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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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