2008 Fummins Air in Coolant stumped
 

New to bombers. Hopefully this is the right section, but if not please let me know. New to diesels in general (coming from yotas, 80s-90s GM gassers) so probably not the smartest decision on my part, but its sort of a dream truck so it was worth the risk to me. I work with po who did entire swap so we have been stumped with the recent cooling issue. Truck pretty much sorted except for a few quirks for last 10k miles, but about 1 month ago after returning from 7hr round trip I smelled coolant and noticed wet fender well, drivers shock tower, and degas bottle. Naturally I refilled it to normal level when cool. Next day same thing. Since then it will push out any extra coolant, it stays low but never empties even after 1 week of around town use. Only while driving and not at idle (tried 45 min idle with ac on). Cap does have pressure even after sitting for 7+ hours to cool. Water pump is dry with no visible drips. No visible drips from hose connectors. Temperature gauge does vary while driving since this started. Once engine warms the needle sits at almost 1/2 way. Stays this way for 2-3 mins then drops to normal operating temp at 1/3rd. Im assuming this is when thermostat opens and the degas coolant is displaced with air from coolant lines. It will stay here infinitely as long as engine is warm. No white smoke at any time from muffler or crank vent. No bubbles in degas reservoir. No milky coolant. No leaks from waterpump or odd noises. Truck runs great. Not fast by any means, but no noticeable loss of power. Previous owner had similar coolant spewing issues when he had it. He assumed worst and replaced head gasket and studded it. He also re-routed main degas heater hose from over fan shroud to under the front crossmember to try and prevent air trapping.

What has been done: Headgasket, studs, thermostat, new degas cap, heater core bypassed.

This is everything I can think of up to this point. We havent done coolant pressure test yet. Any suggestions for further guidance would be greatly appreciated. Thanks!