1990 Toyota Corolla toyota corolla overheating

The fact there's coolant in the reservoir just means it's not leaking out. There's lots of other causes of overheating.

The most common cause is a dead electric radiator fan if that's what your car uses. That is needed at real low speeds. At higher speeds normal air flow is sufficient. If the engine still runs warm when the car is moving, try turning the heater on the highest temperature and one of the higher fan speeds. If that brings the engine temperature down within a minute or two, suspect corroded cooling fins on the radiator. That will reduce its ability to give up the heat to the air. The heater core is a small radiator so running the heater will reduce engine coolant temperature.

thanks i'll have the radiator checked tomorrow. it's generally humid here so i don't have a heater in my car and the radiator fans are working fine so i don't think it's the problem. by the way, when the engine overheats the air conditioning system is on. so what i do is, i'll turn the aircon off and in a few minutes the engine temperature goes down. although, the temperature gauge shows normal i can hear a boiling sound inside the water reservoir. anyway, i'll have the radiator checked first and if the engine still overheats after that, do you think it has a more serious problem?

The next suspect would probably be a leaking cylinder head gasket. Your mechanic can check for that by drawing air from the radiator through a glass cylinder with two chambers partially filled with a special dark blue liquid. If combustion gases are present the liquid will turn bright yellow.