- Staff
- #1
It drives me nuts to hear people say there balance rates are good, so how can they have bad injectors? Well they might THINK there balance rates are good, but in reality they can be quite the opposite. Balance rates are nothing more than how far off of the average fuel flow each injector is. All DURAMAX engines at idle will adjust the injection amount of fuel to each injector to allow the engine to run as smoothly as possible(until the inejction amount drops down below roughly 2MM3 of fuel, then the ECm freaks out and and goes to all 0's for the balance rates). It uses input from the crankshaft and camshaft position sensors to achieve this cylinder balance, and the amount of fuel plus or minus from the average fuel flow is the balance rate for each cylinder. Balance rate numbers can vary from -7 to +15.
To get the actual balance rate of your cylinders, you must know the fuel rate/injection amount at the time the balance rates are taken(this will become VERY clear in a few minutes why you MUST know this number). The engine should be at 170 degrees or hotter, at a normal 680 idle(01 idle at 600, and LMM's idle at 720), rail pressure should be 35MPA for federal LB7's, and 30 MPA for most others, the battery should be charged with no abnormal loads on the engine(IE A/C off), and vehicle speed is 0MPH(the ECm only does balance rates when the engine is idling, and vehicle speed is less than 2 MPH). The fuel rate/injection amount should be at 8-9MM3 of fuel with all of these conditions met(at sea level it is safe to use the 8MM3 of fuel number, at 5000+ feet use the 9MM3 of fuel number. VVT trucks(LLY+) with the turbo closed at idle like in stock tuning will be closer to 9-almost 10MM3 of fuel at idle due to the extra engine load to spin up the turbo at an idle with the veins closed. And your in gear under these conditions should be roughly 10-13MM3 of fuel. Here is where knowing the fuel rate is so important and why it MUST be figured in.
Let's use a hypothetical example here to determine actual balance rates VS the displayed balance rates. All above conditions are met, and here is the balance rates in neutral(I will skip the in gear as it is the same, just with a different +/- spec).
#1 -2.1
#2 1.4
#3 -0.8
#4 1.2
#5 -1.4
#6 -1.2
#7 2.1
#8 0.8
Well these numbers at first glance look great and well within the GM specs of +/- 4 in nuetral and +/- 6 in gear(ideal would be 0, but no 2 cylinders are idnetical so don't expect to see all 0's. If you you most likely have a serious problem and the ECM has reverted to fail safe and not doing balance rates), but lets correct them to ACTUAL. The fuel rate should be at least 8MM3 of fuel, but the fuel rate is 3MM3 of fuel for this example. So we need to add -5 to all of these balance rates to get the actual numbers(the displayed fuel rate is 5MM3 of fuel low, so it is -5 off). So lets see the corrected numbers.
#1 -7.1 BAD
#2 -3.6 borderline
#3 -5.8 BAD
#4 -3.8 borderline
#5 -6.4 BAD
#6 -6.2 BAD
#7 -2.9 getting up there
#8 -4.2 BAD
These corrected numbers look quite different, and according to these numbers we now have 5 bad injectors, 2 on the edge, and one other that is starting to get up there. This is because like stated earlier balance rates are just an average of how far off each injector is from the average fuel flow which is your fuel rate. And if all the injectors are worn equally(like the above example), then the balance rates will stay close to each other but the fuel rate goes down. Once you learn to figure in the fuel rate to your balance rates, you will get much more accurate results of injector/engine health. But balance rates are just a test and not the end all be all of injector tests as it is just a small portion of what is going on and can be influenced by MANY other variables.
Lets do one more example to show how much balance rates can change once corrected to actual.
#1 -3.0
#2 0.4
#3 -2.8
#4 1.2
#5 -1.4
#6 -1.2
#7 4.5
#8 5.8
Now these numbers at first glance without figuring in the fuel rate show that #7 & #8 are out of the +/- 4 spec, but lets correct them for having a fuel rate of 3MM3. We need to add -5 to them again.
#1 -8.0 BAD
#2 -4.6 BAD
#3 -7.8 BAD
#4 -3.8 borderline
#5 -6.4 BAD
#6 -6.2 BAD
#7 -0.5 GOOD
#8 0.8 GOOD
So now #7 & #8 are the best of the bunch with corrected numbers, and we have 5 others bad with 1 borderline. This illustrates how critical it is to have the fuel rate to correct your balance rates to actual numbers, and why balance rates are worthless without knowing the fuel rate when they were taken.
Another good thing to do if you have the capability is to raise the rail pressure while checking the balance rates. Leaking ball seats will show up better at higher rail pressures than they do in the idle range, so by raising the rail pressure you can get a better idea of ball seat condition without doing a return rate test.
Keep in mind these numbers are for stock engines with stock tunes. If you have a modified tune it must use stock rail pressure at idle, and the fueling needs to be stock from 0-20MM3 of fuel(If in doubt put it back to stock). And if you have oversized inejctors you must have the tables corrected in the 0-20MM3 of fuel areas so that the puslewidths give the correct flow for each cell otherwise the balance rates will be inaccurate.
Also balance rates should not be confused with return rates. Return rates is the amount of fuel each injector returns to the tank during a 15 second cranking cycle, and involves a test set with 4 graduated cylinders to perform as well as some engine teardown to perform it. return rates are a much more accurate test of injector health since 01-10 DURAMAX injectors are a pressure differential valve. They open by opening the bypass port, and when there is a 1400-1500 PSI differential between the incoming fuel pressure and the pressure inside the inejctor it will open. This is why you need at least 1500 PSI for one to start as you have to have the pressure differential for the injector to open. Return rates tell you how well the bypass valves are sealing and if the ball seats are worn or not.
