The age old comparison, FASS vs. Airdog, What's different?

You've seen the question on internet forums, "Which is better, FASS or Airdog?"

Which is better? In an effort to abstain from bias, we will never state our opinions derived from this information. This article is to lay out differences between the internal designs of the motors, hardware, and mounting styles. 

What is

FASS Fuel Air
Separation System?

FASS stands for Fuel Air Separation System and is a combination diesel fuel lift pump and fuel filtration system that is designed to improve your diesel truck’s fuel mileage, engine performance and extend the life of your diesel injection system. Our diesel fuel pump and diesel fuel filters are made in America in order to provide complete customer satisfaction. Regardless of the vehicle you drive, and how you power your life, FASS diesel fuel systems provide the cleanest fuel in order to extend the life of your favorite truck.


  • FASS uses 10AWG from Battery to Pump Connector, and 14AWG from Connector to Motor. They use a Weatherpack connector at the pump.
  • FASS uses brass JIC fittings. These threaded fittings have pre-applied sealant where the fittings thread into the base. 
  • FASS includes 14 feet of blue 1/2" push-loc fuel hose.
  • Some model FASS's include a fuel suction tube as the factory pickup has been found to not be adequate for the demand of fuel. (This requires temporary removal of the fuel tank for installation.)
In the Diagram below, you'll see the inner workings of how the fuel flows through a FASS System. 
PureFlow AirDog

The Next Generation Fuel Air Separation System

The advanced 4G technology offers protection and performance like never before! Our patented air separation, along with a water separator and fuel filter makes sure your expensive injection system components get all the clean fuel they need. The AirDog® II-4G also overcomes loss of power due to driving in higher altitudes.

The AirDog® II-4G System Improves Engine Performance by eliminating spray pattern disruption due to air entering the system. This allows the engine to keep its designed injection timing which allows a more complete burn of the fuel entering the cylinder. This increases overall engine efficiency across the power and torque range!


  • AirDog utilizes 12AWG throughout the harness from battery to pump. 
  • AirDog utilizes Quick Connect fuel fittings throughout the plumbing required for the fuel system install. 
  • AirDog includes 20 feet of black push-loc hose. 

AirDog notes these key features their 4G motor offers

  • Low amperage motor provides efficiency
  • Self-aligning motor requires less maintenance and reduces noise levels
  • Intermediate pump shaft that Centers the Gerotor Pump and Shaft Seal
  • LoveJoy Coupling for Smooth Power Transmission
  • Dual Feed Gerotor Pump
  • On Demand Fuel Technology
  • Adjustable Pressure Regulator
  • Small & Compact: ONLY 7"L X 10"T X 3.2"W

Where are these similar?

  • Both include everything needed to install minus tools.
  • Both manufacturers back their product by a lifetime warranty for the original purchaser if purchased from an authorized dealer at MAP Pricing.
  • Both include extra o-rings in the event of a leak. 
  • Both have ports on the pressure side of the fuel flow for a fuel pressure gauge to be installed if the operator so wishes to monitor output. 

Where are these different?

  • On the Duramax, specifically 2011-2016 LML, AirDog mounts on the bracket in front of the tank inside the frame rail, FASS mounts in the bed side.
  • AirDog has an adjustable pressure screw, FASS is pre-set and not adjustable.
  • AirDog does not require a draw straw on the 2015-2016 MY LML's. FASS includes a draw straw with their kit for these years and it must be used unless using a bottom sump. 
  • AirDog uses ORB Fittings and quick connects to plumb to hose. FASS uses NPT fittings and JIC to connect to hoses.
  • AirDog has a drain on the water separator. FASS has an internal filter media to entrap water. 


AirDog Factory Mounting (2011-2016 LML Duramax)  

FASS Factory Mounting (2011-2016 LML Duramax)

 It is worth noting, the differences in mounting can be addressed with a mount that WCFAB manufactures, this relocates the FASS to mount off the fuel cooler bracket as well. It can be found here. Shown below is an example. 

 We can't make your mind on which system is best based on this research, but we hope reading this you learned some insight on the differences to help you decide. 

 We are an authorized dealer for FASS & AirDog here at UnderDog Diesel.

Shop AirDog Brand here

Shop FASS Brand here


The information in this blog article has been gathered from &

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