:iagree:

Here's is what I think happened to darkside4.8's 35d....

<WAG>
10k - BMW tech flushed and filled the small DEF tank. And either because the tech was unaware of the large tank or in a hurry, the tech left whatever DEF was still in the large tank at that point.

14k - The remainder of the DEF in the large tank is now gone and the small tank is down to the warning level. The big question is did the tech only fill the small tank again? If so, that DEF warning will come in less than 4k miles this time :yikes:
</WAG>

The best evidence we are ever likely to see is HOW MUCH DEF is on the paper work.

Funf Dreisig

And here is what I think happened to k80jr's 35d...

<WAG>
12k - BMW tech flushed and filled the small DEF tank. And the tech left whatever DEF was still in the large tank at that point.

18k - The remainder of the DEF in the large tank was gone and the small tank was down to the warning level. Once again the BMW tech only filled the small DEF tank and didn't touch the now empty large DEF tank.

19k - The small DEF tank dropped to the minimum level (roughly 1/3 full) and triggered the DEF warning countdown again. Hopefully the comments "Also says filled both tanks. Says they checked fluid level via ISTA" means that this time they did flush and fill the large DEF tank :dunno:
</WAG>

Funf Dreisig

i'm going to re-check the paperwork and maybe go in and pop quiz the service manager and tech... see what reactions i get when i talk about not one but TWO tanks.
i love the amount of ideas that fly around regarding this, very helpful. this thread's info provides a great arsenal to use on the dealership.

Being someone who is seriously looking into buying a diesel and doesn't own one yet, I'm wondering if someone can answer why a flush is needed if the passive tank is empty and the active tank is giving warnings. Why would you need to flush an empty tank? Since the active tank will be used first, what's the point of flushing this if you drive it daily? I can certainly understand if during oil change time, the active tank is not empty but only the active tank would need to be flushed, no?

This DEF fluid seems to be one more thing to worry about; especially from a potential purchaser from a moderately cold climate (Canada) who doesn't require this system due to government regulations.

Santo

I'm going to bump this thread.

Our diesel is at the dealer once again for low exhaust fluid, only 900 miles after the last message and 1800 total miles. Once again, I don't believe it is really "low" but the sensor messed up again. We do park our vehicles in a garage too. We've been without our diesel for so long it is getting funny. I'm getting quite accustomed to the E90 loaners and like them more! :rofl:

I suspect the passive tank does not empty completely, i.e., using a pump to transfer DEF from passive to active tank probably leaves a few dregs in the tank and line to the active tank.

So a flush may not be completely necessary, but is also probably a good practice.

If I understand your post....

You only have around 1,800 miles on a new 35d and have already had 2 DEF warnings. The first DEF warning happened at about 900 miles. And the second DEF warning at 1,800 miles. Your 35d is still at the dealer. Is that right?

What exactly did your dealer do the first time? Did the dealer add DEF? And if so, how much?

Please post how much DEF (if any) they add this time.

Funf Dreisig

p.s. For completeness, here is a quote from MINIx Guy's previous post in this thread...

I'm still wondering why the tanks require flush.... is it the underhood heat exposure of the active tank?

In my brief discussions with Freightliner service engineering (they write the service manuals), I've seen nothing in the heavy truck world that says this is necessary. With DEF shelf life issues (although Cummins says if kept below 75F, DEF has a shelf life of up to 2 years), a low mileage car could be affected, but any that drives 10-15K/yr its hard to see any reason as to why flush is 'really' necessary because fluid is added during the year. This feels like a padded service work.

I'm with diesel guy, I'll get fluid and fill myself... that is if I don't go buy a pre-SCR cdi first.;)

My X5d also called for DEF at 900 miles. Dealer had me stop in and they found exactly what MINIz Guy's dealer found (100% in main tank, 66% in passive tank). The temperature was 35F outside.

They checked for leaks and didn't find any. So they gave me a bottle of DEF fluid to have at home and sent me on my way.

This dealer has sold quite a few diesels, and I'm convinced they took it seriously. In any case, no warnings for the last two days when the temps have gotten down to single digits - although my X5d is always parked indoors.

THe Active tank in a 35d is not under the hood. It is behind the fake air intake in the bumper on the right hand side (i.e. in the wheel well between the fender and the wheel well cover).

The area near the DEF filler fittings on our 35d has a very small amount of white crusty residue. I think this is dried DEF 'fumes'. I suspect the 'flush and fill' procedure is to make sure this residue does not build up in the various tubes and fittings. But :dunno:.

I also suspect that the DEF systems in large diesel trucks are not as 'complicated' as the multi-tank system BMW designed with hidden tanks and lots of corrugated tubing, several pumps, etc. These on the road trucks would probably fill their tanks pretty regularly and hence their DEF systems would probably not get as 'crusty' as a low mileage 35d. But once again :dunno:

Funf Dreisig

Edit: The active tank is visible in the first photo of this 35d after an accident.