inner electrode is square, with a sharp edge. As the electrode "wears", it becomes rounded in shape as the electrode erodes during spark discharge. By nature, an electric arc jumps easier from a sharp edge than that of a rounded one. The electrons can concentrate easier at a sharp corner and with a shorter distance to jump, results in the arc being established sooner ( i.e., a lower firing voltage ). The effect of a round electrode on a worn spark plug, with a longer distance to jump, produces a higher Snap KV.

The ignition timing curve also has an effect on Snap KV. Under normal operating conditions, the spark is established well before the peak cylinder pressure at high speeds.
for example : envision a spark occurring occurring 28 degrees before top dead center at 2500 revolutions per minute.
From the point of ignition, the crankshaft has to move 28 degrees before the piston reaches top dead center ( TDC ) and peak compression pressure.
Although the overall cylinder pressure is increased by the acceleration load, this is offset by firing the spark earlier, well before the peak pressure is reached. By retarding the timing of the advance curve, the spark occurs closer to top dead center or at a point where cylinder pressure is much higher. As a result, Snap KV becomes much higher.

Another area that makes for difficult Snap KV diagnosing is spark plug " coating" from fuel additives. This coating is usually yellow in color and at cool idle temperatures, it remains a non conductive solid. Under hard driving or rapid acceleration, the increase in cylinder temperature "melts" the coating momentarily and shorts out the spark plug. As soon as the cylinder temperature returns to normal, the coating solidifies and becomes non conductive. A problem such as this will show up on Snap KV measurement as a very low Snap KV reading ( the same reading you would expect from a fouled spark plug ). Usually a short high speed run is enough to burn the coating off and the plug operation will return to normal.

The fuel system can affect the Snap KV readings by delivering either a rich or lean air - fuel ratio. Rich air - fuel mixtures lower the voltage requirement to initialize the spark across the spark plug gap. Lean air - fuel mixtures raise the voltage requirement at the spark plug gap.
Lack of carburetor accelerator pump circuit, incorrect pump squirt nozzles, injector pintle fouling and fuel pressures are a few of the areas to pay attention to when incorrect Snap KV measurements are encountered. Use an exhaust gas analyzer to determine overly rich or lean air - fuel mixture conditions.

The normal range of Snap KV measurement should be at least a 3 KV increase over Average KV, but not more than a 7 - 8 KV increase. Value should typically not exceed 3/4 of maximum available coil output.

Secondary Ignition Snap KV Pattern
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Secondary Ignition Snap KV Pattern
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Oscilloscope set to 20 KV scale. Waveform for 1986 Mitsubishi Cordia AC ( non turbo ) 1755cc 4G37 engine
with Denso Electronic Ignition ( firing order 1342 ) shown

Circuit Gap KV ( Kilovolts )
Circuit Gap KV is the measurement of all gaps in the secondary circuit EXCLUDING the spark plug gap. Normally the only gap that exists external of the combustion chamber is the distributor ignition, cap - rotor gap. Depending on the type of manufacturer's ignition system the voltage utilized to bridge this gap should be between 3 KV - 8 KV and should not vary greatly between cylinders. Be aware that there are ignition systems that operate normally at one end of the good range or the other. Some European systems utilize a resistance within the distributor rotor and a reading of 6 KV - 8 KV can be expected and some ignition systems utilize larger rotor air gaps ( as high as .125 inch ) for ignition noise suppression, again with an expected reading of 5 KV - 8 KV.
Circuit Gap KV is measured under a deceleration condition. During the deceleration mode the cylinder pressure is lower and a richer fuel mixture is developed. It is during this period that the spark plug gap is momentarily below all other gaps and resistances.
A good method to observe Circuit Gap KV is to watch the Firing KV immediately after performing a Snap KV firing test. When interpreting Circuit Gap KV readings it is important to note if all the cylinder readings are high or just a few readings are high.
If all cylinders have high Circuit Gap KV, it is probable that a problem exists in the circuit between the distributor cap and ignition coil.

Possible faults include :
An eroded / broken distributor cap carbon center terminal
A worn rotor ( excessive air gap )
A loose high tension coil lead
A broken high tension coil lead

If the Circuit Gap KV is high for one or just a few cylinders, problem areas include :
Cracked spark plug
An open spark plug resistor
A loose fit of the spark plug high tension lead ( cap/plug insertion )
Distributor cap tower/s corrosion
A broken high tension cable

Secondary Ignition Circuit Gap KV Pattern
Slope Font Parallel Audio equipment Engineering

Oscilloscope set to 20 KV scale. Waveform for 1989 Nissan Pathfinder 2389cc Z24 engine
with Denso Electronic Ignition ( twin coil, 8 sparkplug ), ( firing order 1342 ) shown
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