Cummins Diesel 6.7L Engine Oil Change Interval [Avoid Costly Repairs]

The Cummins Diesel 6.7L is a great engine but there are things to really consider regarding oil change intervals. The important point to consider is that not all drivers are created equally. Do you just make short trips? Are you making consistently long hauls? Are you running conventional 15W/40 oil, or are you thinking of running synthetic? The recommended oil change interval in the owner’s manual should be adjusted depending on usage. Peering under the valve cover of an “extended interval” engine often reveals an accumulation of sludge. A 6.7 engine with full emissions runs hot and dirty, as you can see from the pictures below. Had this owner maintained that engine properly he wouldn’t be looking at a $12,000.00 diesel engine repair bill right now. I would say an oil change is cheap maintenance in comparison.
Straight out of the gate we recommend 6000 km (3700 mile) oil changes if you are running conventional oil 15W/40. This should be a safe interval for city driving, stop and go driving, and idling. Some vehicles can get away with a 8000 km (5000 miles) stretch if you are mostly doing highway driving, consistently long trips where the engine can get nice and warm and run for extended periods.
Running past these distances can really put your engine at risk of damage from a buildup of sludge and soot. For the skeptic that reads this article and drives by his oil change reminder message that pops up in the dash, an important point to know is that the message is triggered by restriction across the filter and / or engine run time, it doesn’t actually monitor oil degradation. The older Cummins powered trucks ran cool, clean intake air and the recommended oil change was 6000 kms (3700 miles). A 6.7 engine doesn’t filter any better, doesn’t hold any more oil, burns more fuel, plus it runs way hotter and dirtier internally, yet the manufacturer has increased the oil change interval recommendation. To us that doesn’t make sense and is evidenced by catastrophic engine failures we have seen in our shop.

How the extended oil change intervals contribute to sludge and soot


Soot build up

Diesel engines are dirty by nature. Because of the way diesel fuel is injected and ignited, oil burners produce more soot than their gasoline counterparts. While most of that soot travels through the exhaust system, some sneaks past the piston rings and ends up in the oil. This is the reason diesel oil still looks dirty right after an oil change. It’s also the reason high-quality, low ash oil is recommended in the Cummins 6.7L engine. Soot can quickly overwhelm any additives in the engine oil, turning that oil into sludge.
Soot is more common in late-model diesel engines, like the 6.7L, because of modern emissions equipment. For example, exhaust gas re-circulation (EGR) valves send emissions back into the combustion chamber. This creates a greater opportunity for soot to get into the oil. Soot build up did not happen as quickly on older diesel engines without such equipment.
Levels of soot can increase because of external factors as well. Excessive engine idling and a rich air/fuel ratio are two examples. Often times, diesel trucks are left idling at the jobsite. They’re also frequently modified with computer programmers and performance injectors. Both of these adds-on can create an overly rich air/fuel mixture, leading to soot.

Oil dilution

Another reason for sludge build up in modern diesel engines is dilution. Much of this has to do with the diesel particulate filter, which traps soot particles flowing out of the exhaust pipe. This device requires a “regeneration” procedure to heat the exhaust enough to burn off the soot. Fuel is injected into the combustion chamber to achieve this increase in exhaust temperature. Unfortunately, that fuel can slip past the piston rings, diluting the engine oil and turning it to sludge.

How sludge affects an engine

Internal engine components must be continuously lubricated to limit friction. And you don’t need to be a petroleum engineer to understand that sludge is not a good lubricant. Sludge allows friction to build up between internal engine parts, causing them to overheat and eventually fail. It also clogs oil passages inside the engine. In the winter this can be even more of a problem as the extreme cold causes the oil to be even thicker. The end result is catastrophic engine failure and a fat repair bill.

Extended drain intervals on Cummins 6.7L Engine

For extended drain intervals, you will need to look at switching a synthetic oil. To properly ensure that your oil is in good shape you need to employ an oil monitoring program. Oil analysis has to be done, but it can give you an indicator on the quality of the oil at any time. In our past experience you can get up to 16,000 kms (10,000 miles) doing this, but there is additional filtration required. A great option is a bypass filter kit, and a primary filter that need needs to be changed half way through the interval.

How to prevent sludge build up

Frequent oil changes are a simple and effective way to prevent sludge build up. Preforming oil changes on a more frequent basis is cheap insurance for your truck’s engine.

Tom Zelinka has been an Alberta Journeyman Automotive Mechanic and Interprovincial Red Seal since 1978. In 1981 he then received his Alberta Journeyman Heavy Duty Mechanic Certificate and Interprovincial Red Seal. He has received Certificates for:  Cummins engine certification on N855/N14/M11 Engines, Cummins B/C/ ISB Series Engines, Cummins B/C/ISB Series Engine Fuel Systems, and Cummins B/C/ISB Series Electronic Engine Controls.

Interprovincial CFC/HCFC/HFC certification, Alberta Liquid Petroleum Gas Certification, Alberta Certified Advanced Mobile Hydraulics, Alberta Certified Diesel Engine Control Systems.

As the technology and models change, Tom continues to stay on top of the industry to be sure that you are receiving superior service for your Dodge Cummins diesel truck.

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