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Bubble, bubble means toil and trouble!

By on September 1, 2015
Air-in-the-Fuel-System

If your engine feels sluggish or you’re seeing reduced miles per gallon, the problem could be in your fuel.

Mileage and performance suffer when fuel does not meet the engine manufacturer’s minimum specifications for quality. Diesel engines require fuel that has little to no contamination, and air is a contaminant. A loss of power, reduced fuel mileage, the engine surging or running rough — all can indicate that air is getting into the fuel system.

This is a bigger issue for diesel engines than gasoline engines. It all comes down to how these different fuels work. When discussing gasoline, a technician might refer to its octane rating, which is related to the amount of heat needed to ignite fuel. Put low octane fuel in a high performance sports car, and the fuel will ignite at low temperatures — before the piston is in the right position to absorb the energy that will be produced. If that happens often enough the vehicle is bound to take a trip to the garage for engine repair or catalytic repairs.

That is not critical in a diesel engine because fuel is injected into the combustion chamber at the precise time needed for ignition. But at that point it’s important that ignition occurs as quickly as possible. Cetane refers to how well the fuel burns. That’s why cetane, rather than octane, is the better indicator of the quality of diesel.

How air gets into the system

During that injection process, diesel fuel mixes with enough air to cause the fuel to ignite and burn efficiently. But a huge problem can arise if there is too much air in the diesel fuel. The air disrupts the smooth flow of fuel through the system, and the engine won’t function at its peak performance.

Air vapor can get into diesel fuel in many ways. When excess diesel fuel returns to the fuel tank, splashing as it drops into the tank, that causes air vapor to be introduced. The normal operation of a truck going down the highway — hitting bumps, stopping and starting at stop signs and red lights — and the general design of fuel filtration systems can introduce air vapor into the fuel system.

Air vapor bubbles are common and usually the engine’s lift pump overcomes them. When bubbles make it to the engine, that’s air, and must be corrected.

A technician can install a sight glass between the lift pump and the engine to see if the vapor bubbles disappear.

I have seen air get introduced in several places such as a cracked pick -up tube in the fuel tank, loose fuel line fittings, cracked fuel/water separator and so on. A quick way to locate the source of the problem is to bypass each section of fuel line with a known good line until the bubbles go away. Clear fuel/water separators allow us to see what is going on in the fuel system, at least from the fuel/water separator back to the fuel tank. This helps us pinpoint the air problem.

While excess air in the fuel line is not a dangerous issue, it has enough of an effect on truck performance to warrant action. If ignored, the vehicle is inefficient and more challenging to drive. So, closely monitor your engine’s performance, including fuel mileage, and take your vehicle to a qualified technician when necessary.

Homer Hogg’s “Maintenance Matters” airs on the Dave Nemo Show (Road Dog Trucking, SiriusXM 146), 8 a.m. ET, the first and third Thursday of each month.

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