I saw you guys were discussing water usage on your engines, with no apparent leaks. We have had many cases where the exhaust seats leak by. You would think the leak would be detectable via EGT’s or other trends, but we have had no luck. We will borescope an engine if we suspect heavy coolant usage.
When repairing the leaking exhaust seats we found the o-rings were completely missing. Since then we had Markisches Werk develop a high temperature o-ring. So, far it has made an improvement.
I hope this helps, but from my understanding exhaust seat leaks is a common issue.