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I have read several post on this site before I began to really understand what the balance rates are telling me. I thought I would share what I have learned.

First let’s define some variables that show up on a scanner:

Fuel rate - average amount of fuel delivered to each cylinder measured in mm3/sec. It is an average. To calculate this value, the fuel rate for each cylinder is added together and then divided by 8 to obtain the fuel rate.

Balance rate – Is how much each cylinder varies from the average. Measured in mm3/sec.

Thus to know the fuel rate for an individual cylinder you must take the fuel rate and add the balance rate. For example if the fuel rate is 9 mm3/sec and the balance rate for cylinder 1 is -2 mm3/sec. Then the fuel rate for cylinder 1 is 9-2=7 mm3/sec. Likewise if the balance rate for cylinder 2 is +3 then the fuel rate for cylinder 2 is 9+3= 12 mm3/sec.

Now how is fuel rate measured? Well there is no little flow meter installed on each injector to measure the flow. Rather cylinder fuel rate is calculated (estimated) based on the width of the fire pulse sent to each injector. Given a perfect (ideal) injector if you know the width of the fire pulse then you can calculate how much fuel would be delivered. Thus fuel rate is really a measure on how long each injector is fired.

Now let’s explain what going on and why balance rates are calculated at idle. At idle (park or drive) for the engine to run smooth each cylinder must produce the exact same amount of power. Engine cylinders are not exactly the same and thus the cylinder may require a bit more or less fuel to achieve the same power. In addition each individual injector varies and thus the width the fire pulse signal sent to each injector must be varied to achieve the same fuel rate.

Given these two variable the desired cylinder fuel rate (or as discussed above - the injector fire pulse width) is varied to achieve a smooth idle. This is only done at idle RPMs because small variations in the actual fuel delivered to each cylinder is more critical as the fuel being delivered to each cylinder is low. Plus there is little inertia in the rotating parts of the engine to smooth out small variations. For example at idle the desired cylinder fuel rate should be about 8 mm3/sec. If the actual flow rate is off by 2 mm3/sec this is a 25% error; however when you are driving in town the fuel rate will be around 30 mm3/sec and now the 2 mm3/sec is only a 7% error.

Now as injectors age and get dirty several thing can happen- the injector flow rate can increase or decrease, the injector turn off time can increase and/or they begin to leak. These issues change the amount of fuel being delivered to a cylinder. Now at idle more or less fuel is being delivered than desired and power produced by each cylinder will vary. The engine computer senses this change and adjusts the width of the fire pulse to each injector to smooth out the power delivered by each cylinder.

Note if an injector is leaking during the cylinder exhaust stroke it will cause the exhaust white smoke that often indicates bad injectors.

For example if 1 injector releases 2 mm3/sec more fuel than required, the computer will decrease the injector pulse width (cylinder flow rate) for this cylinder by 2 mm3/sec. This would show up as a balance rate of -2 mm3/sec for this cylinder. It is IMPORTANT to see that the actual fuel delivered to the cylinder remains constant. If the desired fuel rate is 8 mm3/sec at idle, the pulse width sent to the injector for this example would deliver 6 mm3/sec if the injector was perfect but because the injector releases 2 mm3/sec more fuel than required the actual delivered fuel rate is 6+2=8 mm3/sec.

Now see what happens if all 8 injectors release 2 mm3/sec more fuel than required. The computer will decrease the injector pulse width (cylinder flow rate) by 2 mm3/sec for all eight cylinders. This would show up as a reduction in flow rate by -2 mm3/sec while the balance rate would remain at 0. Remember the fuel rate is the average of all 8 cylinders.
Thus the performance of an individual injector can only be determined by knowing both the fuel rate and balance rate.

When recording fuel and balance rates the engine should be at 170 degrees or hotter, the idle should be at 680 RPM (01 idle at 600, and LMMs idle at 720), rail pressure should be 35 MPA for federal LB7’s, and 30 MPA for most other, no abnormal loads ( e.g. the battery should be charged and AC off) and the vehicle at idle (park or drive). Under these conditions the ideal fuel rate that should be delivered to each cylinder is 8-9 mm3/sec when in park. Typically 8 mm3/sec can be used at sea level and at 5000+ feet elevation use 9 mm3/sec. VVT trucks (LLY+ with the turbo cooled at idle will be closer to 9-10 mm3/sec). In gear these values increase to 10-13 mm3/sec.

Now to see how far from perfect each injector is functioning, calculate the fuel rate(injector pulse width) for each injector and compare to the ideal values above. GM specification is +/- 4 mm3/sec (4-12 mm3/sec per injector) in park and +/- 6 mm3/sec (2-14 mm3/sec per injector) in drive for good injectors. Thus, for each individual injector take the fuel rate (which is an average value) and add the balance rate to come with the fuel rate for each injector.

