Which Def Fluid Should You Use For Cummins, Duramax, And Powerstroke?

Diesel exhaust fluid (DEF) plays an important role in reducing emissions from modern diesel engines. When selecting DEF, it’s recommended to choose a product that meets ISO 22241 quality standards. Fresh DEF helps ensure proper chemical conversion of NOx in the exhaust system. Using a DEF that meets specifications can benefit diesel engine performance and longevity.

In this guide, I have reviewed the best def fluid for Cummins, Duramax, And Powerstroke so that you can make an informed decision.

You can also read my guide on 3.0 Duramax Problems.

What Is Def in Diesel Engines?

Def is a diesel exhaust fluid that contains deionized water, urea, and a small proportion of other additives. Diesel exhaust fluid is also called AdBlue. DEF is 67.5 deionized water and 32.5% urea. The appropriate consumption of DEF in Diesel Engines is around 2% of the diesel fuel used.

Def runs through the selective catalytic reduction (SCR) system. The diesel exhaust fluid is injected into the exhaust stream of the vehicle to remove pollutants like NOx by converting them to nitrogen gas and water.

The SCR system is found on newer diesel engines that meet the stringent emissions regulations set by the Environmental Protection Agency (EPA).

Here’s a video demonstration of the role of Diesel exhaust fluid in the SCR system of a diesel engine.

Also, don’t confuse diesel exhaust fluid with transmission fluid, windshield fluid, or brake fluid. These are totally different and each fluid has been formulated for a specific purpose.

Also, don’t mix DEF with diesel. This is because diesel exhaust fluid contains water and urea that forms crystals in the diesel fuel.

As water is not soluble in diesel, mixing DEF with diesel fuel can also damage the fuel pump as it is not a good lubricant. Moreover, there will be excessive water in the combustion temperature which will affect the combustion temperature.

Can You Use Water Instead of DEF?

You cannot use water instead of DEF. DEF contains deionized water. Even if you want to use distilled water instead of DEF, that will not be acceptable. Distilled water is different from deionized water.

Deionized water does not contain minerals while distilled water contains minerals like magnesium and calcium. If you use water instead of DEF, it will damage the exhaust passages and the SCR catalyst system as the minerals will react with the metal parts of the engine, and impact their longevity.

Moreover, the SCR system also includes sensors that compare the downstream NOx concentration with the upstream NOx concentration.

If you use water instead of DEF, it will de-rate the engine, and the engine control module will also start throwing error codes. This is because the DEF quality sensors will not be able to measure the reduction in NOx.

Ultimately, your truck will fall into limp-home mode because it knows nothing is running through the injectors of the SCR system.

What Happens If You Run Out of DEF?

Diesel trucks need DEF fluid to keep emissions clean. When the DEF tank gets low, bad NOx gases release into the air.

Also, a warning light comes on. The truck’s computer makes the engine slower to limit pollution. This “limp mode” stops big NOx clouds.

Inside the tank, a DEF level sensor watches fluid. If levels drop too low, it tells the computer right away. Then the engine cuts power so the skies stay clean.

What is the Best Brand of DEF Fluid?

Best brand of DEF fluid is the one that meets ISO 22241 standard for urea utility. ISO standards for DEF are 32.5% urea and 67.5% deionized water.

Because of the standard, there really is no better DEF brand than another. Get whatever you like i.e. the one which is the cheapest and freshest.

Best Def Fluid For Cummins, Duramax and Powerstroke

After researching on several forums, several experts are saying that almost all DEFs as the DEF has to meet ISO 22241 specifications and should be API certified.

So, whether you have Ford Powerstroke, Cummins or Duramax engine, you can use Diesel Exhaust Fluid (DEF) that meets ISO 22241 specs and is API certified.

DEFs are not like motor oils where proportion and the type of additives have an importance that set one brand of motor oil ahead of the other. For DEFs, the percentage of urea (32.5) and water (67.5) has been fixed.

So, when you’re buying DEFs, you need to make sure that Diesel Exhaust Fluid is fresh. Under good conditions, the shelf life of DEFs is usually 1 year. DEF will degrade over time depending on temperature and exposure to sunlight.

The optimum temperature to store Diesel Exhaust Fluid (DEF) should be between 20 and 75°F.

Keep out of direct sunlight or hot buildings. Also, you should always note the date code on the label of a DEF jug. Also, check the seal of the jug of DEF before using it.

This is what a person comments in a forum about diesel exhaust fluids:

“DEF is DEF. It has to meet ISO 22241 specifications, therefore it’s all the same. The only concern is if it’s fresh but it last at least a year, two under the correct conditions.”

One person performed a test to determine the relationship between the surrounding temperature and the life of diesel exhaust fluid (DEF). The results are as follows:

TemperatureLife of DEF in Months
≤50 °F36
<77 °F18
≤95 °F6

Several brands are selling DEFs that might confuse you a lot. So, for simplicity, I would suggest you go with the PEAK BlueDEF Diesel Exhaust Fluid as BlueDEF is the most recommended DEF brand by experts and diesel engine owners.

