IGNITION TESTING STRATEGY
The following procedure is a step by step ignition system
testing strategy meant to be used as a 15 minute vehicle test routine. Each
step must be followed exactly and no steps should be skipped to prevent
diagnostic errors.
There are 5 basic types of misfires—electrical, lean,
rich, density and primary missed trigger. All can be detected with the
following ignition tests.
• Electrical misfire is related to the distributor
cap, rotor, wires and plugs. This is called the fixed resistances of an
ignition circuit. Carbon tracking is the most common of all electrical
misfires next to defective secondary components. A carbon tracking problem
will never show up at idle. It is important to stress the ignition system,
by preloading or doing a WOT test, so as to identify any problems.
• Lean misfire is related to an excess of air or lack
of fuel condition as in a clogged injector, vacuum leak, dirty MAF, low fuel
pressure, etc. If propane is added to the engine and the misfire does not
go away, then the problem is not caused by a lean condition. A density
misfire is the probable cause of the missing cylinder.
• Rich misfire is related to a lack air or an excess
fuel condition as in a leaky injector, diluted oil, filled canister, high
fuel pressure, etc. If a vacuum leak is created the rich misfire should
go away.
• Density misfire is related to a jumped timing belt,
ignition timing, EGR valve or mechanically related condition, whereby
cylinder compression usually drops. Propane should always be added to the
engine while verifying that the misfire still persists. If the misfire goes
away when propane is added then the misfire is caused by a lean condition
and not a density problem.
• Primary trigger misfire is related to any sensor
responsible for triggering the ignition spark as in a crank sensor, cam
sensor, pick-up coil, etc. The use of an amp-probe is needed to identify a
defective triggering device. A bad triggering component will show a missing
coil current hump on the scope waveform. A defective triggering device
will also cause the coil turn-on point in the spark primary waveform to
fluctuate back and fourth more than 500 yS. As long as those
fluctuations are bellow 500 yS, they are considered acceptable due to timing
belt/chain, distributor or sensor wear. It is also highly recommended
that the time base be set high on the scope, so as to pack a lot of trigger
pulses on the screen. This will make it easy to see a missed triggering
device. Most technicians tend to spread out the time base too much. In this
case the idea is to do the opposite and pack as many pulses on the screen as
possible.
With the exception of the electrical misfire, the other
types of misfires (lean, rich, density, and trigger misfires) are considered
variable resistance type misfires because said resistances change according
to external conditions.
All ignition system tests take the following format. Test 1
through 5 will identify mostly fixed resistance misfires and test 6
identifies lean, rich and density misfires as well as trigger misfires or
variable resistance. Together with the regular capacitive probe, an
amp-probe is also connected to channel 2 or B of the scope and synched to
the #1 cylinder TDC. By doing so, a spark and current waveform can be seen
both at the same time on the scope screen and a determination can be made as
to proper triggering device operation. . If a triggering device is
suspected, it is highly recommended that the time base be set high on the
scope so as to pack a lot of trigger pulses on the screen. This will make it
easy to see a missed triggering device.
IGNITION TESTING PROCEDURE
Perform a simple visual inspection first. It is absolutely
important to verify that the spark is reaching the spark plug first, before
any tests are performed. Make sure that there are no illegal air gaps,
shorted secondary wires, or carbon tracking in the secondary ignition wiring
system. An open or shorted secondary wire will not show up so readily on
the scope if the problem is after the spot where the capacitive clip is
connected, unless the problem is so severe as to show up on the firing KVs
or spark duration values.
1. Cranking KV’s – A parade ignition pattern is set on the
screen. The engine is cranked at WOT to cut injector pulsation and raise
combustion chamber pressures to its highest level, thereby, stressing the
system to its maximum potential ( raising KV demand ). Any compression
problems will be identified with this test by not showing the high KV’s
needed to fire the lean air/fuel mixture. Any low firing line KV’s indicates
low compression. The more air molecules in the combustion chamber, the
higher the resistance and the more the KV demand. The more the amount of
fuel in the combustion chamber, the lower the resistance and the less the KV
demand. Air acts like an insulator and fuel as a conductor.
2. Idle Firing Line KV’s – The engine is let to idle and a
parade ignition waveform is analyzed. All firing lines are compared for
uniformity and proper KV amplitude. If a triggering problem is identified,
attention should be shifted to that particular cylinder or group of
cylinders. (Normal KV firing lines should be between 8 and 12 KV ).
It is also important to analyze the individual firing KV’s, spark line KV’s
and spark duration as well. (Refer to test #6). The firing KV’s,
spark KV’s and spark duration are closely related and should be taken into
account as a group.
3. 2500 RPM KV’s – The same analysis of the spark waveform
is made as in step #2. Since the engine is now at 2500 RPM, the system is
stressed more.
4. WOT snap – A WOT snap test is made to verify cylinder
compression and mechanical integrity. All firing KV’s should go up (off the
scale). A bar graph feature is desirable when conducting this test. This
test is done using a parade pattern and/or bar graph so as to be able to see
all firing KV’s at the same time. Clogged injectors tend to show up is this
setting, in which case, the spark duration would go well bellow 0.50 mS
during the test.
5. Pre Load KV’s – The engine is pre-loaded and the
firing line is analyzed. Most ignition problems are identified while
pre-loading the engine, due to the stress on the ignition system. It is very
important to always stress the system to its maximum.
6. Individual Cylinder Spark Line – This test is made to
identify any lean, rich and density misfires. The spark line and the spark
duration are taken into account very carefully in this test. The system
should be tested at idle no load and loaded, 2500 RPM no load and
pre-loaded. It is always important to pre-load or stress the system while
testing. The more air molecules in the combustion chamber the higher the
resistance and the more the KV demand. On the other hand, the more fuel in
the combustion chamber the lower the resistance and the less the KV demand.
As stated before air acts like an insulator and fuel as a conductor.
• In a distributor ( DI ) system ( 2 air gaps ) around 2
to 3 KV should be seen at the spark line and a minimum of 1.3mS spark
duration.
• In a DIS ( EI-waste spark ) system ( 2 air gaps ) GM
type-II, around 2 to 3 KV should be seen at the spark line and a minimum of
1.0mS spark duration.
• In a COP system ( 1 air gap ), around 1.5 to 2 KV
should be seen at the spark line and a minimum of 1.5mS spark duration.
