urgent

Hi, check that the thermostat is opening, that the coolant reservoir is clean, and that the cap is replaced if possible. If it heats up quickly, it could be the thermostat, or in the worst case, the water pump. Check that the coolant is circulating properly and that the coolant or oil isn't cloudy; in that case, it's the cylinder head.
Good luck!

I agree with all the points Pescaditto mentioned. Also, check the oil color; you can do this with the dipstick. If it's grayish, the cylinder head gasket might be blown. Water is leaking into the hottest part of the engine, causing the temperature reading, and it's also mixing with the oil in the crankcase. The solution is to remove the cylinder head, resurface it if necessary, replace the gasket, and the problem will be solved.

I replaced the water reservoir cap, the oil is fine and water is reaching the reservoir, but could the gasket still be blown?

Hi, these engines need to have the cooling system thoroughly purged, removing all the air.