Maintenance Q&A – July 2019

Kenworth oil, DEF-SCR

July 2019

Paul Abelson

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Q: I have a new T680 with a Paccar MX-13 motor, 500 horsepower, 1,850 lb-ft. For years, I’ve been using 15W-40 oil, because it worked well on all my Kenworths. I’m coming up on my first oil change and people at the dealership are telling me to use 10W-30, but they don’t say why. What do you say and why?

A: I say go with 10W-30, but even I don’t take my word for such matters. To be sure, I checked with Kenworth’s engineers, and they said that for your new truck, API grade CK-4/CJ-4 10W-30 oil is specified for normal, over-the-road trucking at ambient temperatures above 5 degrees. For colder temperature operations, 5W-30 is allowed, provided it meets API CK-4/CJ-4 specs. In cold weather, they recommend that biodiesel blends not be used or, if unavoidable, be kept to a minimum blend ratio. Their initial factory fill is with 10W-30.

Motor oil does more than just lubricate. It cools, cleans and seals the engine’s moving parts, too. As a coolant, it transfers heat from the cylinders to the sump and on to the oil cooler. As a cleaner, it has detergents and dispersants that dissolve soot and sludge and carry them to the oil filters. And as a sealer, it creates a film barrier around piston rings and on cylinder walls to reduce combustion blow-by into the oil sump.

In the old days, manufacturing tolerances were relatively large. Machining was not as precise as we have today, and metals were not as durable and were subject to far greater thermal expansion. Consequently, thicker oils were needed to fill the wider gaps and stand up to the demands of those engines. And just as you are questioning the switch to the improvements offered by 10W-30, operators back then questioned the change from straight SAE 40 and SAE 50 to 15W-40.

With modern materials and manufacturing processes coupled with synthetic base stocks and additives in today’s oils, the oils stand up to the stresses on them better then ever before.

When I entered the trucking industry, the then-new CD oil was all the rage. Each later advancement represented a major step forward in engine oil properties. Since then, we have progressed through the alphabet to CK-4 (C for compression ignition or “commercial,” -4 for the four-stroke cycle, and the letter designating the generation of oil properties), the latest and greatest oil to date.

Engineers assure us that even at the lighter SAE viscosity it has ample film strength to lubricate and seal today’s engines while reducing drag on moving engine parts because of its lower viscosity. The result is improved fuel consumption by 1% to 2% and a recommended oil drain interval up to 75,000 miles (as long as idling time is less than 20%). If you idle more, your new engine can still go 50,000 miles between oil changes in on-highway operations.


Q: I drive a 2015 Mack Granite daycab with a 455 hp MP8 and 1,760 pounds of torque. I haul mostly dirt and rock locally and regionally, with occasional loads of sand or ash. I get home almost every night. I have a big shed for my truck, like a garage but bigger. Lately, my check-engine light has been coming on quite a bit, and it says I have problems with my SCR system. It costs a small fortune to get it fixed, not to mention the hauls I lose when it’s in the shop.

I get my DEF from my dealer, so I know it’s pure. It’s delivered in 2½-gallon jugs. I keep it in the shed and top off every night when I shut down. I also keep a jug in the cab for when I’m out overnight. Why is my truck acting up, and what can I do to stop it?

A: The SCR (selective catalytic reduction) system is easily contaminated, and the DEF (diesel exhaust fluid) is sensitive to contamination and degradation. The process converts the urea into ammonia, which gets chemically reduced, broken down in the presence of the catalyst. (A catalyst causes a reaction but is not part of it.) The result is nitrogen gas and water vapor go out the exhaust instead of smog-forming nitrogen oxides.

Specifications for DEF call for 32.5% technically pure urea mixed with purified water. If one uses agricultural-grade (fertilizer) urea, which has impurities, or if the water has mineral content, it will contaminate the injectors and coat the precious metal catalysts, degrading the process. When sufficiently degraded, check engine lights come on and engine shut down will eventually occur. You are using good DEF, so we can eliminate that as a factor.

You may possibly allow contamination in when you refill. DEF is used at around 2% of diesel use, less than the contents of a jug each day. Contaminants may be getting into your DEF. With your shorter hauls than over-the-road drivers, you probably use only part of a DEF container each day. Dirt or dust can enter when the jug is opened and reopened.

Temperature and time are also factors in maintaining stability of DEF. When stored at temperatures between 10 and 90 degrees, shelf life is about one year. If always below 75 degrees, it can be up to two years. But if storage temperature gets above 95 degrees, which it did often last summer, DEF can break down after 6 months, less if it’s hotter. Summertime cab temperatures can get well above that when you park for lunch. Also, DEF is corrosive to copper and brass, so avoid buying from bulk systems with copper pipes or brass fittings. They, too, degrade the DEF and hinder the SCR reaction.

You may want to consider buying and storing fewer jugs each time to keep your stock fresh. LL

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Paul Abelson, senior technical consultant, is a longtime contributor to Land Line. He’s a member of the Society of Automotive Engineers and a member of the Technology and Maintenance Council. In 1995, TMC awarded him its Silver Spark Plug award. In 2006, he received the Lifetime Achievement Award from the Truck Writers of North America. Although he’s “retired,” he still makes a popular contribution to Land Line readers with “Maintenance Q and A.”

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