My 2014 E250 Bluetec has a DEF tank that holds 6.5 gallons. I drive the car about 5000 miles per year and that translates into about 2 gallons of DEF per year.

My understanding is that, if properly stored, DEF can last up to two years from manufacture date. But I'm assuming that DEF in the tank will degrade a bit faster. What are the potential adverse effects of having DEF fluid that's a few years old in the tank? I'm wondering if I should periodically drain the tank and put fresh fluid in there.

Maybe I'm worrying unnecessarily....but the SCR system is complicated and repairs can be expensive. I want to do everything I can to ensure its longevity.

Thanks.

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Lake Speed Jr. has a You Tube channel called the; "Motor Oil Geek."

It's a very informative channel, and might address Def.

In regards to motor oil aging in the original containers? Lake devoted a video to that. His answer was if you see coagulated product at the bottom of the container, don't use it.

If your mileage is using 2 gallons of Def per year, I would not keep a whole lot of Def in the reservoir.

You can always pump some out occasionally.

Are the Def reservoirs easily removed to inspect? If so, it'd be an exercise to remove one to see if any coagulation is taking place in it. Just an idea.

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'06 E320 CDI
'17 Corvette Stingray Vert

Water down your old def and apply it to your grass. Best use for the stuff. Remember it's just urea.

Unused DEF needs to be replaced every 2 years.

I'd still pour it on the grass.

If you're driving a diesel that needs DEF, you need to reverse-engineer your thinking of driving a diesel that needs it.

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'06 E320 CDI
'17 Corvette Stingray Vert