Bottom Line
Maintenance Q&A

Paul Abelson
technical editor

Land Line reader John Meaney, Carpinteria, CA, called me a short time ago with a question about tires. A friend of his lost two steer tires from the same manufacturer recently. When they blew out, one caused very little damage, but the other tore up the fender badly. Both tires had between 60,000 and 70,000 miles. John’s question was …

What could have caused these steer tires to blow out? Is it a defect, normal wear or driver-related?

I called Jim Garratt at Bridgestone/Firestone and conferenced John into the call. Jim didn’t deny that tires occasionally have manufacturing defects, but if they cause problems, it is usually in the first 15,000 to 20,000 miles. At 60,000 miles or more, the chances are the cause was either road hazard damage or driver inattention, resulting in low air pressure.

During the call, I found out that whenever a complaint comes in that a tire failure caused damage to equipment or property, Bridgestone/Firestone (and probably the others, too) has a formal procedure to ensure the complaint is thoroughly investigated. The flip answer from the dealer, “Oh, those things happen to Bridgestones,” was totally unacceptable to John and his friend, as well it should be. It indicates the dealer employee either was undertrained, undermotivated or overworked, with no desire to look into the problem and help retain a customer. Now John’s friend will be working with factory representatives.

A driver I met at a truckstop had plugged fuel filters. The shop told him it was due to a load of bad fuel. His question was …

What is bad fuel and how does it get bad?

When diesel fuel comes through the last valve at the refinery, it is clear and free of contamination, certified to meet specifications. As soon as it starts the process of traveling to your engine, things can and do happen to it. It is exposed to air, and air contains moisture, humidity. Air also carries both organic and inorganic matter. In polluted areas, air carries chemicals and solids. These enter fuel storage and transport facilities all along the delivery system — at the pipeline, barges, bulk storage facilities, tank trucks and storage tanks. And of course, air comes in contact with fuel when you open your cap to add diesel.

Water condenses from the air as it cools in the evening. Particulate matter found in fuel includes dust, dirt, sand, metals from fuel pumps, tanks and pipes, resins from containers and pipes, paper and cotton from filters, fungus, yeast and bacteria from the air. Living organic matter grows in diesel fuel, taking nourishment from the hydrocarbons and oxygen from the water. Left to grow, these colonies of living organisms, which we see as slime, can plug filters or affect pumps. Oxygen and heat help diesel oxidize, forming gum and sludge that clog fuel systems.

The longer diesel fuel is in transit and storage after refining, the more prone it is to have problems. Diesel can also go bad when truckstops or refueling stations don’t practice good housekeeping. Some fail to cover fill holes when it’s raining. Some have fillers where the ground has settled, so water pools instead of running off. Some don’t “stick” the tanks using water-detecting paste often enough, or they don’t pump out water that accumulated in the tanks.

Instead of having a regular supplier, some facilities call around to see what suppliers have surplus diesel, fuel that may have been in storage too long. They get it at reduced prices and pass some of the “savings” along to you. But when you buy fuel from the cheapest place, you risk getting a load of bad fuel. Saving pennies per gallon may wind up costing many dollars in engine repairs. Housekeeping, essential for fuel maintenance, costs money. The large chains follow strict procedures. The low-priced outlets may or may not.

A friend told me he travels with a supply of clean, dry baby food jars. When he pulls into a truckstop, he goes to a pump and lets a little residual fuel drip from one or two nozzles into a clean jar. He looks at it to see if it’s dark or cloudy. If he’s not sure, he’ll cap the jar and leave it sit for 10 or 15 minutes. If there’s any sediment or water visible at the bottom, he won’t get fuel there.

Additives will control moderate amounts of water in diesel, but they can’t do much about dirt, rust, sludge or other contaminants. That’s why it’s so important to get good fuel in the first place.

Johnny Wollman, Graceville, MN, wrote me asking:

How does the air system work? How does the engine brake function?

