There may be few common causes of coolant (fresh water or various mixtures) fluctuations:
1. Air pockets in cooling spaces or pipes due to insufficient air release. This often happens after engine maintenance when coolant was drained and refilled but cooling system was not properly vented.
2. Static pressure at the cooling water pump suction reduced. Some valve on the return pipeline could be partially closed or expansion tank level insufficient.
3. Exhaust gas entering the cooling water space due to a defect (crack in the cylinder head or cylinder liner). Check the expansion tank for increased level and bubbles - the escape of exhaust gases. Stop the engine but keep the cooling pump running. Inspect combustion chamber for possible water leaks through cracks.
4. Overheating of the cooling water. In this case, steam produced accumulates in cooling spaces and causes pressure fluctuation just in the case with trapped air from item 1. Overheating could be judged by increasing the temperature of the cooling water at the output of the individual cylinders. However, if there is an air pocket at the temperature sensor its readings may be incorrect. Reconfirm temperature rise with laser gauge.
Possible causes of overheating:
1) The valves on the respective cylinders were mistakenly closed or defective.
2) Cooling system not properly vented and air pockets prevent coolant from proper circulation.
3) Cooler dirty
4) Cooling system contaminated and clogged.
5) Overload of the respective cylinders.
6) Overheating of the piston.
7) Cooling pump capacity reduced due to wear.
