955 cooling system problems

[Rch]

jonny955, a broken headgasket or cracked head can lead to overheating two ways. The coolent can leak out via the combustion chamber, crankcase or to the outside and overheat from low coolent. Another mechanism that happened to me was a headgasket leak between the combustion chamber and the water jacket that caused exhaust/ gas vapor to be pumped into the water jacket that would form a bubble that blocked flow. A vapor lock situation but not in the gas line! High RPMs would move the bubble along and eventually it would be relieved by the cap into the coolent overflow, taking some coolent with it and overwhelming its capacity. Hopefully your problem is less fundamental. Overheating can warp aluminum casting a lot easier than cast iron ( remember those Chevy Vegas- the only car in decades to have a coolent level indicator. They had an aluminum block that was junk with a bout of overheating. They had a "siliconized" cylinder wall, i. e. no cast iron sleeve to stiffen them up). I don't know if the 955's Yanmar engine has cast aluminum components like the head or not. In a tractor engine there is no advantage in the weight saving but these engines are put into boats, etc.