Question on Balance rates ?

[jbplock]

Questions on Balance rates...

Per the following description of balance rates quoted from the Sept04 GMTechLink, when the balance rate for a given cylinder is positive does that mean it is getting extra fuel? Also, would an over limit positive balance rate indicate the injector is leaking or returning too much fuel? Or is the sign (+/-) of the balance rate not significant?

"Balancing Rates
Normal fuel delivery to each cylinder is around 1 cubic millimeter (mm3) for the pilot injection and 7 mm3 for the main injection. The balancing rates are the measurements of the amount of fuel removed from or added to these normal fuel quantities during each combustion event.

Balancing rates are available and accurate only during stable idle. These numbers tell you whether or not you have a possible compression, injector control, or injector issue.

Once the misfire is detected, the balancing rate of the cylinder that has the misfire will be as high as 15 mm3. The misfire codes (P0300, P0301-P0308) will set only after the first 90 seconds on Federal LB7s (30 seconds on California LB7) of engine operation and will set only when the balancing rate goes above 15 mm3.

The normal values that should be seen on the balancing rate parameters will range between -4.0 and +4.0 in Neutral or Park (-6.0 to +6.0 in Drive when the brake is fully applied). Suspect a possible problem when the balancing rate of a particular cylinder starts to increase out of the normal operating range. An example of a possible problem cylinder would be a balancing rate of 7 mm3 or so. Once the numbers start to increase out of the normal range, the cylinder is progressively contributing less power than the other cylinders."

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[OC_DMAX]

Bill,

A good question. I'll supply what info I have come across. Maybe something can be pieced together.

In Bosch literature, I have seen what GM calls balance rates described in several different ways. Probably depends on the application (car or truck). Balance rates appear to be the output of what Bosch describes as "Smooth Running Control (SRC)" or Adaptive Cylinder Equalization (AZG)". These are functions that are programmed in the engine controller and called during program execution. From a Bosch document I quote:

"Presuming the same duration of injection, not all the engine's cylinders generate the same torque. This can be due to differences in cylinder-head sealing, as well as differences in cylinder friction and hydraulic injection components. These differences in torque output lead to rough engine running and an increase in toxic emissions."

"The smooth-running control function use the resulting rotational-speed fluctuations when detecting such torque fluctuations. By selected variations of injected fuel quantities at the cylinder concerned, they compensate for the torque variations. Here, the rotational speed at a given cylinder after injection is compared to a mean speed. If the particular cylinder's speed is too low the injected fuel quantity is increased and if it is too high the fuel quantity is reduced."

So above is a very top level description of how the software function works that generates the balance rates. Now read what GM wrote "Once the misfire is detected, the balancing rate of the cylinder that has the misfire will be as high as 15 mm3".

Lets speculate now: For the moment lets assume they meant a positive (+)15 mm3. This would make sense because if a cylinder miss fired, the instantaneous rotational rate calculated for the cylinder would be low and the ECM would try to compensated by adding fuel. Adding fuel would be a plus sign (+).

So the vast majority of my balance rates are negative,,,, hmmm what does that mean? (but fortunately all are between 0 and -2, with six of them between 0 and -1.) If the above holds, then one could assume a positive balance rate means fuel is added and a negative balance rate means fuel is removed from the nominal baseline.


Anyone else have some info they would like to present?


Alan

[jbplock]

Originally posted by OC_DMAX:
… If the above holds, then one could assume a positive balance rate means fuel is added and a negative balance rate means fuel is removed from the nominal baseline...

Alan,

Thanks for good Bosch info! I agree with your conclusion… Based on the Sept 04 Techlink and the info you quote above, it does seems that the positive balance rate indicates fuel is being added… Hmmm…

Six of the injectors on my 03 at 33 kmi are running negative and two are positive as follows:

Cyl Park Drive
#1 -1.5 -2.0
#2 -1.6 -2.6
#3 -0.1 +3.2
#4 +1.8 +5.3
#5 -1.0 -2.6
#6 -1.6 -3.1
#7 -2.0 -2.0
#8 -1.1 -2.0

Since the acceptable limit is +/-4mm3 in park and +/-6mm3 in Drive at idle I’m keeping a close watch on #’s 3 & 4. Could this be an early indication of a pending injector failure or are there other variables that make this normal? i.e. “differences in cylinder-head sealing, as well as differences in cylinder friction and hydraulic injection components”

I thought it might be interesting if folks who have access to their balance rates with a Predator or similar scan tool could post and/or email their balance rate data. We could then statistically summarize and plot the data in hopes of identifying some trends. I would be happy to pull this together if anyone is interested… As a starting point I think the pertinent data would be: Truck Year; engine model (LB7/LLY); Mileage; Balance rates at idle in Park and Drive; If injectors have been replaced – mileage on current set of injectors. Any further thoughts?
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[OC_DMAX]

