X15 Random Coolant Loss

[Rawze]

Your description so far is exactly the symptoms of a partially blown head gasket due to a dancing liner... not so much the egr cooler.

If the engine was de-mandated improperly (90%+ chances for improperly) then that is a heavy contributor of blown head gasket.

(Things that will effect engine life related to liner failure) ...
Is this truck de-mandated (deleted).. and if so, who did the work/programming?.
What make.model truck?.
How many miles (or miles since last inframe)?.
How much HP/Torque?.
What is the trucks rear end ratio?.

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You should also pressurize the coolant system to 20-psi and fix any leaks first too.

next step would be pressurizing the coolant system to 20-psi and letting it sit over night with the oil pan off the engine. Then the next day, look for droplets of coolant seeping out from around the piston rings on each cylinder at each compression stroke, as someone turns the engine by hand. You will either hear a dry hiss (good) ... or a gurgling sound(bad) + dropplets on the cylinder that is blown.