X15 possibly dropped liner? input please

[Val]

Hi fellow members as the title states i have a x15 cm2350 500hp an am in need of some input i noticed i was loosing excessive coolant on my way home from a trip out west and it was not leaking external, to put into perspective from MT to MN i used a total of 4 gallons to keep the level above min in the coolant reservoir during this whole time coolant temp levels where normal and no overheating issues. What i observed was while the truck was running no visible white smoke but when i shut her down for ten min and restarted a big white cloud would immerse out the pipe than clear away in about a min. When i made it home i figured i would rule out the egr cooler so i pulled the egr cooler rear tube off placed a small dish and pressured the coolant system to 20psi overnight next morning no signs of any coolant so at this point things are looking bad:s. Further inspecting i did not see any coolant in the oil while pulling a sample (sending oil analysis tomorrow) i did however while the truck was in the shop and dry notice dried coolant under the oil pan and undercarriage of the tractor while i followed it back it appears it was coming from the overflow tube off the coolant reservoir as well i must of not caught this at first while i was driving from MT because most the roads where wet so i start up the truck and looked closely at the coolant reservoir and could see it bubbling inside i shut the truck off and removed the cap and notice higher than usual pressure as well. Ran a Combustion gas detector test and it read positive for combustion gasses. So here i am now :too_sad: with my x15 at 540K miles and obviously out of warranty thinking could i have dropped a liner? i hear of isx engines dropping liners but closer to a million miles. is it more common with the x15s? i take good care of my machine and equipment with scheduled routine maintenance and top of the line fluids but most of all not a lead footer, maybe i am the few unlucky ones. I Would really appreciate some input or advise here on what else i can check before i really have to pull the head off and to confirm whether its a liner or something else like cracked head or bad head gasket all i know at this point is i am getting only getting combustion gas in my cooling system and its not coming from my egr cooler. Thanks in advance.