Diesel Engines have long been known to lose power, increase fuel consumption and have increased exhaust emissions... As the fuel filter plugs with use,  

WHEN operating at higher altitudes, and...

Especially when air/vapor is present in the engine’s fuel system!



AIR BECOMES ENTRAINED in diesel fuel from sloshing and agitation. Vapor develops from pump cavitation.


Air & Vapor in Diesel Fuel

Retard Injection Timing!

Air & Vapor are compressible. When air bubbles and/or vapor are present in the fuel, the injection is delayed until the air/vapor is compressed to the injection pressure. The delayed injection results in Retarded Injection Timing, Which causes...Increased Fuel Consumption, Lower Power Output and Increased Exhaust Emissions!

University of Illinois

Department of Mechanical and Industrial Engineering

College of Engineering... 1994

The following spray pattern pictures were taken by the University of Illinois, School of Engineering during an in-depth study and analysis of the Fuel Preporator® Fuel Air Separation System. In each of the pictures, the injector plunger was in exactly the same position when the camera lens snapped. The difference in the growth of the spray pattern was caused by the presence or lack of entrained air and/or vapor. Through the hundreds of pictures taken, it was determined that invariably, one out of three injections was delayed as in the "before" picture and the other two, on average were somewhere in between!  This explains the origin of the "rough idle" and also, the "wet idle" of diesel engines utilizing injector pump or unit injector systems.

Although entrained air and vapor do not affect the injection timing of the "Common Rail" engine. However, entrained air and vapor does negatively affect the injection rate and spray pattern of the "Common Rail" engine. 



                     After the AirDog Installation

This picture of the spray pattern taken after the installation of the AirDog® shows definitively the timely injection and full spray pattern when all air and vapor have been removed from the fuel! The diesel engine now, with Correct Injection Timing Restored, will be able to deliver the fuel economy and the power output the engine designers intended!    



                      Before AirDog® Installation

This spray pattern picture was taken before the installation of the AirDog®. The delay of pressure buildup caused by the presence of air effects the timeliness of the injection. This injection is delayed and will result in a late ignition, loss of power and a lot of unburned fuel exiting the exhaust in the form of black smoke! Additionally, it will take more fuel to create a greater combustion for the engine to produce acceptable power output.


Who Knows About Air in Diesel Fuel?

Cummins® Engine Co., 1965 Service Topic 5-135 states:

"Like water, (diesel) fuel contains a certain amount of dissolved air depending upon the fuel temperature, pressure on the fuel, specific gravity and the amount of aeration to which the fuel has been subjected."

NOTE: This service topic explains the source of the low fuel level usually found when changing a primary ‘spin on’ fuel filter.


Caterpillar®, Special Instruction 651-1250 points out:

"Normally No. 2 Diesel Fuel contains about 10% air in solution..."

 NOTE: This Special Instruction describes the use of the 2P8278 Fuel Flow Tube (Sight Glass) to observe the fuel flow and realize that engine power loss is due to air in the fuel. 


Parker - Racor Technical Support claims:

"Fact #1: There is Air entrained in diesel fuel."

“Fact #2: A very slight pressure drop can cause air to form visible bubbles.”

“Fact #3: Air can cause problems.”

“Fact #4: Air entrained in diesel fuel is not the same as diesel fuel vapor.”


Milwaukee School of Engineering, Handbook of Hydraulics states:

"Hydraulic oils also contain varying degrees of entrained air."

    It goes on to state...

"There are two types of contaminates, Fluids and Solids. Probably the most destructive fluid contaminate is Air.”


What Are the Symptoms of Air & Vapor in the Fuel System? 


Cummins Engine Company’s Troubleshooting Guide Explains It Perfectly!

