December 2021 - 37

of the waveforms indicated very high firing
kV. This was indicative of the ignition
system having to work very hard, simply
trying to fire the spark. Conditions like
high compression, lean air-fuel mixture/
large air gaps, and retarded ignition timing
can create this situation. This capture
is not from the subject vehicle but from
similar circumstances.
I decided it was best to inspect the
spark plugs, as they were not too difficult
to remove, and the results of ignition
analysis justified it. Not only were
the spark plugs worn to an extremely
large gap, but they were also white as
ash, which provided evidence of a lean
air/fuel mixture. I replaced the secondary
ignition components to be sure
they were not going to contribute to the
symptom I was pursuing. After starting
the vehicle, the engine still exhibited the
same symptoms. Although the firing kV
was still abnormally high, the properly
gapped new spark plugs and fresh secondary
components lowered the firing kV
considerably. Of course, we should realize
not only that the replacement of the
components was necessary but also that
their wear was a result of the underlying
lean condition.
IN PURSUIT OF THE
LEAN CONDITION
This is the point in the analysis where
many technicians get ahead of themselves
and take on the idea that a cylinder(s)
is under-fueled because of an injector
restriction. For those of you that were
thinking this, I'm not going to say you
are incorrect. However, I will suggest that
rather than jumping to a pinpointed test
like " injector balance, " consider if the
injectors are under-delivering. It may be
due to an insufficient command from the
PCM (an insufficient injector pulse width
or INJPW).
To capture this data is quite simple.
With the use of the lab scope and referencing
the wiring diagram, we can
Photo courtesy of Brandon Steckler
determine where to place the probes so
that the scope can display the fuel injector
waveforms for all the fuel injectors, with
only two test locations. This same diagram
allows us to also see how the PCM
strategizes to control the fuel injectors.
It can be seen that all eight fuel injectors
are controlled on the ground side by only
two individual injector drivers within the
PCM. One driver provides the ground
path for all four banks 1 fuel injectors.
The other driver shares the same responsibility
but for all four banks 2 fuel injectors.
Simply back probing these two wires
allows us to see the command for all eight
injectors as well as the pulse width. A
fuel injector, being an inductive device,
takes on the current through the bobbin
(a winding of wire to carry an electrical
current and take on a magnetic field). As
the ground side controlled injector is provided
a ground path, the voltage on the
controlled side (ground side) drops across
the injector. During this time the circuit is
held near ground potential and is known
as the injector on time. When the PCM
determines the end of the injector operation,
the ground path is removed and the
Connecting the ignition scope and the distributor
coil input cable using a
capacitive lead.
current ceases to flow. As a result, the
magnetic field collapses and is converted
back to electrical energy. This is the high
inductive kick visible in the pattern.
Viewing the amperage trace on the
lab scope can be carried out as well, but I
chose not to in this case (for no particular
reason). If I chose to deploy the vertical
measuring cursors, I could Plot them
between the point in the waveform where
the injector was energized and where the
injectors were de-energized to offer me a
measurement of delta time (the elapsed
time between the cursors). This would
offer the actual measured INJPW. However,
simply viewing the three successive
injector events, it's clear to see that their
on-time varies greatly.
* YELLOW= CKP/CMP input
(profile ignition pickup-
described shortly)
* GREEN= Bank 1 Injector driver
at PCM
* BLUE= Bank 2 Injector driver
at PCM
VehicleServicePros.com DECEMBER 2021 PROFESSIONAL TOOL & EQUIPMENT NEWS 37
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December 2021

Table of Contents for the Digital Edition of December 2021
