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Old 05-20-2019, 07:36 PM   #1
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Join Date: Jul 2017
Location: Albuquerque
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DEF question

Every evening on the road, I am top off the DEF container under the hood on our Mercedes Benz Sprinter Class C RV. The capacity is 4-plus gallons. Usually, I add between one and two quarts.
A friend told me I should let the DEF level drop to almost empty.....because, he says, the DEF will crystalize if I keep the container full.
Any truth to this?
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Old 05-20-2019, 09:22 PM   #2
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It crystallizes when it dries. Seems to me keeping it full prevents it from crusting the sides and the sensors. I use to wait till the light said to refill but now I just keep it topped off. DEF does have a shelf life and they do recommend draining and refilling as part of routine maintenance. Not sure of the intervals. I have a tester that I use and so far both Sprinters have tested good.
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Old 05-21-2019, 03:58 AM   #3
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There has been lots of discussion on this topic over on There seems to be 2 schools of thought, run it out and then fill or keep it topped off. There doesn't seem to be a clear winner.

I don't use much DEF in my 2015 Sprinter, approximately a 2.5 gal box per year. I also use a DEF refractometer to test mine. As long as it is good, I just add a box every 8K or so and keep going. I have yet to drain and fill, but will if it tests poorly.
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