How to Put DEF (Diesel Exhaust Fluid) in a Truck & Which is Cheapest?

How do you put DEF fluid in a truck without spilling a drop? Here are a few tips for diesel truck owners out there as well as lots of helpful info about Diesel Exhaust Fluid, what it does and where we found it’s cheapest to buy.

Since 2010, diesel pickup truck engines have relied on Selective Catalytic Reduction technology (SCR), which uses Diesel Exhaust Fluid (DEF), to meet the EPA emmissions standards. DEF is a mixture of 1/3 urea and 2/3 de-ionized water, and it is sprayed into the exhaust system of diesel trucks to reduce the nitrous oxide (NOx) emissions into harmless nitrogen and water.

Pouring Diesel Exhaust Fluid DEF fluid into truck

Late model diesel trucks require refilling the DEF tank!

Each manufacturer designs their trucks with a tank to hold the DEF, and you have to replenish it every so often. Our 2016 Dodge Ram 3500 has a 5 gallon DEF tank. The filler hole is under the gas cap next to the diesel filler hole. On some brands of trucks the filler hole is in the engine compartment under the hood.

Dodge Ram 3500 DEF and diesel gas cap

The Dodge Ram has the DEF filler hole next to the diesel filler hole under the gas cap

Dodge Ram Diesel gas cap

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As a side note, opening the gas cap on a Ram truck is interesting because there is no screw-on dust cap covering the diesel filler hole inside. There’s just a spring-loaded flap. We find we need to wipe down the whole area after we’ve taken the truck out on four wheel drive roads in dusty places like southeastern Utah.

There is a nifty aftermarket diesel gas cap available that uses a magnet to keep it handy during fill-ups. Very cool!

While it would seem trivial to refill the DEF tank, we struggled the first few times with the bulky jugs and awkward spouts, and it dripped here and there and was basically a pain in the neck.

We’ve also shopped around quite a bit to find DEF that is fairly inexpensive. One of the more common brands is BlueDEF. Others are Command DEF by Prestone and Blue Blood by Cam2. You can also buy it in bulk (see below).

However, the best deal we’ve found is the big blue 2.5 gallon jug of SuperTech DEF sold at Walmart (in-store) for $7.88. The jug is easy to use and it’s drip-free.

SuperTech DEF Diesel Exhuast Fluid

SuperTech Diesel Exhaust Fluid (DEF)

SuperTech DEF comes with a flexible pouring spout that is wrapped around the handle. Simply unscrew the jug’s cap and screw the spout on until it’s really tight.

Diesel Exhaust Fluid nozzle

The SuperTech DEF jug comes with a flexible pour spout wrapped around the handle

Diesel Exhaust Fluid installing the nozzle

Screw the flexible pour spout on tightly

Diesel Exhaust Fluid nozzle-2

Ready to pour.

There is a small black vent hole on the spout, and this little guy is essential to making the whole process go smoothly and cleanly.

Vent on Diesel Exhaust Fluid DEF container

The black air vent on the pour spout is the key to doing this chore
without dripping or spilling

After putting the flexible spout securely into the filler hole on the truck, give the jug a small squeeze to start the flow of fluid. This will make the vent hole on the jug such air in, which vents the jug and allows the fluid to flow easily.

Pouring DEF fluid Diesel Exhaust Fluid into the truck

Once the spout is inserted securely in the filler hole, squeeze the bottle slightly to start the flow.

Make sure all the fluid has been poured into the tank, and then you’re done!

Pouring DEF fluid Diesel Exhaust Fluid into the truck

Empty the contents into the DEF tank.

The first time we did this, we didn’t squeeze the jug first, and DEF dribbled out the air vent. But if you get the siphon going properly, by squeezing the jug as you start to pour, it’s a cinch.

Our Ram truck has a dashboard gauge that indicates the fill level of the DEF tank. When it gets down to half full, we put a 2.5 gallon jug of DEF into the tank to fill it back up again. We typically do this every 1,000 miles or so.

DEF has a minimum shelf life of a year. We keep just one 2.5 gallon jug on hand at all times. Since we refill our tank about every four to six weeks, we never remotely approach its shelf life.

Buying DEF in Bulk

Another option for DEF fluid is to get it in bulk at a gas station. This is not common yet, and most gas stations don’t have DEF in bulk. However, Flying J and Pilot truck stops carry it at some of their travel centers (see links at the end for locations).

In our travels, we don’t often fill up at Flying J or Pilot, simply because they tend to be on the interstates and we tend to be on back country roads. So, we haven’t yet seen or been able to take advantage of their bulk DEF at the pump.

However, for RVers that use these travel centers a lot, this is a super way to go. The current bulk Flying J / Pilot DEF price at many of their centers is a little less than the SuperTech bottled price (~$2.79/gallon vs. $3.16/gallon).

Pilot / Flying J offers a discount card for RV travelers that gives a few cents off on fuel and 50% off on RV dump station fees, and special discounts for Good Sam Club members.

Pilot Flying J RV Traveler Discount Card

Pilot and Flying J have DEF in bulk at the pump AND a discount card for RVers

What are the pros and cons with a truck that uses DEF fluid? An obvious disadvantage is that you’ve got to do this extra little chore every so often.

However, there are benefits too. The fuel mileage is slightly better on SCR equipped engines than on older trucks that used a Diesel Particulate Filter (DPF) to meet the EPA emissions standards (like our 2007 Dodge Ram 3500). Cummins, manufacturer of the engine in the Dodge Ram line of diesel trucks, says the improvement can be up to 5%.

This increase in fuel economy is because the improved conversion of NOx emissions allows the newer engines to be fully optimized. Also, the older engines go into a “Regen” frequently to burn off the particulates in the Diesel Particulate Filter. In a Regen, a higher quantity of fuel than normal is pumped into the engine to make it run hot so the particulates can be burnt off. This wastes fuel and ultimately lowers the truck’s overall fuel mileage.

The newer engines that use SCR technology (and DEF) don’t need to go into a Regen quite as often.

For more info on all of this, there are lots of links below that explain the history and mechanics behind Diesel Exhuast Fluid.

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4 thoughts on “How to Put DEF (Diesel Exhaust Fluid) in a Truck & Which is Cheapest?

  1. Thanks for your post. We have a GMC diesel with the DEF fill under the hood and have not been as skilled when filled….always manage to spill a little ;-( Safe Travels, Jeff and Deb

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