State and federal emissions regulations propelled changes in diesel engine technology that complicate bus maintenance routines. More than ever, a vigilant program of preventative maintenance (PM) is essential to the safe and reliable operation of fleets. Among the manufacturer’s goals in developing new engines are increased reliability and durability, reduced maintenance and extended oil change intervals, says Dave Sarcona of Detroit Diesel Corp. However, careful attention to the basics of a PM program remains the foundation of any successful maintenance effort. Fluid and filter changes can have a greater impact for this next generation of engines because of their higher performance standards. “Maintenance does have a definite effect on engine durability,” Sarcona says. Sarcona suggests that oil and oil filters be changed every 75,000 miles for motorcoaches and every 6,000 for transit or shuttle buses. Clark Ahrens, of Cummins Inc., breaks down the oil change interval for stop and go service buses even further. His recommended interval is 4,000 miles for buses with an average speed of eight to 10 mph, 5,000 miles for buses averaging 10 to 12 mph and 6,000 miles for those averaging 12 to 14 mph. Sarcona says that between changes the oil level should be checked daily and used oil should be analyzed. Oil analysis, while useful, should not be used as a means with which to extend oil change intervals, Ahrens says. Synthetic oils do not stretch the recommended mileage between oil changes either. When intervals are extended, “you’re adding soot and your package will break down,” says Ahrens. Newer synthetic oils are beneficial for control of soot, say both men. Ahrens says he recommends a CH4 or CH4 plus oil, which have holding capacities for soot of 5% and 6.5%, respectively. Synthetic oils are also recommended for buses that have cold weather starts below -13 degrees Fahrenheit. The quality of coolant makes a significant impact on fleet reliability as well. Proper coolant PM and monitoring of air flow and heat transfer in the engine can make serious strides in preventing overheating. Coolant should be drained every 60,000 miles or two years and filters should be changed every six months, or 12,000 miles, for buses with stop and go duty cycles. Supplemental additives should be checked at every filter change. Modern antifreeze and high quality water are the foundation of the coolant program, say both Sarcona and Ahrens. Ahrens recommends low silicate antifreeze because it provides protection down to -34 degrees Fahrenheit. The coolant mixture should be half antifreeze, half water. Ahrens’ benchmarks for coolant water are that it be no more than 170 parts per million (ppm) Carbon and CO3 plus MgCO3, no more than 40 ppm Cl and no more than 100 ppm SO4. Sarcona adds that the water should be deionized. Using preformulated antifreeze in coolant mixtures provides protection against corrosion and increases lubricity, concerns that are especially prescient as more and more bus fleets turn to low-sulfur diesel fuels. Winter blend fuels also increase concerns with lubricity. Low lubricity can cause costly wear to engines and should be closely monitored. Ahrens also highlights other elements of fuel PM, including changing filters every 6,000 miles. He says that fungicides and biocides are acceptable additives for fleets with black slime problems. Aside from PM programs, maintenance personnel should be rigorous in warming up and cooling down engines. Engines should warm up for three to five minutes before loading and should not run at more than an idle until oil pressure is indicated. Allowing buses to warm up increases engine lubricity. For buses that are out of service for more than 30 days, the same standard applies. If those buses do not achieve oil pressure within the first 15 seconds, their engines should be turned off. Cold weather starts require special care. Air intake heating elements are helpful to fleets operating in cold climates, Ahrens says. If intake heaters are used, maintenance should not use ether in the buses, he says. Once warmed up, buses should avoid idling to decrease engine wear. Idling is essential, however, to cooling down buses that have run at full throttle. Three to five minutes should cool the engine and protect the seals on turbochargers, Ahrens says. The greater the variation in the duty cycle, the more fatigued the turbocharger will be. Turbocharger wear is highest among the most responsive models.