My bro has an 89... They have the d16a1 or a6 Dohc can't remember but anyways. My bros did the exact same thing. I kept putting coolant in it and checked everything. Finally found out the water pump was leaking but you couldnt tell. And yours might be going bad. Cuz my bros did for like a month, until we changed the water pump. Driven like a champ since. And let's just say they a pain in the ass to change too. But good luck. And if you do change the pump, do the timing belt. Might as well. Hope that helps.