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Message: <blockquote data-quote="drexchan" data-source="post: 1063695651" data-attributes="member: 6936">I understand the motive as we've discussed today in the shop. But I forgotten to mention to you that, there are hot spots in the water channels especially those casting marks with sharp edges.The thermostat is only activated by the coolant temperature around it, but not sensitive to the temperature in those hot spots between the pump and the thermostat (basically the entire block and head). The thermostat does open at the rated temperature, but there are hotter spots especially in the water jacket around the cylinders liners.Depending on the chemical content of the coolant (boiling point), engine speed (heat-up rate) and A/F ratio (exhaust temperature), these hot spots can cause heat shock and the resulting temperature may exceed the boiling point of the coolant. Internal micro-boiling must be prevented and the easiest way is by suppressing thermal expansion.Thermodynamically, leaving the cap opened allows an undesired thermal expansion (I bet that you saw the coolant expanding and overflowing from the radiator mouth as the engine heats up), and micro-boiling can occur at the hot spots. Micro-boiling is what created the fizzy situation after the bubbles (trapped air) have been removed entirely.I am not saying that your method is wrong but it was unnecessary. I must also remind you that the situation didn't get worse because you've used ethylene glycol coolant, and the small 1.3 engine wasn't generating enough heat to cause a bizarre boiling. If you have done that with tap water on your or my Wira 1.5, you may get a home-made little Yellow Stone.So my point is, top the water level after the first thermostat opening, and cap it back. Then fill the reserve tank to the mark and call it a day.---------- Post added at 12:32 AM ---------- 6 hour anti-bump limit - Previous post was at 12:26 AM ----------bera and D7zul,Thanks for sharing your experience and knowledge.Point taken but not accepted. ;]---------- Post added at 12:51 AM ---------- 6 hour anti-bump limit - Previous post was at 12:32 AM ----------Before it's open, the hot coolant is under higher pressure but not boiling. The moment the cap is opened, the pressure is released and the coolant is allowed to expand. However, it's not the thermal expansion that's causing the high pressure steam jet and coolant burst. It's the sudden pressure drop (due to expansion) that has reduced the boiling point of the coolant below its temperature at the time. This subsequently boils the coolant abruptly in the entire cooling system, and water is turned to steam at 1700 times of expansion rate (no joke). Therefore, even the slightest amount of the coolant got boiled up, the bursting result can still be phenomenal.</blockquote>

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