* Blown Head Gasket: This is the most probable cause. A failed head gasket allows coolant to leak into the cylinders, causing the white smoke. The pressure from the combustion process can also force oil into the coolant system, contributing to the oil consumption.
* Cracked Cylinder Head: Similar to a blown head gasket, a crack in the cylinder head allows coolant to enter the combustion chamber. This is often accompanied by overheating issues.
* Damaged Piston Rings: Worn or broken piston rings allow oil to leak past the pistons into the combustion chamber. This would explain the significant oil consumption, and some of the combustion byproducts could mix with coolant to create white smoke, although it's less likely to be as voluminous as with head gasket issues.
* Valve Stem Seals: While less likely to cause *this much* oil consumption, severely worn valve stem seals can allow oil to leak into the combustion chamber, contributing to the smoke. However, 3 quarts per 100 miles points to a far more serious issue.
Why it's urgent: Continuing to drive the truck in this condition will cause significant and irreversible damage to the engine. The risk of catastrophic engine failure is very high. The engine is likely overheating and potentially damaging internal components further.
What to do: Immediately stop driving the truck. Have it professionally diagnosed by a mechanic experienced with Duramax engines. Ignoring this problem will only lead to a much more expensive repair or even a need for engine replacement. Trying to troubleshoot this yourself is strongly discouraged given the severity of the situation.
