Bakit ka pumayag na tanggalin ang thermostat, what was the objective?

Your cooling system was designed to work with a coolant which is not pure water. Why and when did you stop using this? If you will notice, the boiling will stop once you increase the rpm of your engine.? This boiling is caused by hot spots in your engine which is due to scaling inside your engine ( very similar to the scaling you would see in public comfort rooms using hard water or deep well water source) Do you live in a village with a deep well water source?

The use of ordinary water in your cooling system is the cause of all this, in other words nabubulok na cooling system mo. You have actually significantly altered your cooling system, 1st removing your thermostat 2nd changing your coolant medium. Pero me magagawa ka pa,
you should drain your cooling system of this water and replace it with a proper coolant. Better if you could flush your radiator for a few weeks to dissolve the scale and rust, sana d pa sobra rust and scaling.

The use of a proper coolant is not to increase the cost of maintenance,
it is to increase heat transfer, rust protection and to aid your mechanic in
failure analysis (dyes).

You can start by cleaning your reservior (expansion tank). This is to help you in your failure analysis. This may sound ridiculous to mechanics in the
the US but hey we are in a third world country.

Now again, me putik ba o puro putik reservior mo?