Diesel engines have long been known to lose power, increase fuel consumption, and have increased exhaust emissions when operating at higher altitudes, and especially when air and vapor are present in the engine’s diesel fuel filtration system. Air becomes entrained in diesel fuel from sloshing and agitation. Vapor develops from pump captivation, which ultimately impacts diesel engine performance.

University of Illinois
Department of Mechanical and Industrial Engineering
College of Engineering

The University of Illinois took the following spray pattern pictures, School of Engineering during an in-depth study and analysis of the AirDog® Fuel Preporator® Fuel Air Separation System. In each of the images, the injector plunger was in 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 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 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, entrained air and vapor does negatively impact the injection rate and spray pattern.

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 of 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 states:

“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:

“There are two types of contaminants: 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 poor response
  • Cranking fuel pressure is low
  • Engine difficult to start or will not start (exhaust smoke present)
  • Engine power output low
  • Engine runs rough or misfires
  • Excessive black or white smoke
  • Engine shuts off unexpectedly or dies during deceleration
  • Engine speed surges at low or high idle
  • Engine operating fuel pressure is low
  • Engine difficult to start or will not start (no exhaust smoke present)
  • Engine runs rough at idle
  • Engine speed surges under load or in operating range
    • The engine will not reach rated speed (RPM)
  • Engine starts, but will not keep running
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 currently, 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 and maintains a net positive pressure flow of fuel to the transfer pump, preventing pump cavitation and 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

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