i have a 93 gs 300 recently its been acting crazy, i thinkh its a cracked head gasket when i start it starts boiling over in the resevoir and gets real hot but its not showing it on the reader, i also recently rplaced the thermostat, radiator and the crank shaft pulley, do you think its a crack, and would it be cheaper to replace it w a whole new engine, or to fix the crack in the head?

 

if youve runn it hot to many times you could have blown the head.

but it sound like you didnt bleed all air out of the system.
the sensor wont read unless coolant is touching it.

other possibility is a cloged radiator
or a leak causing air to get into the system.