Coolant system failure, otherwise known as coolant seal failure. Coolant seals in a rotary function much the same as a head gasket in a piston engine, but this failure in a rotary requires full teardown. This can allow coolant to enter the combustion chamber, and compression pressure to enter the coolant system and cause excess pressure and overheating. Coolant seal engines are often fully rebuildable unless they’ve been severely overheated, which can cause warpage and cracks in the iron housings. However, if a coolant seal engine is left to sit for a few weeks or months with water inside, rust will form quickly and cause most of the internals to become junk quickly and not rebuildable, increasing the cost of the rebuild later. To avoid this, pump plenty of oil, atf, or wd-40 into the chambers while turning the engine to lube all metal surfaces and displace water to prevent rust, also turning the engine every week or 2 will be helpful in preventing rust and stuck seals until the rebuild.