Eg. In Park the following data is given on the scanner:
Fuel rate =8 mm3/sec
Balance rate =
#1 = +1 mm3/sec, then cylinder fuel rate = 8+1=9 mm3/sec - good
#2 = -3 mm3/sec, then cylinder fuel rate = 8-3=5 mm3/sec- good
#3 = -2 mm3/sec, then cylinder fuel rate = 8-2=6 mm3/sec- good
#4 = +2 mm3/sec, then cylinder fuel rate = 8+2=10 mm3/sec- good
#5 = +3 mm3/sec, then cylinder fuel rate = 8+3=11 mm3/sec- good
#6 = 0 mm3/sec, then cylinder fuel rate = 8+0=8 mm3/sec- good
#7 = -1 mm3/sec, then cylinder fuel rate = 8-1=7 mm3/sec- good
#8 = 0 mm3/sec, then cylinder fuel rate = 8+0=8 mm3/sec- good

As a check average the 8 cylinder flow rates and they should equal the scanner flow rate.
(9+5+6+10+11+8+7+8) / 8 = 8 mm3/sec

For a second example let’s reduce the fuel rate read by the scanner.
Fuel rate =5 mm3/sec
Balance rate =
#1 = +1 mm3/sec, then cylinder fuel rate = 5+1=6 mm3/sec - good
#2 = -3 mm3/sec, then cylinder fuel rate = 5-3=2 mm3/sec- bad
#3 = -2 mm3/sec, then cylinder fuel rate = 5-2=3 mm3/sec- bad
#4 = +2 mm3/sec, then cylinder fuel rate = 5+2=7 mm3/sec- good
#5 = +3 mm3/sec, then cylinder fuel rate = 5+3=8 mm3/sec- good
#6 = 0 mm3/sec, then cylinder fuel rate = 5+0=5 mm3/sec- good
#7 = -1 mm3/sec, then cylinder fuel rate = 5-1=4 mm3/sec- good (border line)
#8 = 0 mm3/sec, then cylinder fuel rate = 5+0=5 mm3/sec- good

As a check average the 8 cylinder flow rates and they should equal the scanner flow rate.
(6+2+3+7+8+5+4+5) / 8 = 5 mm3/sec

Thanks to the following post to provide a majority of this information:
Therermanator post dated Aug 2005.
 

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Thank you! Very good information
Jim
 

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Good information. Thanks for taking the time to write this up and explain it. Very helpful.


I have no way to see flow rate but my balance rates using torque pro are:
#1 = 0.1
#2 = 0.2
#3 = 0.3
#4 = -2.3
#5 = 0.7
#6 = 3.8
#7 = -1.1
#8 = -1.3


I thought #6 was going bad, I've seen it go up to 4.2, I've also seen #4 go as low as -3.8. Now I believe my flow rate is heavily restricted and basically all but #6 are have trouble with leaking and that #4 is the worst. This set of injectors was replaced at 50k under warranty, it has 183k on it now so it would make sense that they are all leaking. I see white smoke at idle when its warmed up. Several times I have seen the balance rates all go to 0 and the truck shakes a little, which to my understanding happens when the flow gets below about 2 mm3, basically the ECM quits trying to balance the cylinders.


What do you all think? Should I spend money having a shop hook up to it just so I can see the flow rate or should I just spend the money on new injectors? It seems logical that they are all leaking to me but I really don't have extra money laying around to spend on unnecessary injectors.
 

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What is the most accurate scan tool to check balance rates.? I use my EFI V2 and my rates have changed after replacing the alternator, they do not bounce around as much now, more of smooth transition now..


jdh111, is this the thread you are referring to by Ferman in your first post.? He has been a member since 2005 and the thread below was posted in 2014...


http://www.dieselplace.com/forum/63-gm-diesel-engines/7-duramax-first-generation-2001-2004-lb7/576505-proper-way-check-balance-rates-3.html
 

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An excellent description of a technically complex situation!

I thought I read (somewhere) that the ECM also uses the time between firing pulses, as measured at the flywheel, to assess cylinder balance. That would seem to make sense since the ECM has no way of measuring injector leakage, although come to think of it, leaked fuel will not effect idle smoothness since it is not affecting the power stroke.

My LB7 idled beautifully, while creating a smoke cloud.

Your thoughts?
 

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An excellent description of a technically complex situation!

I thought I read (somewhere) that the ECM also uses the time between firing pulses, as measured at the flywheel, to assess cylinder balance. That would seem to make sense since the ECM has no way of measuring injector leakage, although come to think of it, leaked fuel will not effect idle smoothness since it is not affecting the power stroke.

My LB7 idled beautifully, while creating a smoke cloud.

Your thoughts?
Run some good injector cleaner through it or do the injector cleaning procedure in the sticky above.
Just got mine to stop looking like I was fogging for mosquitos.
Also check the coolant temperature the ECM is seeing to make sure the thermostats are not stuck open and its running too cool.
 
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