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Old 05-30-2018, 04:07 PM   #21
ptheland
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Default Re: No DEF warning light

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Originally Posted by Vinz View Post
I have been wondering... do you just top up the DEF fluid up to the fill cap. I have looked for a level indicator in the reservoir, but I see nothing.

I saw someone mention that they do top it up and give it a bump to burp it and add some more. Is that the general consensus? I don't want to have to think about it, I just want to go out once a month and top the thing up as full as I can and forget about it until the next month...
There are several different DEF tanks and layouts. My specific DEF tank is under the side door - roughly opposite the fuel tank - with the filler under the hood. That means there's several feet of tubing to get from the filler to the tank. Others have a tank closer to the engine. Still others (cab and chassis models, often made into motorhomes) have a tank and fill near the house entrance. These all have different capacities. And there might be another configuration or two that I've missed.

Frankly, it doesn't matter how you fill it up. "Burping" probably only applies to layouts like mine - with a long tube between the tank and the fill neck. And even then, I'd only worry about a burp if the fill was considerably less than I'd expect. (BTW - it helps to fill while parked on a good slope with the nose uphill in my layout.) The point of the fill-often crowd is to keep the DEF tank mostly full, which hopefully will lessen future problems with the electronic components inside the tank. It doesn't matter if you get the last cup of DEF in or not, as long as you keep the tank mostly full.

Unless you're driving 5k miles a month or more, your plan to top off once a month sounds good.
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Old 05-30-2018, 05:07 PM   #22
avanti
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Default Re: No DEF warning light

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Originally Posted by ptheland View Post
The point of the fill-often crowd is to keep the DEF tank mostly full, which hopefully will lessen future problems with the electronic components inside the tank.
That is one theory, but I don't think it is the dominant one (at least it is not the one that convinced me). The most substantive argument for keeping the DEF tank full seems to have originated by a series of posts by Dennis, including this one. In brief, Dennis' argument is as follows:

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Originally Posted by lindenengineering View Post
...Overall it would seem that the system would lay dormant until it got woke up by a low DEF fluid levels then any faults that were there in stored codes then became hard active problems.
Of course communication in Sprinters and MB vehicles in general (in fact all German brands) now and again do present some difficulties and the trade mechanics who work regularly on these imports will explain repairs these vehicles in one word--- Quirky!
Dennis
Mechanic
So, the argument as I understand it is not that disturbing sensors (or whatever) in the tank was the problem, but rather that letting the "add DEF" warning occur triggers buggy diagnostic routines that stir up latent issues that otherwise would remain benign.
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Old 05-30-2018, 05:31 PM   #23
ptheland
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Default Re: No DEF warning light

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Originally Posted by avanti View Post
That is one theory, but I don't think it is the dominant one (at least it is not the one that convinced me).
Ok.

But the point of my way-too-long screed is that it's not necessary to dribble the last little bit of DEF into the tank. Drive a few miles, and the level is going to drop a bit anyway as the fluid is used.

What matters - no matter why you believe in it - is to top off regularly.
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Old 05-30-2018, 06:22 PM   #24
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Default Re: No DEF warning light

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Originally Posted by ptheland View Post
Ok.

But the point of my way-too-long screed is that it's not necessary to dribble the last little bit of DEF into the tank. Drive a few miles, and the level is going to drop a bit anyway as the fluid is used.

What matters - no matter why you believe in it - is to top off regularly.
Yep. I aim for a refill when it is roughly half-way down.
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