december2021 - 55

testing location. The engine was started and the data
was captured for a few seconds and stored within the
scope buffer. A view 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 analysis point where many techs 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).
Capturing this data is quite simple. With the use
of the lab scope and referencing the wiring diagram,
we can 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 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
December 2021 | FleetMaintenance.com
55
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