1988 Nissan Sunny coolant

If you don't see an external leak around the engine, and no white smoke out the tail pipe, add a small bottle of dark purple dye to the coolant, then check with a black light a day or two later. The dye will show up as a bright yellow stain that you can follow back to the source. Be sure to check the tail pipe. If you see the dye there, the cylinder head gasket is leaking.