december2021 - 54

ANALYSIS
Data from another angle
Sometimes it's the tools we use that give us a different
perspective and lead to a diagnosis.
By Brandon Steckler
I
remember the days when data-gathering and
analysis were something I considered to be
very challenging. This was before I learned to
implement that game plan that we frequently
reference in the pages of Motor Age. Of course,
having that game plan in place keeps us focused
and avoids those times where we tend to drift off
into the weeds. On occasion, though, I still find
myself faced with a challenging driveability fault.
Sometimes it's the tools we choose to implement
and the results of a specific test that brings it all
to the surface for us and provides that " nail in the
coffin, " a diagnosis. Stick with me and I'll show
you how viewing a piece of data from a different
perspective gave me the confidence I needed to
condemn an ECU, while the other available data
left me pacing back and forth.
TODAY'S CHALLENGE
The vehicle that drove me nuts on this particular occasion
was a beat up contractor's van. Yes, we all have that
neighbor that we like to assist. Well, this was that neighbor's
van, and I agreed to assist. The subject vehicle
was a 1994 Ford E250 with a V8 engine. The complaint
was that the vehicle idled rough and, on occasion, the
engine would cut out and then regain consciousness.
After speaking with my neighbor, it was determined
that the engine would actually " stall and come alive
again as if someone had shut off the key momentarily. "
One thing I will state about a vehicle of this era is
that it is pre-OBD-II, and the available data is very slow
and not what we are privy to on today's more advanced
vehicle technologies. With that said, troubleshooting
from the driver's seat will not yield me what it typically
does. I will have to get my hands dirty on this one.
Performing some preliminary research is where I
always begin my diagnostic approach. Understanding
what systems the vehicle is configured with allows me
to determine what preliminary testing can be carried
out easily, especially considering I'm at home, not at the
shop. Of course, I began with a basic visual inspection.
What I discovered was a catalyst (CAT) that was not
doing its job. The fumes from the tailpipe were enough
to choke a horse! Many of you realize that a CAT is a
sensitive device and needs proper feed gas (from the
engine's output) to function properly. I was not yet concerned
with the CAT's performance, as I had realized
it was likely a victim of the rough-running condition.
Monitoring the HO2 sensors (pre- and post-catalyst)
showed them to be operating erratically, and there were
no DTCs flagged in the PCM's memory. To me, this was
a clue of a loss in fuel control. Again, I figured this to be
the effect of the rough-running condition but not the
cause. The vehicle was more frequently in open-loop
than in closed-loop, so " faulty HO2 sensors " was near
54 Fleet Maintenance | December 2021
A neighbor complained his van would cut out and then regain
consciousness.
Photos courtesy of Brandon Steckler
the bottom of my list of components to test.
This vehicle is equipped with a distributor ignition
system. With that being said, the result of secondary
ignition analysis was definitely of interest to me. Let's
face it-it's easy to obtain and easy to analyze. A lot
can be learned about the conditions within the combustion
chamber derived from ignition analysis. With
the capturing of the secondary ignition waveforms and
performing a throttle snap test, we can stress the ignition
system to simulate a light load and also one that
demands a lot of fuel. Reflecting on the results of the
captured waveforms and a bunch of practice, we can
learn to determine many things due to an understanding
of the physics involved:
* Low cylinder compression/timing faults
* Worn ignition components (cap/rotor/spark
plugs)
* Rich conditions
* Lean conditions
* Weak coil output
* Open/hard spark plug cables/poor connections
* Excessive resistance
* Short circuits
* Bent distributor shaft
Since the distributor is at the front of the engine,
Connecting the ignition scope and the distributor coil input cable
using a capacitive lead.
it was very easy to access. A simple connection was
made between my ignition scope and the distributor
coil input cable using a capacitive lead. Simply clipping
this lead around the coil input cable offers a view of the
electricity within the cable. This serves as a " cousin "
of the actual kilovolts (kV) traveling within the cable's
insulation. Using the coil input cable, I was able to
view the ignition events for all cylinders from just one
december2021

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