DEF question

My truck burns DEF at the rate of a gallon per 750 miles driven. If I drove less than 5,000 miles in total during the summer months with 100 degree temperatures I would avoid refilling the DEF tank until the fall.

DEF deteriorates from heat and sunlight so in hot weather it is best to buy what you need at the pumps at truck stops and Shell gas stations with diesel and not buy the 2.5 gallon containers - you also pay less that way.

Test Data on Thermal Stability of DEF from my DEF Technical Library:



This test data correlates well with the Arrhenius equation for the rate of DEF hydrolysis.

Note that in the vehicle's SCR system, the water in DEF rapidly evaporates in the exhaust stream resulting in fine urea particles which then decompose to ammonia which reacts in the vehicle's catalyst to convert oxides of Nitrogen to Nitrogen, water, and CO2.

DEF Storage guidelines apply to storage in the original packaging, not the vehicle tank. Per ISO22241-3, the international standard for DEF:



DEF is pure water (distilled, deionized, reverse osmosis) and technical urea (very pure urea made on a dedicated chemical process line). DEF decomposes to ammonia (and other compounds) when exposed to heat. The issue in sealed packages that decomposition pressurizes the container and can cause issues with leaks. The DEF tank in clean diesel vehicles is vented so no pressurization can occur.

ISO22241-1 (the technical specification for DEF) limits ammonia content to .2% (1/500) by mass at the end of storage life. This is primarily to assure that transportation and storage limits for ammonia (which is classified by the EPA as a hazardous material) are met. DEF exposed to higher temperatures in a clean diesel vehicle tank decomposes slowly, even at 140F, and can be subject to slight evaporation. Ammonia vents from the tank. Neither of these effects are important for the SCR system which compensates by injecting slightly more DEF to convert NOx to nitrogen and water vapor.

Temperatures in F:

10C = 50F
25C = 77F
30C = 86F
35C = 95C

DEF life expectancy
Temp( deg F), life expectancy
32, Infinite
50, 75 Years
68, 11 Years
86, 23 Months
95, 10 Months
104, 4 Months
122, 1 Month
140, 1 Week.

I had been adding DEF every 3-4,000 miles - will start adding when the low level warning first comes on - will add about 2 gallons. There are no drains on the GM tanks and it can not be siphoned out - tank needs to be dropped to empty it

Unless you don't drive your truck much you will need to fill up your DEF every 7,500 miles or so. Less if you are towing. I wouldn't stockpile a supply, but I wouldn't worry about it going bad in the truck.

The bigger issue is sometimes finding it when you really need it. Last year towing in the West I had to break down and buy some at a NAPA. Normally I try to get it at a truck stop in the big boy pumps they have it at the pump for a much nicer price.

brooks379 wrote:
Impact on the quality of DEF per Transliquid technologys LLC

Q. What is the shelf life of DEF? A. The shelf life of DEF is directly related to the ambient storage temperature. DEF will degrade over time depending on temperature and "exposure to sun light". If stored between 10 and 90 deg F, shelf life will easily be one year. However, if the maximum temperature does not exceed approximately 75 deg F for an extended period of time, the shelf life will be two years.........

IMO....If you don't drive it much don't fill it all the way just put in a gallon at a time so you're always running good def.

I agree. I will never top mine off again. I had a few issues, from here on out it will be 2.5 gallons max when it show low. My truck goes about a 1000 per gallon of DEF. They only way I would top it off is if I was leaving on a long trip and even then not really necessary and you can get DEF at many many locations.

Impact on the quality of DEF per Transliquid technologys LLC

Q. What is the shelf life of DEF? A. The shelf life of DEF is directly related to the ambient storage temperature. DEF will degrade over time depending on temperature and "exposure to sun light". If stored between 10 and 90 deg F, shelf life will easily be one year. However, if the maximum temperature does not exceed approximately 75 deg F for an extended period of time, the shelf life will be two years.........

IMO....If you don't drive it much don't fill it all the way just put in a gallon at a time so you're always running good def.

Not really, at 100 degree's it has a shelf life of many months - or several years.