Big Donny Brought My Diesel Truck Back to Life

In diesel engines, blow-by (combustion gases escaping past piston rings) causes oil to enter the exhaust system, resulting in white smoke when the exhaust heats up and burns off the oil residue; after an engine swap, it's normal for the exhaust to contain oil and produce some smoke initially, but persistent smoke indicates the engine needs further attention or the exhaust system may require maintenance.