I'll try to explain in some detail to help understand how the system works. When it's understood how the system works, then it's easier to appreciate its simplicity and makes it easier to maintain.

Because DEF freezes, you can't have the same type of level indicator that say is used in the fuel tank. So a different type of system was put to use.

The DEF tank uses 3 level indicator rods, and 1 ground rod. When the level indicator rods are submerged, the idea is that the PCM can "see" that the tank is at least above those marks.

The thing is that the first level indicator rod is around the halfway point. At any time above half-full, there is no way for the PCM to see the actual level of the tank.

For EPA mandated speed/idle limitations when the DEF tank is empty, the system employs a miles-to-empty countdown. When you fill up an empty or near empty DEF tank, that countdown resets because the system now sees a "refill event". As the DEF levels drop below each level rod, the PCM will re-adjust that countdown to follow along the DEF usage. When it gets close to empty, put DEF in and another reset happens. And so on and so on.

Now let's say you keep topping off the DEF tank before it reaches the first level indicator. The system cannot see that the tank is continually being filled, and the miles-to-empty keeps counting down. At some point the system will just assume the level indicators aren't functioning and force a count down, and that's when you get folks with completely full tanks but with warning indicators showing less than 500 miles to empty. At that point they are required to take their trucks to the dealer for a reset.

By simply following the owner's manual as to how to properly fill DEF, you avoid the countdown issue (unless there's some sort of system failure of course). The system is simple yet very reliable when proper fill procedures are used. Altering those procedures can at times cause adverse effects, similar to any other maintenance or driving issue.

The dealer employee told you it was ok to fill because he/she hadn't personally seen issues with topping off DEF tanks. Plenty of dealer employees have never experienced "death wobbles", or the 15+ regen engine shake issue, etc. But just because that employee never saw an issue, doesn't mean it hasn't popped up somewhere else.

Topping off the DEF tank is a known issue, albeit not a widely known one because it doesn't happen extremely often. Some people continue to top off the DEF tank and never see a problem. Others are not so lucky. In the end it's your truck, so you can decide if you want to keep the tank topped off or not. Just know that if the miles-to-empty countdown doesn't reset, you'll at least know the reason behind it.

Hope this helps.

 

Great explanation. I don't think it gets any clearer than that.

 

My dealer not only told me that topping off is not an issue, they actually top off the DEF tank for me at every oil change.

I've never had an issue and will continue topping off. From time to time, I travel to parts of my state where I don't know where I would get fresh DEF from without a long drive. I'd prefer to leave home with a full tank of DEF and not worry about it.

For others that are passionate about running the tank dry, I approve. Sounds like a great plan for others, just not me. If I have DEF trouble, I will report back on the problems and admit that my topping off was the cause.

bruce...

 

Great explanation!

I have a 16' F-250 Lariat Ultimate, I recently about 3-4 weeks ago did my first oil change at 3500 miles, got everything from Wal-Clown (MC 10w-30 oil - 3 5quart jugs and a MC Oil Filter) & I got a 2.5GA of ST DEF.

After doing the oil change, I remember hearing from the ford dealer where I purchased my truck that they typically top off the tank anytime after performing a OCI. So I once done w/the OCI filled up my DEF tank, surprisingly at 3500 miles, it took all of the 2.5 gallons.

Now on my dash I didn't see any indication telling me that I was down on DEF and was actually surprised that it took all of it. I did see a date stamp on the DEF jug and I got one that was the latest I could find.

Just figured I'd share my experience.

~Val

 

Now that you know it is probably close to full, don't fill again till you get the count down reading.

 

Next OCI will be done at the Ford dealer at 10k for Free and they will do the DEF fill for me.

 

Curtis - Thanks for the explanation on how the system works. Yes it makes sense (the engineering). What does not make sense, to me, is the lack of distinct verbiage in the owners manual that would instruct us to not add fluid until prompted.
Apparently my dealer was under the same belief I was and treated the DEF reservoir in the same manner as windshield washer fluid as they top off both as long as the truck is being serviced.
I'm curious now if I should just siphon out half a tank just to get the system in sync?

Thanks..

P.S. I will be printing off this explanation of yours and sharing it with the owner and service manager at my dealership.

 

I guess that's where you and I differ on opinion, as to me it seems to be very distinct to not add fluid until prompted.

But I do see where you're coming from. I guess what you're looking for is "don't refill until prompted or else 'X' could happen". But then again there's plenty of other scenarios in the owner's manual that say to do something without fully explaining the why portion.

I'm sure there's someone who goes over the wording of the owner's manual to give enough direction on usage without creating concerns for the average owner. Sometimes that leaves more to be desired from the manual.

 

Man, am I glad I have a gasser!

 

Takin' a walk on the WILD side I see.....

 

Once you burn oil, you don't go back.

 

Once you get BURNED you go back!

 

LOL look at this video what they say at 5 :43 in too video i am done 40,0000 miles no issues

 

Val, did you really buy oil in 5qt jugs? That is usually gas engine oil, not diesel oil. I've only ever seen diesel oil in gallon containers. There is a different spec oil for diesel engines.

Also, if you put 15qts in, you are overfilled by 2 qts.

Don't want to see you damage your truck!

Dave

 

Topping off the DEF tank before it reads less than half full, and filling up the tank when it's below half full, are two completely different things. The video did not recommend topping it off before it showed it was below half tank.