How Does DeeZee Aux Tank Fill Main Tank of Vehicle
Updated 10/12/2026 | Published 12/09/2024
Question:
Looking at your DeeZee auxiliary fuel tank. DZ91752X My question is how does it transfer fuel into your regular fuel tank. I saw a picture of a tee that was plumbed into the main fuel line and is it gravity fed, because to me it would over fill your main tank. Does it have a pump? No one has quite answered that question or anybody looking at these is a whole lot smarter than me.
asked by: Eddy
Expert Reply:
Hey Eddy, so the DeeMax aux fuel tank part # DZ91752X is a gravity feed design and does not use any pumps. So what does that mean as far as over filling? If the fill line has a check valve built into it then you are okay to leave it open as that would prevent it from over filling. If there isn't a check valve then you'd have to open it, let the tank fill till it's near full and then close it.
(click to enlarge)
Brian W.
10/9/2026
Actually, that “check valve” in the filler neck works off of head pressure. As the factory tank fills up, the fuel begins climbing up the filler neck. Assuming the fuel cap is working correctly, as the fuel climbs up the filler neck, the volume of air in the filler neck is compressed. Due to the laws of physics being what they are, the pressure inside the filler neck increases to match the pressure of the fuel line coming from the auxiliary tank. Once the two pressures are in equilibrium, the fuel flow from the auxiliary tank stops, and will remain stopped until the filler neck pressure is reduced by the usage of fuel. One thing that’s very important when using this setup: DO NOT OPEN the factory fuel cap. This will release the filler neck pressure and the fuel from the auxiliary tank will begin to flow once again. Only open the factory fuel cap will when the auxiliary tank valve on the fuel line is closed. Until recently, I’ve been using this setup for years and can say hat it works very well. When it was time to fuel, the auxiliary tank is empty, and the factory tank is less than full. I would close the aux tank valve, then put a few gallons of diesel in it. I would then pour in the appropriate dose of fuel additive into the aux tank, then continue to fill with diesel. I would take the factory tank fuel cap off, pour in the appropriate amount of fuel additive, and top it off with diesel. After which, replace the fuel cap and make sure it is securely tightened down. Only then, open the auxiliary tank valve. Hope this helps.
Jameson C.
10/12/2026
Product Page this Question was Asked From
DeeZee Truck Bed Auxiliary Tank - Rectangle - Aluminum - 39 Gallon
- Fuel Transfer Tank
- Auxiliary Fuel Tank
- Rectangular Tank
- Aluminum
- Small Capacity
- Diesel Only
- Silver
- DeeZee
more information >
Continue Researching
- Shop: Tool Box
- Shop: DeeZee Truck Bed Auxiliary Tank - Rectangle - Aluminum - 39 Gallon
- Shop: DeeZee Specialty Series Truck Bed Transfer Tank - Rectangle - Aluminum - 39 Gallon - Silver
- Shop: Connector Kit for DeeZee Truck Bed Auxiliary Tank - Later Model Chevy/GMC, Dodge, and Ford
- Q&A: How To Add a Fuel Tank To a Truck with 5th Wheel Hitch In Bed
- Search Results: hammerlocks
- Shop: Trailer Wiring
- Video: Setting up the DeeZee Truck Bed Auxiliary Tank on a 2014 Chevrolet Silverado 3500
- Shop: Durabilt Hammerlock Coupling Link for 3/8" Chain - 7,100 lbs - Qty 1
- Video: Beginner’s Guide to Installing the DeeZee Truck Bed Auxiliary Tank on a 2018 Ram 3500
- Video: DIY 2012 Ford F-250 Super Duty Installation for the Durabilt Hammerlock Coupling Link for 3/8" Chain
- Shop: Gooseneck Hitch
- Shop: Suspension Enhancement
- Search Results: timbren spacers
- Search Results: timbren
- Search Results: air lift spacer
- Search Results: in bed wiring
- Search Results: air lift spacers
- Video: Review of Timbren Spacer Kit - T58AV
- Q&A: Correct SumoSprings for a 2018 Ram 2500
- Video: DIY Install: Pollak 5th Wheel / Gooseneck T-Connector with 7-Pole on your 2015 Ram 2500
- Q&A: Choosing Between Air Lift Load Lifter 5000 or 7500 For 2018 Ram 2500 Towing 5th Wheel
- Q&A: What Size Spacer Do I Need for a 2018 Ram 2500 w/ a 4" Lift?
- Q&A: Best Air Bag Kit For Lifted 2020 Chevy Silverado 1500
- Shop: B&W Turnoverball Underbed Gooseneck Trailer Hitch w/ Custom Installation Kit - 30,000 lbs
- Shop: Air Lift Angled Universal Air Spring Spacers for LoadLifter 5000 and 7500XL - 4" Lift - Qty 2
- Shop: Pollak 5th Wheel / Gooseneck T-Connector with 7-Pole - Ford, GM, Dodge, Nissan w/ Factory Plug
- Q&A: Can the DeeZee Transfer Tank # DZ91752X Use the Drain Plug to Gravity Feed Fuel for Primary Tank
- Q&A: Why Does Valve for the DeeZee Connector Kit 97962 Have to be Closed During Travel on 2016 Ram 2500
- Search Results: diesel tank
on Fuel Transfer Tank
Jameson C.
11/17/2025
Hey Eddy, how did things go with the gravity feed setup on your DeeZee auxiliary fuel tank? Did the check valve help keep your main tank from overfilling? Your update could make it easier for others looking at the same design.