December 2021 - 36

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 datagathering
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 or down into that rabbit
hole. 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 " -
to sum up, 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 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
36 PTEN DECEMBER 2021 VehicleServicePros.com
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, 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 testing
location. The engine was started and
the data was captured for a few seconds
and stored within the scope buffer. A view
http://www.VehicleServicePros.com
December 2021

Table of Contents for the Digital Edition of December 2021
