24 AIR REPAIR OBDII REVIEW 2008
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.
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 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 carbon and varnish buildup.
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 (blue 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 returned to
its seat? If we look closely at the voltage pattern, we’ll see the
command for the injector to close. As we follow the voltage spike
downward as it returns to system voltage, we can see another
pintle bump (the lower bluev arrow in Figure B). Now we have
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
3.69 TIME DIFF: 2.66 MS
A
V
1
0
2.0
.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
3.69 TIME DIFF: 2.66 MS
Command Signal
