# Do we really have to purge DEF if it's not old?



## mecodoug (Nov 30, 2007)

If I drive 20,000 miles per year, the DEF will never get old. Won't I just be able to top it off every 6 months or so myself and avoid the time consuming and expensive (after warranty) trip to the dealer?


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## d geek (Nov 26, 2008)

mecodoug said:


> If I drive 20,000 miles per year, the DEF will never get old. Won't I just be able to top it off every 6 months or so myself and avoid the time consuming and expensive (after warranty) trip to the dealer?


Is there a specified shelf life for this stuff? If not, I don't know why you couldn't just keep it topped off.


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## Penguin (Aug 31, 2003)

d geek said:


> Is there a specified shelf life for this stuff? If not, I don't know why you couldn't just keep it topped off.


The shelf life is generally considered to be 1-2 years, depending upon the storage conditions (2 years means keeping it below 75 degrees F). So it would seem a periodic flush would be advisable on low mileage diesels.

Here's a handy reference:

http://www.fleetguard.com/pdfs/product_lit/americas_brochures/MB10033.pdf

That previous link was Cummins; however, I expect that due to the rather precise and stringent specifications for DEF, all brands of DEF will likely have similar stability.

http://www.truckline.com/AdvIssues/Energy/Diesel%20Exhaust%20Fluid%20Documents/Diesel%20Exhaust%20Fluid%20%E2%80%93%20Frequently%20Asked%20Questions%20%28May%202009%29.pdf


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## d geek (Nov 26, 2008)

thanks for those informative links on DEF, Penguin.

If the OP drives 20K mi/yr, and BMW has presumably sized the tank to refill coincident with oil change, then why would he need to purge?


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## Penguin (Aug 31, 2003)

d geek said:


> thanks for those informative links on DEF, Penguin.
> 
> If the OP drives 20K mi/yr, and BMW has presumably sized the tank to refill coincident with oil change, then why would he need to purge?


That why I said, "on low mileage diesels."

But if one is a purist, some of the old fluid will mix with the new fluid if it is not purged. You could "run it until it is virtually empty" to minimize this effect, but even then the heated tank would have some old fluid in it, and I suspect the unheated tank does not get pumped-out completely in normal operation. The impact of this residue of old fluid may be significant or not. I suspect it is not, but I do not know the stability chemistry of DEF and whether or not the presence of a small amount of of fluid would promote the breakdown of new fluid.


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## d geek (Nov 26, 2008)

Penguin said:


> That why I said, "on low mileage diesels."
> 
> But if one is a purist, some of the old fluid will mix with the new fluid if it is not purged. You could "run it until it is virtually empty" to minimize this effect, but even then the heated tank would have some old fluid in it, and I suspect the unheated tank does not get pumped-out completely in normal operation. The impact of this residue of old fluid may be significant or not. I suspect it is not, but I do not know the stability chemistry of DEF and whether or not the presence of a small amount of of fluid would promote the breakdown of new fluid.


I see.
Well it seems that BMW has planned ahead knowing that the DEF has a shelf life, and incorporated the flush/fill process into their service plan. I'd think that people who drive > 15K mi/yr (default OCI?) would be OK topping off the DEF, and (per your informative links) the system would let you know if the NOx reduction were not up to snuff.

After the "free" maintenance I'd just top it off if I were you, mecodoug.


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## mecodoug (Nov 30, 2007)

d geek said:


> After the "free" maintenance I'd just top it off if I were you, mecodoug.


That is my tentative plan, subject to the next 2 or 3 years' of others' experiences and issues.

The dealer I like to use is 70 miles away, and after warranty being the cheap yankee I am I will do most of the general (fluids filters brakes etc) maintenance myself, so I'm hoping just filling up DEF at the ~250 miles left point will work. If that works others who drive less might be able to do the same but never fill it more than halfway.


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## finnbmw (Jul 6, 2008)

*How to fill the passive tank?*

Does anybody know how to fill yourself the passive tank? If so, can I ask to put the DIY instructions as separate thread for the DIY inclined...

OP's question is interesting to me as I drive typically over 25k a year.


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## d geek (Nov 26, 2008)

what does the owner's manual say?
the MY09 online version of the x5d has step by step instructions.


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## DC335i (Nov 2, 2006)

You may want to try and compile experiences from others on this - I believe the fluid is used in many diesels aside from the 335d, including the larger VW diesel (the one in the suv).


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## Penguin (Aug 31, 2003)

d geek said:


> what does the owner's manual say?
> the MY09 online version of the x5d has step by step instructions.


I believe that instruction is only to fill the smaller active, not the larger passive tank.

Though I suspect a funnel might work on the passive tank, if filled slowly.


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