Duramax Diesel Truck Balance Rates
Duramax Diesel Truck Balance Rates
If you’re big into diesel engines, especially the Duramax diesel engines, like we are, you have probably heard of balance rates. You are likely even familiar with checking them so that you can determine the health of the injectors in your Duramax. But do you know what they really mean, and how they work? Did you know that the simple numbers can be misleading without looking a little further? Balance rates can even be used to identify and help fix other mechanical problems, too. Since we see many people online searching for "lb7 injector balance rates", "duramax injector balance rates", "lbz injector balance rates" and many more phrases along those lines... we wanted to put together a great explanation to help bring some clarity to everyone.
To start, let’s briefly touch on what a balance rate is. A Duramax truck with stock injectors and tuning, at operating temperature, should idle with a calculated fuel rate (CFR) of 7-9 mm³ – and that is the baseline, i.e. balancing point, from which each of the individual cylinders gets its rate from. So, for example, if a truck is running at an 8 mm³ CFR, and there are cylinders at +3.4, -2, etc., it means that those cylinders are 8 + 3.4 (11.4), 8 - 2 (6), and so on. This means that even though those balance rate numbers are within the acceptable industry standards of +/-4 mm³, there is a variation of almost 100% in the actual flow rate of fuel between the two!
Now, the variation itself isn’t the biggest issue we see with balance rates. With aging diesel engines, we often see them belching smoke under load, suffering a lack of top end performance, or throwing out a haze under idle conditions - despite acceptable balance rate numbers. Why is this?
A lot of the time, the answer is just plain old worn parts. As the injectors wear out, the high fuel pressure (in excess of 26,000 psi) causes fuel to leak past the worn areas, and more fuel reaches the cylinder than the computer is commanding. So to compensate for the extra fuel, the ECM dials the CFR back to say, 2 mm³ instead of 8 mm³, meaning that there’s an actual discrepancy of 6 mm³ in the reported amount needed to maintain a 700 RPM idle. These trucks will start fine, and run fairly smoothly…but then the problems begin, when more power is applied or they idle for a while.
Say you have a truck running at 83° C with a 2 mm³ CFR, a tolerance of +/- 2 mm³, and your balance rates are shown as:
On the surface, these are all within the industry specs, but when you factor in the low CFR (i.e., adding in the missing 6 mm³ that should be present in the balance rates), it becomes clear how these “good” balance rates are actually skewed very heavily:
Out of those, how many are within two mm³ of the CFR? Only Cylinder 5, at 4 mm³, is even close – which means that 7 out of 8 of these would be considered beyond saving, and we wouldn’t even do just 7. That last one would be replaced to match the rest, because we want it done right the first time, ensuring you don’t have to come back in a month to do it again.
This lays out why a truck with good balance rates can still be in need of all new injectors in their Duramax. But that’s just one area – the worn injectors! The balance rates can also indicate mechanical issues with the engine itself, because they are a function of cylinder contribution – meaning that you can use the numbers to find out if a specific cylinder is low on compression due to valve issues, a bent connecting rod, a cocked piston, broken rings, and so on.
Let’s take a look at the engine stats below, for the same engine at 83° C with an 11 mm³ CFR:
TuningWe can see that Cylinder 6 has a huge compression loss issue, from something mechanical going on, and the truck is running that cylinder at 25.5 mm³ (11 + 14.5), while the others are around 4-5 mm³. Even the high CFR itself, at 11 mm³, is often a sign of something going wrong – it’s common on engines with compression differences of this size, and may be mistaken for a bad injector. Further tests may be required to really get to the root of the problem.
At Schultz Diesel Sports, we strive to diagnose the engines we see, properly and quickly. We hope that this gives you some insight to the calculations behind balance rates, and how they work and what they mean – so that if you encounter trouble of this type, you know the background of the problem and how to identify it, too. That way, it can get fixed faster, and you’ll be back on the road enjoying your Duramax in no time.
A product that we recommend is Edge Insight. It shows you your balance numbers and your CFR in one easy-to-use display, which is why we sell them off the shelves and put them in our own trucks. Of course, there’s still a lot more to learn about diesel engines and all the science that goes along with keeping them running at their best – so if you ever have any questions about your Duramax balance rates or anything else to do with your truck, don’t hesitate to reach out to Schultz Diesel Sports... especially if it is about Diesel Truck Tuning!
7/18/2020 05:22:31 am
I have a 2004 Chevy duramax 3500 series. It has 320.000 miles on it. When it broke down I thought it just blew up. Engine had no power and idling violently. Parked it for a couple of weeks and decided to start it up. Ran like a top.but after a few test runs it did not again. I don't want to sell it for scrap.my local mechanic put it on the computer and said that #5 was week . Could the injectors be the problem? Would it be worth it to replace the injectors ?
1/30/2021 03:31:07 pm
When I run it down the road around 50-60 speed, it' lose power I got to pull over and cut it off then start it up and it's ok for a while then it does it again just a few miles, but it doesn't do it in town only on the highway
1/20/2022 07:00:27 am
This sounds like a problem that I had. Mine turned out to be a common issue with two of the injector wiring harness plugs going bad. There was a pigtail replacement kit that is made specifically for that issue. Fairly simple fix....just cut the old connectors off and wire in the new pigtails. Not sure if this is your problem or not but it is something to check into.
10/9/2020 01:43:09 pm
I am learning about diesel engines and I love the way Duramax ones' feel. I don't know a lot about balance rates. I'll keep what you said in mind about learning more about them and all the science that goes into an engine.
10/16/2020 04:27:49 pm
Great blog :)
12/16/2020 09:51:59 am
Do you do 2.8 ?
2/22/2021 03:26:24 pm
I have an lly duramax with new injectors put in about 15k miles ago.
5/3/2022 06:22:43 pm
Hi, I have a 2014 Duramax and after deleting and tuning, it is giving codes of P01DA-Cylinder 8 injection timing performance-over advanced, P02DB-Cylinder 8 fuel injector offset learning at Max Limit, and P02CD- Cylinder 1 injector offset learning at max limit. Balance rates all showed very close to 0 with the worst being 1.6. No codes before deletion and tuning. Could this be sign of bad injectors that only appeared after tuning? Thanks!
6/28/2022 05:56:44 pm
I have my first diesel truck a 2019 dmax it just gave me my first headache with it. It has a service emissions soon power derated that reset once or twice. 70,000 miles and the dealer says it showed p0205 and says for just $2400 they will replace the injector high pressure pipe the connector and the air filter. Would the balance rate be able to tell the difference between the injector and the electric connector? Internet forums suggest replacing the connector and re evaluate. My worry is if the injector is pouring diesel into #5 cylinder it will take the whole engine out.
12/12/2022 01:15:48 pm
I'm contacting you with a tuning question. I have a 2005 Silverado with a 6.6L Duramax LLY. It has EFI LIVE and a tune from Relentless Diesel. It runs great, but it only has a single tune, haul ass. I would like to keep that tune or have a similar tune but add at least an economy tune. I get about 15mpg on that tune, I'd like an economy tune with some better fuel economy.
1/6/2023 09:49:24 am
Interesting article.......and sort of helpful.
2/8/2023 01:26:06 pm
You're correct that the math above is incorrect. And you're math is correct (except that the first CFR difference is -6). The formula for sea level is:
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