Bill,

You got me motivated, I hooked the Predator up to the truck today and measured the Balance Rates. I have not done this for six months or so. Below is what I measured:

Cyl Park Drive
1 -2.01 -3.14
2 -0.77 -0.96
3 -0.24 +0.87
4 -0.96 -2.04
5 -1.43 -2.77
6 -0.26 +0.79
7 -0.46 +0.58
8 -0.63 -0.20

I wish I had of written all the previous numbers down. I do remember the #1 cylinder was -1.6 before, so looks like things are drifting a little bit. My truck has 22.4K miles / 598 hours. I suspect as the components have more miles/hours on them, the standard deviation between the balance rates will grow.

Probably the best thing to do is to take regular readings (maybe every quarter). Log the readings and look for trends.

I remember DMAXALLI_Tech posting some time ago that anything above 3.0 in park and he became suspicious. It would be interesting to record the balance rates for every vehicle that has a bad injector.

Alan

[Jomar]

Here`s my Bal. rates from 9-18-04
2002 Chevy, 57,613 miles, 1641 hrs., original injectors.

Neutral Drive
1 -1.51 -2.21
2 -0.48 -0.90
3 -0.78 -0.71
4 +0.48 +2.60
5 -1.49 -2.28
6 -0.75 -1.38
7 -0.80 -0.77
8 -0.86 -0.90

Jomar

[jbplock]

I noticed my truck has been running noticeably quieter lately so I checked Balance rates and the #4 cylinder values have dropped significantly.

Previous values at 33Kmi
Cyl Park Drive
#1 -1.5 -2.0
#2 -1.6 -2.6
#3 -0.1 +3.2
#4 +1.8 +5.3
#5 -1.0 -2.6
#6 -1.6 -3.1
#7 -2.0 -2.0
#8 -1.1 -2.0

Latest values at 35 kmi
Cyl Park Drive
#1 -1.5 -1.9
#2 -1.8 -2.5
#3 -0.9 +3.2
#4 -.3 +.35
#5 -1.1 -2.1
#6 -.8 - .94
#7 -1.0 -1.4
#8 -.9 -1.3

For the last few months I’ve been running a double dose of Stanadyne performance formula and 3-4 oz of FPPF Injector Cleaner (per 15 gal)… Could this be helping keep the injectors clean?? Fuel knock at cold idle and between 1500-1800 rpm is also greatly reduced.

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[CHVYMAN#9]

THE BALANCING RATES ONLY OCCUR AT IDLE TO SMOOTH IT OUT. GENERALLY I FIND THAT IF IT IS SMOOTH JUST OFF IDLE(OR NOT IN BALANCING MODE)THAT THE INJECTORS ARE OK. IF IT SEEMS TO HAVE A MISS YOU EITHER HAVE A RICH OR LEAN ONE-THIS CAN USUALLY BE FELT AS SOON AS THE BALANCING RATES GO TO 0 JUST OFF IDLE. I HAVE SEEN A FEW LIKE THIS BUT MOST TIMES HAVE LOUD FUEL KNOCK ON ACCEL. ALSO THAT WHEN USING TECH 2 AND POWER BALANCE MODE WHILE DRIVING YOU CAN USUALLY FIND THE SUSPECT INJECTOR.(AS IT USUALLY GETS RID OF HARD KNOCK).

[OC_DMAX]

They are either being cleaned or the extra lubricity is helping. Next time, run your tank low and then fill up without adding any additive. Take a measurement and see what happens.

[jbplock]

Chvyman & Alan,

Thanks for the replies … my truck runs very smooth at idle and under acceleration. Mileage is also excellent… However, I do occasionally notice what I think is a slight fuel knock from one cylinder. It’s been this way since the truck was new and I always thought it was the nature of the beast until I started checking the balance rates with the predator. I also listened to a few other Duramax trucks while idling and I noticed some had the slight knock at idle and some didn’t. The noise comes and goes and really is not that bad but it seems that it’s become louder with time… I should probably just take it to the dealer and let them use the Tech-II to isolate/disable each injector and see if the knock goes away. The main reason I haven’t taken it in sooner is that I’m not sure how they will react to my added filters. When I asked the service writer at the dealer where I bought the truck if I would have a problem adding a supplemental filer, he said it would void my warranty … needless to say I haven’t been back there. There is another dealer in the area that has experience with the Duramax so I may inquire with them. Alan, I may also try your suggestion to run with out the additive on one tank but I’ll probably wait until spring … it’s getting cold here (17deg this morning ) so I want to make sure there is enough anti-gel protection in the fuel.

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