The design and operation of the air system, including brakes, suspensions, seats and actuation of pump-offs and hoppers is long and involved, but I’ll try to hit the high spots. Air is a marvelous tool to have on a truck. It is abundant and free, it can be compressed and stored, to be released to do work at a later time. And it provides a great deal of force to do work.

The air compressor, powered by the engine, sends compressed air through a dryer where most moisture is removed. It then goes to a tank system, where the air is stored under pressure until it is needed. The compressed air, at 100 to 120 psi, can be distributed to the seats, the suspension, the horn, fan clutch, transmission, differential lock, fifth-wheel slide and, of course, the brakes. Air stored at the trailer can be used to operate a slider, pump-off equipment, the suspension and brakes.

Since moisture condenses in the air tanks, it is important to drain them regularly. Water is not compressible, so any water takes away from the volume of air. In winter, it can freeze and destroy air tanks and valves. Change desiccant cartridges in your air dryer once a year (or according to manufacturers’ instructions) when you do your winterizing or summerizing.

Air goes through valves and hoses to a supply tank in the trailer, where it can do work there. When you apply brakes, compressed air is sent through a hose to control valves. They release air from the storage tank to the brake chambers. Air is used because of the tremendous force it can apply. With 100 pounds per square inch pressure in your system, a 30-square-inch brake chamber applies up to 3,000 pounds of force. There are four 30-square-inch chambers on a tractor and another four on a trailer in a five-axle rig. Steer-axle chambers are usually 24 square inches, applying up to 2,400 pounds of force each. That’s up to 28,800 pounds of force applied to the brake pushrods during a maximum panic stop. Even at a mild 20 psi application, almost 6,000 pounds is applied. We could never stop an 80,000-pound rig without Mr. George Westinghouse’s invention, the air brake.

Since braking converts motion to heat, and excessive heat makes brakes fade, we need a way to control speed without heating up the drums. There are several ways today, but it was not always so. Clessie Cummins, founder of the engine company that bears his name, was driving down a long, steep grade when his brakes gave out. After a scary ride, he decided to do something to help the brakes. His invention, the compression brake, was sold to Jacobs Manufacturing to become the famous “Jake Brake.”

When your foot is off the throttle, no fuel goes to the engine (except what is needed to idle), but air still goes in through the intake valve. Intake air is compressed in each cylinder, absorbing some of the truck’s momentum. Without a compression brake, the air releases its compressed energy to the piston, turning the crank and adding back the absorbed energy to the momentum of the truck. With a compression brake, the air is released to the exhaust after it has absorbed some momentum but before it can release the force to the pistons. The net effect helps slow the truck.

There is no harm done to the engine because the mechanism opens the exhaust valve as the piston passes top dead center of the compression stroke. The air follows the path of least resistance out the exhaust. Retarding force is controlled by the number of cylinders activated. The driver can select between 3 or all 6, 2, 4, or 6, or with Cummins ISX engines, 1, 2, 3, 4, 5 or 6 cylinders. Compression brakes are more efficient at higher engine speeds. At 2,100 rpm, some brakes generate as much as 600 retarding horsepower.

As you pointed out in your letter, there are differences in the way mechanical and electronic engines activate the “Jake.” There are also differences between compression brakes and different makes of trucks using the same brand of brake. Electronic engines have more ability to vary the controls. No matter what the brand, all auxiliary brakes, both compression and exhaust types, need some form of annual inspection and maintenance. Omit that and you very well might damage your engine.

I hope this answered your questions, Johnny. If not, just let me know.

Do you have a maintenance question?
You can write to Paul Abelson, technical editor, in care of Land Line Magazine, PO Box 1000, 1 NW OOIDA Drive, Grain Valley, MO 64029; or you can fax information to (630) 983-7678; or e-mail your question to Please mark your message Attention Maintenance Q&A. Although we won’t be able to publish an answer to all questions in Land Line, we will answer as many as possible.