Smooth Sailing

| February 07, 2003

Improper oil drain intervals can be the cause of turbo bearing trouble. While most owner-operators change oil more frequently than engine makers’ common recommendations of 15,000 to 25,000 miles, it’s wise to check maintenance records or to get oil analysis when acquiring a used truck.

Engines can act up because of the failure of some tiny component or deterioration in any number of places. Here are some common engine problems and the maintenance techniques that can keep you out of trouble.


One of the most common unit injector troubles results from a problem in a related system. Symptoms such as a gradual reduction in power output, normally without a lot of smoke, and a drop in fuel economy can be traced to failing to adjust the overheads – the closest thing there is to a tune-up on a heavy-duty diesel.

The overheads are adjustable rocker levers that operate the injectors and valves via the camshaft. After a lot of wear, the position of the injector will not be in the right relationship to the camshaft.

“You won’t be using the full travel of the injector,” says Doug Wilson, a Caterpillar analyst. The result is a lack of atomization of the fuel and reduced fuel delivery that you experience as a loss of power and fuel economy.

Engine makers require an adjustment of the overheads after the initial break-in period and then normally at infrequent intervals, especially on the latest engine models. Wilson says Caterpillar recommends doing them between 15,000 miles and 60,000 miles or after the third oil change. After that, go to 300,000 miles.

Peter Blonde, drivetrain engineer with Volvo Trucks of North America, says Volvo recommends an initial adjustment after three to six months or 50,000 miles. After that, the interval moves to 150,000 miles to 175,000 miles and 18 months.

Detroit Diesel’s national service manager, Gene Walker, recommends an initial adjustment at 60,000 miles to 100,000 miles. “After that, it does not tend to change,” he says. “If fuel economy and performance are good, the adjustment is likely OK.”

Cummins’ Zack Ellison, director of technical support, recommends the interval listed in the owner’s manual. Not only do intervals differ between engines with in-block camshafts, like the N14 and those with overhead cams like the ISX/Signature, but Cummins has made modifications that have lengthened the intervals in recent years.

Wilson says it’s hard to find misadjusted valve overheads, and when they are out of whack, “the symptoms don’t exactly jump out at you.” While wear in the mechanism can result in noisy valves that restrict air and exhaust flow, the most common wear pattern is valves that pound into the head and get too tight. This can result in overheating and burning.

Adjusting the valve overheads along with the injector rockers at recommended intervals will not only maintain engine performance and economy, but it will also prolong valve life.


Experts say it’s critical to buy filters that meet the micron rating for your particular engine and to change filters at the specified interval. The recommended micron rating relates to the clearance between the injector plunger and body. Particles that exceed the rating will get trapped between the surfaces and score them.

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