My temperature gauge fluctuates, and a hissing sound comes from near the coolant tank when car is hot, what could be the problem?

. Purge any air from the cooling system.

. Check the radiator cap. The link below should help:

https://www.askmehelpdesk.com/cars-trucks/faq-how-troubleshoot-repair-maintain-hondas-selected-other-vehicles-46563-3.html#post235973