Driveability Corner - MOTOR Information Systems

Driveability Corner
Driveability diagnosis is rarely simply checking DTCs, reading a fault
tree, then replacing a defective part. It’s wise to follow a consistent
diagnostic routine, especially when no codes are recorded.
Mark
Warren
smwarren@motor.com
Afriend asked for some help
on a troublesome Nissan. It
seems that intermittently
the Nissan would go full
rich, blow black smoke and
run very poorly. Of course,
no codes. We all do logical no-code diagnosis,
plotting our own map for diagnostics.
This is a good example of creating that diagnostic
strategy.
The black smoke was a likely indicator the
engine was running rich, but it never hurts
to have confirmation with a gas analyzer. My
friend had bypassed this test and watched
the scan tool when the problem was occurring.
The injector pulse width jumped to
20mS when the problem occurred.
The diagnosis always starts with a list of
possibilities and a process of elimination.
There are two paths through the possibilities—first,
the most likely failures that
could cause the problem, and second, the
easiest to test. Judgment dictates which
comes first. Would you test fuel pressure
first or use the scan tool you already connected
to check for codes?
It does look like a bad input is causing
the fuel injector pulse width (IPW) to go
way too long. For this exercise, let’s list the
other possibilities, not associated with a bad
fuel delivery input. They are:
•High fuel pressure.
•Leaking injector.
•Purge system (evap) issue.
In our diagnosis, we don’t see cause and
effect; we see effect (running rich) and
have to backtrack to the cause. The IPW
calculation is the end of the line for a number
of inputs to the powertrain control
module (PCM). Let’s run through some
continued on page 14
Screen capture : Mark Warren
12 February 2013

Driveability Corner
possible defective inputs that would
cause this problem:
Mass Airflow (MAF) Sensor. In
the scan capture of a defective
MAF signal shown on page 12, note
the dropouts in the MAF signal
(green in the top graph), the corresponding
drops in IPW (red) and
the drop in short-term fuel trim
(red, in the lower graph).
In this example, of course, the
drop caused a lean condition, which
is more common than a rich condition
on this Nissan. Our Nissan’s
MAF signal was perfect when the
defect occurred.
Oxygen Sensors. Next, did the
O 2 sensors respond correctly when
the problem was not occurring and
when it occurred? According to the
information provided to me, they
ranged correctly in response to a
snap throttle test when the problem
was not occurring and stuck rich
when the Nissan ran rich. Don’t
forget to test the postcatalyst O 2
sensor; generally, the trim authority
for the rear O 2 sensor is limited and
cannot create such a dramatic rich
condition. But on some vehicles,
the rear O 2 sensor has unlimited authority
and can cause a stuck-rich or
stuck-lean condition.
Temperature Sensor. Start disconnecting
temperature sensors on
late-model vehicles and you’ll note
that when running, the temp sensor
generally has no effect on determining
fuel delivery. This is unlike
the early days when the temp sensor
had unlimited authority and an
open sensor would cause a stuckrich
condition.
Throttle Position Sensor. A
wide-open throttle command may
cause a rich condition, but this is
pretty unlikely. On a throttle-bywire
system, the system is moni-
tored very closely for safety reasons
and will always set a code. Also, the
redundancy in the system would
never allow this rich condition to
occur.
Crank and Cam Sensors. The
crank and cam sensors time and sync
the injector signal. Corrupted signals
can cause double-injection events or
other strange behavior. An oscilloscope
is mandatory here, but any
aberration in a tach or rpm input
may signal a problem in this area.
Power and Grounds. These
guys can never be left out of almost
any no-code diagnosis. The effect of
bad power or grounds can be wideranging
and extreme. After seven
years or 70,000 miles of vehicle use,
issues in this area become more
common.
Crank Signal. I know of a similar
intermittent running-rich case
where the PCM received an inter-
Circle #13
14 February 2013

mittent crank signal while running.
During crank, generally the PCM
commands extra fuel for starting.
The crank signal wire from the
starter on this vehicle was intermittently
shorting to 12 volts, signaling
the computer that cranking/starting
was occurring. The computer obviously
did not have a rationality test
to compare the crank signal to current
rpm and commanded fuel enrichment.
This is one of those crazy problems
right out of left field. Using an
eight-trace scope to monitor many
signals and extreme persistence
fixed this problem. The advantage
of the eight-trace scope is for monitoring
and eliminating the most
likely possibilities during the intermittent
event.
Vehicle Speed Sensor. I had a
computer-controlled Honda many
years ago that gave me fits over a
pig-rich condition. The O 2 sensors
worked fine. When I restricted fuel
they went lean, and they correctly
indicated rich when the Honda ran
rich. I could turn in the idle mixture
screw and watch the gas analyzer
go lean and then watch as the
system commanded it back rich.
The system was commanding this
vehicle rich. The question was:
Why? The customer request was to
fix the emissions problem and, after
the cost was known, look at the
speedometer and estimate the cost
to repair it as well. After two days of
no luck with the fueling problem, I
decided to look at the speedometer
issue just to have some repair success
with this vehicle. The vehicle
speed sensor (VSS) on the back of
the transmission was bad and cost
about 25 bucks. I fixed the speedo
and took the vehicle for a test drive
to confirm the repair. Lo and behold,
the fueling problem was fixed!
It turns out this Honda had a catalytic
converter strategy that drove
the system rich if the vehicle remained
stationary (no VSS signal)
for more than two minutes with the
engine running. It seems these vehicles
had a problem with the converters
getting hot at idle and burning
the carpet in the vehicle. After
sitting and not moving, with no
VSS, the system went rich to cool
and unlight the catalytic converter.
With a failed VSS, the system was
stuck in this mode.
There was a code set for the VSS.
Even if a code seems logically unrelated
to your problem, take a hard
look at that component and consider
making the fix.
Remember, a logical diagnostic
procedure, thinking out of the box
and a good dose of luck never hurt
in the diagnostic hunt.
Circle #14
February 2013
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