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Discussion Starter #1 (Edited)
For those who contemplating the DEF filler mod for fueling a diesel heater in a gasoline van, or have already done so...I've been studying the blue DEF cap and I think the the following modifications are possible to vent the auxiliary tank and clarify the purpose of the fill port. This mod works if the fill hose to the auxiliary tank slopes down to the tank at all points so the venting pathway is always clear to atmosphere.

It might work for @ranchero who put a 90 degree bend in the fill hose and brought it out through the side of the fill channel, depending on the height of his auxiliary tank fill port. I think it probably won't work for @HDH who ran the fill hose down to the exit of the fill channel and back up to the top of the auxiliary tank. The dip in the hose holds fuel like a drain trap between the tank and atmosphere. That configuration needs a separate vent hose rising from top of tank to a point at or above the highest point of the total diesel fuel volume, ideally with no low point between to trap condensation.


1. Venting the DEF cap: The cap has a hollow cylindrical post in the center to hold the seal for the filler neck. The ID of the cylinder is 0.39", which is the standard ID for 1/4" female threads of tubing compression fittings. The closed top of the cylinder is the bottom of the channel for the cap tether. Drilling the top of the cylinder to open it up into the tether channel will vent the cap at the highest point of the diesel storage volume. Insert a temporary strip of tin can to protect the tether when drilling. The vent hole will be hidden, yet open to atmosphere and somewhat protected from debris. Next, using a triangular file, cut a couple V-channels in the threads on one side of a brass 1/4" compression union. Using it as a tap, screw it carefully into the central cylinder of the cap, backing off every half turn to clear cuttings. Unscrew the "tap" side of the union, clean out residual cuttings, flip the union and screw it into the cap. Put a small disk of fine mesh brass screen into a 5/16" compression nut and screw it onto the "tap" side of the union. You now have a vented DEF cap.

2. Changing the color of the DEF cap: The DEF cap is labeled PA-6, which means it is polyamide 6 plastic, aka nylon, which is very easy to dye using an acid dye such as All Purpose Rit. Dye colors are transparent, so I think a yellow dye will turn the cap green, the standard color for diesel fill. I will try Rit color Lemon Yellow. Degloss the cap with acetone, which also removes the DEF lettering. Prepare the dye bath and heat to 185 degrees F in an old saucepan. Drop in the cap (it sinks), add vinegar after 5 - 10 minutes and observe for color change. You now have a green vented diesel fill cap.

3. Lettering: A friend who makes signs professionally will print and cut labels out of traffic grade 3M sign vinyl for me. HEATER FUEL / DIESEL ONLY. Maybe put NO GAS on the center bar. This completes transformation of the cap from DEF to DIESEL.

4. Note: One advantage of a separate vent is that it can be terminated with a roll-over vent valve. They need to be upright to vent. I'm not sure if it's possible to modify one of these vents to replace or attach to the union. Aluminum dirt bike fuel tank vents are cheap and look to be about the right size. I'll get a couple and experiment.

Photos pending completion of this experiment. Comments?
 

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Discussion Starter #3
DEF cap conversion to Vented Diesel

Photos and notes - DEF cap conversion

Fitting is 1/4" compression union - 2 for $4 at Advance Auto Parts
Dye is Rit, regular all purpose for natural and nylon, not "More" for synthetic fabric. Color is Lemon Yellow. $4.98 at Walmart, in the laundry section.
I got the pack of 5 screens for 35 cents total at a local head shop.

I cut three v-notches in the threads - the hex flats made it easy to hold steady. I used a finger ratchet and socket to screw the "tap" into the cap, backing all the way out a few times to clear cuttings. Cleaned up the completed threads with the brass tube brush.

Dye bath recipe: 2 quarts water at 185°F + 2 ounces dye. Add 1/2 cup vinegar after cap soaks 10 minutes - the color will intensify. Compare final color to the blue shavings from drilling the vent hole.

I screwed the "tap" side of the fitting into the cap and used the full thread side for the screen. Trimmed the screen to just barely fit into the cap, pushed it in with a Q-tip, then screwed on the cap. I could feel the screen grab and dig in at the last turn - it felt a bit like setting a star lock washer.


This was a very satisfying mod (i.e. it worked)
 

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It might work for @ranchero who put a 90 degree bend in the fill hose and brought it out through the side of the fill channel, depending on the height of his auxiliary tank fill port. I think it probably won't work for @HDH who ran the fill hose down to the exit of the fill channel and back up to the top of the auxiliary tank. The dip in the hose holds fuel like a drain trap between the tank and atmosphere. That configuration needs a separate vent hose rising from top of tank to a point at or above the highest point of the total diesel fuel volume, ideally with no low point between to trap condensation.

I like what you did to solve the venting issue. It hasn't been an issue for me, but after reading your post I went ahead and drilled a very small hole in the DEF cap towards the tether channel. To prevent dirt from entering the tank I plan on ordering a 1/4" pneumatic exhaust (muffler) or a small 1/4" check valve that only opens if there's a small vacuum in the tank.


I have a few comments regarding the statement you made about routing of the fill hose and the affect that has on venting.


The issue that I have (had) with venting is that the pump pulls a vacuum when the tank level drops.
I do have a separate vent line from the top of the tank to the top of the DEF fill/vent hose assembly. This works great for filling of the tank, when the cap is removed, but it doesn't help when the cap is installed since there is no vent in the cap.
So to break the vacuum we need a hole in the cap or a vacuum breaker elsewhere in the system. You came up with a clever solution. However, it doesn't matter how the fill line is routed and if there's remaining fuel in the low spot of the fill hose. Once there's a vacuum in the tank, it will be everywhere since there's a separate vent line to the top. Even if there wasn't a separate vent line you would still break the vacuum by opening the cap (or drilling a hole).
I was always running my heater with the cap a little loose. There have been times that I forgot to loosen the cap and had the heater running all night without issues. The next morning I would open the cap to break the vacuum. Luckily the heater uses so little fuel that the vacuum didn't reduce my pretty 3 gallon tank into a 1 gallon tank. :)
 

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Discussion Starter #5
That certainly works if the vent hose is clear and dry, which positive drainage back to top of tank makes pretty much certain. If the vent line follows the same path down and up as the fill hose, two worries as to how a fluid trap might develop. It seems fairly easy for some fuel to splash into the vent line when filling, and condensation if it occurs could accumulate in the dip. Slow fill and a long fill nozzle or funnel can help with the first. Sounds like no problems for you with condensation, so it’s working out. No worries, stay warm!
 

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Liquid in the fill tube or vent tube doesn't matter. If there's a vacuum in the tank it will suck whatever is in the pipes right out as long as the cap is loose or as long as there is a path for air to come in.
 
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