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Old 07-31-2019, 07:31 PM   #1
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Default DEF replacement

I have a 2017 sprinter Pleasure way Ascent. it's about time to add DEF and I want it full before leaving for our next trip. Is it ok to just add as needed or best to wait until the light turns on to add DEF?
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Old 07-31-2019, 07:45 PM   #2
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Add as needed is when the light turns on. I think it is best to top off before any trip as routine. Generally most trips then you will not need to add DEF while on the road nor need to carry a spare container in you B though you can easily buy it on the road at Walmart, any auto store or truck stop.
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Old 07-31-2019, 07:48 PM   #3
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That is a highly fraught question. I really don't think anybody knows for sure.

I used to be a strong advocate for waiting for the light to come on. That was until I got a CEL light along with a "10 starts remaining" message 5 miles after filling our near-empty DEF tank with cool DEF from the truckers pump along the highway on a very hot Texas day. Of course, it could have been a coincidence, but...

There is a credible theory that there are bugs in the DEF system firmware that can be triggered by a normal "add DEF" condition, and that it is best to keep it topped off in the spirit of letting sleeping dogs lie. Who knows?

There is another theory that letting the tank get empty "exposes the sensors" (or some such) causing them to fail. I do not believe this theory.

I now refill my tank when I estimate it is down about halfway and don't wait for the warning. May or may not be necessary, but there are few arguments that frequent fillings are harmful, as long as you are careful not to contaminate the system in the process.
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Old 07-31-2019, 08:58 PM   #4
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I filled my tank to full regularly before any warning lights, in fact I never had any warnings about low DEF before my DEF saga begin in Texas early this year.

The DEF assembly has 2 level sensors from some kind metal -2 rods, temperature sensor is fully encapsulated in plastic. The heater is the only one which could possibly fail with a very low DEF level, heater is located on the bottom of the DEF assembly.

When we got stuck in Texas, we heard from the service manager that “the tank was dried so the heater fried”. I wouldn’t be shocked if this statement is used as a talking point. In my case I paid for replacing the heater even thou it was the temperature sensor which I learned after measuring the replaced assembly.

Being a little paranoiac after this warranty ordeal I tend to fill my tank as often as it is practical.
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Old 07-31-2019, 09:06 PM   #5
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I wait until the gauge shows slightly less than half. I then open and add a complete 2.5 gallon container of DEF from Wally World. That way I stay away from the potential problem of dry sensors, and I can add the complete container with no overflow or spillage. I find that I'm using about 1 gallon of DEF for every 1,000 miles driven.
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Old 07-31-2019, 09:14 PM   #6
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Quote:
Originally Posted by DUTCH in Atlanta View Post
I wait until the gauge shows slightly less than half. I then open and add a complete 2.5 gallon container of DEF from Wally World. That way I stay away from potentially dry sensors and I can add the complete container with no overflow or spillage. I find that I'm using about 1 gallon of DEF for every 1,000 miles driven.
That is similar to what I try to do. Note however that:
1) Older Sprinters do not all have DEF gauges, so sometimes you just have to use dead reckoning.
2) There are several (at least three) different size DEF tanks in Sprinters, so one's strategy has to be adjusted accordingly.
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Old 07-31-2019, 09:21 PM   #7
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Thanks everyone for your input. I'll be erroring on the side of caution and filling before my light comes on. The dealer service folks told me that I should only need to fill every 8,000 miles and there no gauge to show me a level before the light comes on. That just drives me crazy as I always like to know my fluid levels before a trip.
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Old 07-31-2019, 09:29 PM   #8
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Quote:
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there no gauge to show me a level before the light comes on. That just drives me crazy as I always like to know my fluid levels before a trip.
I am surprised that a van as new as yours doesn't have a gauge. Are you sure it isn't buried in the menus somewhere?
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Old 07-31-2019, 09:36 PM   #9
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I agree! I may have to dig a little deeper. But the person at the Mercedes service said there wasn't anything on the menu so I stopped looking.
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Old 07-31-2019, 10:14 PM   #10
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Quote:
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I agree! I may have to dig a little deeper. But the person at the Mercedes service said there wasn't anything on the menu so I stopped looking.
I will refrain from editorializing about the likely quality of anything a Mercedes service advisor tells you.

Check your manual.
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