Medium and Heavy Trucks
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If i was to diagnose the issue it would be for failing or poor performing injectors that are not spraying the fuel in the cylinder properly. Injectors have a lifespan of 500,000 miles and begin to fail. You should have the injectors replaced and I would also have the unit ran on a dyno for a blowby check. You at the max miles for an engine overhaul. Cummins recommends between 500,000 and 700,000 miles to overhaul the engine. That would include new head,injectors and cylinder kits. as well as bearing replacement. Optional but recommended are oil pump, water pump, and oil cooler. You have already done the turbo. Please don't forget to rate my service.