To get the actual balance rate of your cylinders, you must know the fuel rate/injection amount at the time the balance rates are taken(this will become VERY clear in a few minutes why you MUST know this number). The engine should be at 170 degrees or hotter, at a normal 680 idle(01 idle at 600, and LMM's idle at 720), rail pressure should be 35MPA for federal LB7's, and 30 MPA for most others, the battery should be charged with no abnormal loads on the engine(IE A/C off), and vehicle speed is 0MPH(the ECm only does balance rates when the engine is idling, and vehicle speed is less than 2 MPH). The fuel rate/injection amount should be at 8-9MM3 of fuel with all of these conditions met(at sea level it is safe to use the 8MM3 of fuel number, at 5000+ feet use the 9MM3 of fuel number. VVT trucks(LLY+) with the turbo closed at idle like in stock tuning will be closer to 9-almost 10MM3 of fuel at idle due to the extra engine load to spin up the turbo at an idle with the veins closed. And your in gear under these conditions should be roughly 10-13MM3 of fuel. Here is where knowing the fuel rate is so important and why it MUST be figured in.
Let's use a hypothetical example here to determine actual balance rates VS the displayed balance rates. All above conditions are met, and here is the balance rates in neutral(I will skip the in gear as it is the same, just with a different +/- spec).
#1 -2.1
#2 1.4
#3 -0.8
#4 1.2
#5 -1.4
#6 -1.2
#7 2.1
#8 0.8
Well these numbers at first glance look great and well within the GM specs of +/- 4 in nuetral and +/- 6 in gear(ideal would be 0, but no 2 cylinders are idnetical so don't expect to see all 0's. If you you most likely have a serious problem and the ECM has reverted to fail safe and not doing balance rates), but lets correct them to ACTUAL. The fuel rate should be at least 8MM3 of fuel, but the fuel rate is 3MM3 of fuel for this example. So we need to add -5 to all of these balance rates to get the actual numbers(the displayed fuel rate is 5MM3 of fuel low, so it is -5 off). So lets see the corrected numbers.
#1 -7.1 BAD
#2 -3.6 borderline
#3 -5.8 BAD
#4 -3.8 borderline
#5 -6.4 BAD
#6 -6.2 BAD
#7 -2.9 getting up there
#8 -4.2 BAD
These corrected numbers look quite different, and according to these numbers we now have 5 bad injectors, 2 on the edge, and one other that is starting to get up there. This is because like stated earlier balance rates are just an average of how far off each injector is from the average fuel flow which is your fuel rate. And if all the injectors are worn equally(like the above example), then the balance rates will stay close to each other but the fuel rate goes down. Once you learn to figure in the fuel rate to your balance rates, you will get much more accurate results of injector/engine health. But balance rates are just a test and not the end all be all of injector tests as it is just a small portion of what is going on and can be influenced by MANY other variables.
Lets do one more example to show how much balance rates can change once corrected to actual.
#1 -3.0
#2 0.4
#3 -2.8
#4 1.2
#5 -1.4
#6 -1.2
#7 4.5
#8 5.8
Now these numbers at first glance without figuring in the fuel rate show that #7 & #8 are out of the +/- 4 spec, but lets correct them for having a fuel rate of 3MM3. We need to add -5 to them again.
#1 -8.0 BAD
#2 -4.6 BAD
#3 -7.8 BAD
#4 -3.8 borderline
#5 -6.4 BAD
#6 -6.2 BAD
#7 -0.5 GOOD
#8 0.8 GOOD
So now #7 & #8 are the best of the bunch with corrected numbers, and we have 5 others bad with 1 borderline. This illustrates how critical it is to have the fuel rate to correct your balance rates to actual numbers, and why balance rates are worthless without knowing the fuel rate when they were taken.
Another good thing to do if you have the capability is to raise the rail pressure while checking the balance rates. Leaking ball seats will show up better at higher rail pressures than they do in the idle range, so by raising the rail pressure you can get a better idea of ball seat condition without doing a return rate test.
Keep in mind these numbers are for stock engines with stock tunes. If you have a modified tune it must use stock rail pressure at idle, and the fueling needs to be stock from 0-20MM3 of fuel(If in doubt put it back to stock). And if you have oversized inejctors you must have the tables corrected in the 0-20MM3 of fuel areas so that the puslewidths give the correct flow for each cell otherwise the balance rates will be inaccurate.
Also balance rates should not be confused with return rates. Return rates is the amount of fuel each injector returns to the tank during a 15 second cranking cycle, and involves a test set with 4 graduated cylinders to perform as well as some engine teardown to perform it. return rates are a much more accurate test of injector health since 01-10 DURAMAX injectors are a pressure differential valve. They open by opening the bypass port, and when there is a 1400-1500 PSI differential between the incoming fuel pressure and the pressure inside the inejctor it will open. This is why you need at least 1500 PSI for one to start as you have to have the pressure differential for the injector to open. Return rates tell you how well the bypass valves are sealing and if the ball seats are worn or not.
Last edited:
): How do I determine the actual fuel rate? Is this something that my Edge can read, or do I need some other software / device? (And do not presume I am asking purely from the 6.5 perspective 
