Same happened to my 87.
Head bolts were not torqued down tight enough out of the factory.
Sad part is I actually had the valve covers off while replacing the gaskets and the bolts were right there in front of me.
If I had only known..
Still got over 300K out of that Toyota though.
It was weird the coolant temp would go up and then go down I replaced the thermostat but no change. I bought this car new, the odometer stopped working but I got at least 200k miles out of it.