I am planning on changing the oil before I go on my vacation next week. The oil only has about 3000km but its been in there since last fall. I added DEF then as the display told me it was time.
I would like to fill it up at home before I leave. Will this mess up the DEF readings?
I have read that you should not add until the display tells you to.

 

I've done it quite a bit in the summer before towing with no issues.

Just watch it close cause before you know it...it's all over the side of your truck.

 

I keep mine topped off all the time. No issues after 60,000 miles over two trucks.

Bruce

 

Good to know. Thanks!

 

I've topped p often and let the truck tell me... no issues either way - other than the truck took a little longer than I would have liked to reset once. If I am going on a long trip, I like to top off prior to leaving regardless of what the truck says.

 

likely you have a bunch in there and there is no need for more unless you are gonna tow for the next 2-3K miles.

I would not bother myself.

 

Can you get away with it? Of course.

But the chances of it throwing off the miles to empty and going haywire on you increases exponentially. There is a reason Ford doesn't recommend filling until the truck tells you to.

It's like smoking 2 packs a day. You may be one of those who never gets lung cancer, but it doesn't mean others who also smoke won't get it.

 

This! As one who tows heavy often and far, I never bother to fill it till it gets down to 200 to empty.
I take two 2.5 blue def and throw them in the tool box. Replace them after use and drive on.

 

I'm assuming the DEF "gauge" is akin to the fuel gauge meaning that it is a "dumb" gauge (like a dumb terminal) as it solely reports what it sees (full/less than 1/2, etc). Based on that, we fill up our trucks with fuel before the idiot light tells us we're almost empty, we fill up our windshield wash before an idiot light reminds us (if there is one), some people change their oil before the idiot light reminder...what is the harm in filling up DEF before the truck says we have less than 500 miles to go? Does Ford void our warranty or say they won't fix something because we filled up the DEF too soon? Is the technology in our trucks not smart enough to adjust and has a "dumb" countdown based on mileage as it relates to DEF? I'd love to know the truth behind this myth or fact.


Thanks

 

The DEF level is measured completely different than fuel level.

Inside the DEF tank you have 4 measuring "rods". 3 of them are level indicators, and one is the ground if you will. The first measuring rod is about halfway.

This means that if the DEF level is more than halfway full, the truck has no way to sense when you add more DEF. Makes sense so far?

Ok, so when you properly fill the DEF tank when it's empty or close to empty, the computer "resets" the miles to empty. It's an estimate, but it's re-adjusted as the DEF level drops below each marking rod. Refilling an empty DEF tank assures that the computer can see that the levels have changed by going over the indicating rods.

Now the issue. In the background the computer is counting down the miles to empty. However, let's say you keep topping it off before it ever even gets to the first marking rod. Since the truck has no way to ever tell that you filled it back up, it continues to count down. At some point, the truck will just assume that the level indicators are not working, and eventually will get confused and reach that >500 miles to empty. The tank is full, but the system doesn't know it is, and will continue to count down and put you in limp mode.

The computer needs to see the tank emptied and then refilled in order to reset. Continually topping off the tank prevents that from occurring, and may eventually cause problems. Hence why it's recommended to not fill the DEF tank until the indicator on the dash tells you to.

 

Just so everyone knows...Ram has a def guage. But it too goes off of the same tech and depends on the rods as well. They just use an estimate based upon an algorithm.

 

Thank you. That gives me a simple to understand explanation.

 

Well, I think I will throw a couple jugs in the back then.
I got enough trouble keeping my old FJ40 together after playing in the rocks. No sense in messing up the tow vehicle.
This makes sense now, thanks Pocket.

 

Curtis, wouldn't the same be true if you just let it drop below the first rod to less than half full. Once it drops below the first one, it knows it's using DEF and should reset if you fill at that point. I completely understand why you have to let it drop below one sensor but is there actually a difference between that and letting it go further down?

 

Having it go down below half and then filling it gives the computer a chance to see a "refill event". So that should be ok, assuming it's being filled all the way.

Of course... anything can go haywire at any time. Personally I let mine run down until it starts telling me to fill, and that's when I put the 2 jugs in. That just assures me that I know for certain it saw a refill. But to each their own.