No one will argue that the
lab scope has become the
diagnostic tool of choice for
today’s technician. This can make
diagnosing a problem with today’s
automobile a lot easier. We know
that dirty fuel injectors can be a real
problem for our clients’ vehicles, but
proving it is not always easy. So let’s
assume at this point that you suspect
the injectors to be a problem. Some
clues could be: fuel trims have drifted;
a trouble code, P0300, has been stored;
your client complains of performance
or fuel economy issues; or Lambda is
just no longer at 1. In any case, you feel
that your client’s vehicle problem may
be caused by dirty injectors. Now you
would like to know if fuel injection
cleaning is needed, and you want to be
able to prove whether it did any good.
channel and voltage leads on the other.
I prefer to have the low amp pattern of
my injector on the top of my screen. I
set the scale to 1 amp, which handles
most applications. Now set the injector
voltage signal below with the range set to
20 volts (see figure A). Notice that I am
not interested in the voltage spike of my
injector pattern. This part of the injector
pattern is the most significant and gets
me the most detail. The time base is
set to 1 ms per division. Set your scope
up so that you fill your screen with one
complete injector pattern. We are now
displaying the two patterns for a single
injector, amperage on top and voltage
on the bottom. I refer to the voltage
signal pattern on the bottom as the
command signal (see figure A). This is
the signal given by the PCM in response
to all the interpreted (interpreted being
the operative word here) inputs. The
amperage is the injector’s response to
that command and is mostly affected
by ca rbon and va rnish bui ldup.
Now, before we examine this pattern
any further, we need to determine if
we’re using a good base line. Does
the command time in your injector
signal pattern (injector pulse width)
as measured on your lab scope match
the PCM command time displayed in
your scan data? In our sample, the
data stream PID shows 3.1 ms, which
is what our lab scope verifies. Since
we are in agreement, we’ll proceed.
When system voltage is pulled to
ground, the amperage begins to climb
in response to the injector’s resistance
to open. We have all been taught that if
we look closely at the amperage signal
along its inclining ramp we should see
a small dip known as the pintle bump
(red arrow along the amperage ramp
in figure A). The pintle bump indicates
when the mechanism of the injector has
finally popped open. And we can assume
(sorry, I know how much we all hate that
word) that fuel has begun to flow. The
O2 sensor is going to measure the fuel
delivered and report back to the PCM.
Most everyone agrees that the pintle
bump should occur within the second
third of the pattern. Let me explain. If
you take the time base of the amperage
pattern and divide it into thirds, the
middle third, or some will say the second
third (grayed area of the amperage
pattern in figure A), will be the general
area that you will usually find the
injector’s opening pintle bump. Some
believe that the location of the pintle
bump along the inclined ramp is an
indication that the injector is dirty and
needs to be cleaned. I won’t argue that
point. The question I have is how dirty
is it and how do I know that it has
been cleaned properly. I can’t assume
(there’s that dirty word again) that just
because we moved the pintle opening
up or down along the ramp that we
have cleaned the injector properly.
Let me offer another point of view.
Let’s look at our pattern a little closer.
When the injector closes, shouldn’t
there be an indication that the pintle has
January
2008
Air
Repair 6
Diagnosing Dirty Injectors
3.69 TIME DIFF: 2.66 MS
A
V
2nd 3rd
Load Response
Signal
Command Signal
1
0
2.0
0
Figure A
1 ms
3.69 TIME DIFF: 2.66 MS
A
V
Pintle Bumps
Open \ Close
On \ Off
1
0
2.0
0
Figure B
1 ms
3.69 TIME DIFF: 2.66 MS
A
V
Response Time
2.66 ms
1
0
2.0
0
1 ms
Command Time
3.1 ms
3.69 TIME DIFF: 2.66 MS
A
V
1
0
2.0
0
1 ms
.5
Command Signal
3.69 TIME DIFF: 2.66 MS
A
V
2nd 3rd
Load Response
Signal
Command Signal
1
0
2.0
0
Figure A
1 ms
3.69 TIME DIFF: 2.66 MS
A
V
Pintle Bumps
Open \ Close
On \ Off
1
0
2.0
0
Figure B
1 ms
3.69 TIME DIFF: 2.66 MS
A
V
Response Time
2.66 ms
1
0
2.0
0
Figure C
1 ms
Command Time
3.1 ms
3.69 TIME DIFF: 2.66 MS
A
V
1
0
2.0
0
Figure D
1 ms
.5
Command Signal
In this model, we will be using two
channels of your lab scope to view the
operation of a single injector (I’ll show
you a method for single channel lab
scope at the end of this article). Your
lab scope has to have a good resolution
or sample rate in order to get enough
detail from the displayed patterns. We
will be using a low amp probe on one
