GeoffR Gadsden

Trying to put simply the internal combustion engine (ICE) is an air pump, using expansion and contraction of the air the piston inside the engine moves the connecting rods, in turn rotating the crankshaft thru the gear box to the drive wheels. We use heat to expand the air inside the cylinder, we use valves to allow the air I and out of the cylinder in time for the piston movement. Metal is used in the manufacture of most ICE motors. Metal has a thermal operating limit, as therefore the temperature needs to be controlled with in limits. Consequently we have a engine cooling system, usually air eg many motor bike or lawnmower engines, or water cool (Radiator) as in most motor car engine, as is your case. My thoughts on your problem is the engine has suffered heat stress at some point. I would check the cylinder compression and the water cooling system to make sure the are with in the manufactures specification. This will highlight why you are having a problem. you also need to ensure the water thermostat is working correctly. Hope this helps.