I will also review some other brands of DEF because the fresher the DEF, the better will be its performance. You can buy DEF from any brand I’m going to list below.

1. BlueDEF Diesel Exhaust Fluid By PEAK

This BlueDef Diesel Exhaust Fluid is the best DEF fluid for Cummins, Duramax, and Powerstroke in the market. Here is what a user says about BlueDEF by Peak:

“I’ve always used the BlueDef in the box(es) with no issues. My local auto parts stores and Wally world turn over quite a bit of it so it’s always pretty fresh. I’ve had the same def in the tank of the truck for 2 years before having to fill it. No issues there either.”

BlueDEF By Peak also comes with a convenient new nozzle so that you can easily pour the fluid into your vehicle.

Now, here comes the most complicated thing about BlueDEF, which is the date code so that you can know about the freshness of the BlueDEF. BlueDEF does not mention the expiration date on the packaging. They just mention the date of manufacture. Since the life of a DEF is around 1 year. You can easily understand whether you should use the DEF fluid or not.

BlueDEF uses two different date codes on the packaging of their box. I’m really annoyed why they can’t stick to the one. I think they scramble the product dates of DEF fluid because they do not want you to know if you are buying old DEF.

Usually, the date code on the packaging of the BlueDEF is something like GA233590089.

  • GA: The designator of the plant that manufactured the DEF
  • 23: It is the year of production plus 1. So, the year in which DEF fluid was manufactured is 2022.
  • 5th, 6th and 7th digits: 3, 5, and 9 show the day of the year on which the DEF fluid was produced. Subtracting 359 from 365 (no. of days in a year) gives 6. So, it means the DEF fluid was manufactured on the 6th day of the year i.e. 6th January 2022. 
  • 0089: It is the batch code.

I heard that nowadays i.e. at the time of writing this guide, the date code stamped on the packaging of BlueDEF fluid is changed. If you find the date code on the BlueDEF box as “122100362274″, it can be read in the following way:

  • 12: It is the facility code where DEF was made
  • 21: Same as the previous code i.e. year of production plus 1.
  • 003: It shows how many days are left in a year. So, it means the DEF was manufactured on the 29th of December 2020.

2. AC Delco DEF

AC Delco is recommended by GM for its Duramax engines. Users have had great experiences with their products. However, sometimes I think there is a market tactic behind due to which General Motors is recommending AC Delco DEF in the owner’s manual.

One user faced the issue that using BlueDEF in Silverdo triggered the bad DEF quality trouble code. But, according to this forum, people are also satisfied with BlueDEF.

I don’t think there is any chemistry behind it as all DEFs have the same proportion of deionized water and urea. So, after reading reviews on different forums, AC Delco and PEAK BlueDEF diesel exhaust fluids are exactly the same.

You can get the fresher one. However, let me clear one thing, AC Delco sells a pack of two 2.5-gallon jugs in one box. There is no need of ordering 5 gallons at a time.

AC Delco does not use a funny date code like BlueDEF. You should find the date about 3/4 the way down the back of the jug.

Important Points When Using DEF

Here are some important points when using diesel exhaust fluid.

  • When there is overdosing of diesel exhaust fluid, DEF may not fully hydrolyze which leads to the crystalized deposits in the exhaust or injector nozzle. Over-dosing of diesel exhaust fluid can result from a partially open DEF injector nozzle, plugged DEF lines, or DEF pump issues.
  • Contaminated DEF can cause abrasion of the DEF pump. Rust, dirt, or scaling on the walls of the DEF tank can occur with improper DEF storage or handing. That’s why it is always recommended to first check that the container of DEF is properly sealed before you use it. When you use DEF, make sure that it is crystal clear. Check the inside of the jug/storage container of DEF for any dirt or debris.
  • Make sure that DEF is not too diluted (too much water). Otherwise, urea will not be able to react with NOx in the exhaust gases and neutralize it. You should have a refractometer to check the dilution of DEF fluid. Refractometer tells you specific gravity which basically indicates the percentage of water vs. urea. If the refractometer shows the specific gravity of DEF as 32.5% +/-1.5% by weight, it will be considered good.
  • At colder temperatures, DEF can freeze which results in preventing the engine operation if there is no required concentration of urea and deionized water in DEF. At 32.5% of specific gravity, both the urea and deionized water in DEF will freeze at the same rate at 12°F, ensuring that as it thaws, the fluid does not become diluted, or over concentrated. The freezing and unthawing of DEF will not cause degradation of the product. That’s why it is not recommended to add additives in the DEF to thaw the DEF as additives can change the concentration of the DEF fluid and damage the SCR system.

What About Bad DEF Trouble Code?

So, you might be wondering what you should do if the engine starts throwing a bad DEF fluid error code? Well, there are quite less chances that DEF is bad. DEF quality error code can be due to the following reasons:

  • Bad DEF sensor
  • Faulty NOx sensor
  • DEF tank is overfilled
  • Crystals around DEF cap and injector

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