Cummins® Bulletin No. 3666239, Section TT-Engine Performance Troubleshooting Tree:

• Engine Acceleration or Response Poor                •  Engine Speed Surges at Low or High Idle
• Cranking Fuel Pressure is Low                             •  Engine Operating Fuel Pressure is Low

• Engine Difficult to Start or Will Not Start             •  Engine Difficult to Start or Will Not Start

      (Exhaust Smoke Present)                                             (No Exhaust Smoke Present)

Engine Power Output Low                                    Engine Runs Rough at Idle  

• Engine Runs Rough or Misfires                              •  Engine Speed Surges Under Load or in   

• Smoke, Black - Excessive                                           Operating Range                                                     • Smoke, White – Excessive                                   •  Engine Will Not Reach Rated Speed (RPM)

•  Engine Shuts Off Unexpectedly or                        • Engine Starts But Will Not Keep Running                    Dies During Deceleration



For all of the above symptoms,

 TROUBLESHOOTING STEP 4 is recommended first!

                                           STEP 4: Fuel System Checks

                                           STEP 4A: Check for air in the fuel.

                                           STEP 4B: Check fuel inlet restriction.



The “Vacuum Feed” system, THE SYSTEM ON YOUR ENGINE NOW, passes the entrained air on to the engine and does not provide a positive pressure flow of fuel to your engine. Additionally, It restricts the INADEQUATE FLOW even more as it plugs with use! Literally speaking, the vacuum feed system on your diesel engine is the source of Degraded  Fuel Economy, Poor Performance, Increased Exhaust Emissions and Potential Damage to the INJECTION SYSTEM of your engine!


AirDog®  removes the ENTRAINED AIR from the fuel flow going to your engine & also maintains a Net POSITIVE PRESSURE FLOW of FUEL to the transfer pump, preventing pump cavitation & vapor!

 Air & Vapor Can Cause Permanent Injector Damage:

Caterpillar® Bulletin PEHP7046: "Damage can be caused by anything in your fuel system that should not be there, including.......air bubbles".


TROUBLESHOOTING -SECTION 15.2: “An engine will lose horsepower with an increase in altitude. The percentage of power loss is governed by the altitude at which the engine is operated.” © Detroit Diesel Corporation


PureFlow® Technologies, Inc.

5508 Business 50 West

Jefferson City, MO 65109

Toll Free:1.877.GO DIESEL


Direct: 1.573.635.0555

Fax:     1.573.635.0778



Hours of Operation


8:00 AM-4:30 PM

Saturday & Sunday



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How Does Air Become Entrained In Diesel Fuel?


As in all liquids, air becomes entrained in diesel fuel from sloshing and agitation! Air can be on the surface of the fuel in the form of foam or it can be in the bulk fluid in the form of tiny bubbles. Entrained Air is an issue of an "in use" engine in operating equipment!


What is Pump Cavitation?


Pump cavitation, simply put, is the pump not having enough pressure flow, or "Net Positive Pressure Head" of fuel coming into the inlet to completely fill the vacuum chamber of the pump.


Diesel fuel, as is true with all petroleum base liquids, will give off vapor when subjected to a vacuum. The amount of vapor depends upon the level of vacuum and temperature.


How Does Fuel Filter "Restriction" Effect Cavitation?

In the diesel engine industry, "fuel filter restriction" is a term that refers to the vacuum level at the inlet to the transfer or lift pump. It is measured in inches of mercury (in hg).


As the filter plugs with use, it further restricts the flow and increases the vacuum level in the pump.


The amount of vapor produced depends upon the level of vacuum.


What Else Increases Cavitation and Vapor? 


As the fuel levels in the tank(s) goes down as the fuel is burned, the "Dry Suction Lift" increases. This reduces the flow, increasing the vacuum.  


Operating at higher altitudes, where atmospheric pressure is less, reduces the fuel flow and increases vacuum. For example, at Denver,  atmospheric pressure  is about 17% less than at sea level and at Eisenhower Tunnel, it is approximately 32% less. Having less pressure to drive the fuel from the tank to the transfer pump, reduces the flow and increases the vacuum levels.


Higher fuel temperatures cause more vapor to form than cooler fuel under the same vacuum!