Duramax LB7 White Smoke Diagnostics
Posted to Technical Theory Forum on 11/6/2011
23 Replies
Recently in the Heavy Duty/Fleet forum there has been
discussion about white exhaust smoke on these engines at
idle. When injectors are the root cause of this issue, they
are allowing too much fuel to enter the cylinder over too
long a period of time. This is a mechanical fault of the
injectors which the PCM can not account for and why no codes
are generally set.
If a technician connects to the vehicle and performs all
related tests they may find cutting out one or more
cylinders may cure the smoking issue but they are also
adding more load to the the remaining cylinders which can
give a false result. If the entire set is warn however this
may not work and leave the technician at a loss. Two PIDs
that are commonly over looked in these cases are "Calculated
Fuel Rate" in mm3 and "Injector Command" in ms.
Below is an example of a smoking truck's balance rate with
several warn injectors.
Cyl.1_ 0.1mm3
Cyl.2_ -1.0mm3
Cyl.3_ 2.5mm3
Cyl.4_ 1.3mm3
Cyl.5_ 2.5mm3
Cyl.6_ -1.6mm3
Cyl.7_ -3.0mm3
Cyl.8_ -0.8mm3
In the above example no injectors are falling out side of
the +/-4mm3 limit set by GM at idle. Some may flag #7 right
away for being close but lets look at this in a little more
depth. Do you notice that the average of the all eight
injectors equals zero? The real question is what figure are
these balance rates zeroed to?
This is where the "Calculated Fuel Rate" comes in. On the
truck in my example this figure is 3.0mm3. When you apply
this figure to the balance rates you get the calculated fuel
rate for each injector.
Cyl.1_ 3.1mm3
Cyl.2_ 2.0mm3
Cyl.3_ 5.5mm3
Cyl.4_ 4.3mm3
Cyl.5_ 5.5mm3
Cyl.6_ 1.4mm3
Cyl.7_ 0.0mm3
Cyl.8_ 2.2mm3
Now you can see a much clearer picture of what is really
going on. A new set of injectors will run around 5.0mm3 at
idle, on a warm engine. Injectors 1-2-6-7-8 are commanded to
deliver less fuel because they are faulty. The other method
of catching this fault is by watching the "Injector Command"
PIDs.
Inj 1_ 0.29ms
Inj 2_ 0.26ms
Inj 3_ 0.37ms
Inj 4_ 0.34ms
Inj 5_ 0.37ms
Inj 6_ 0.24ms
Inj 7_ 0.14ms
Inj 8_ 0.27ms
I have found that any injector running under 0.30ms will
lead to the white exhaust smoke in question. It is always a
good idea to watch your rail pressure when checking all the
above PIDs as higher than normal pressure will cause the
injector delivery rates to drop in an attempt to maintain
idle speed.
One variable in this situation is the engine itself. The PCM
is trying to balance each power stroke of the engine for the
smoothest possible idle. Engine compression and what is
happening in companion cylinders can have an affect on these
readings.
I would not use this information to only replace the
injectors deemed faulty. This truck received a set of
injectors because five where faulty but I could not say that
the remaining three were any good. This may come in handy
when a customer does not want to replace an entire set due
to the expense. I have found however that replacing the
worst bank can fix the issue for a period of time until the
other bank deteriorates further.
Ryan from British Columbia
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