have 87 Sentra that runs hot, but can't find any water leak.Any ideas?

You will have to pressure test the cooling system. Coolant doesn't evaporate. You have a leak somewhere. You need to find the leak before you have to do major engine repair. Get it checked out now.

It could be a stuck thermostat or many other things causing the engine to run hot but it needs to be investigated before more serious damage occurs. Is the engine loosing coolant? The way an automotive shop would locate the leak is by removing the radiator cap and attaching an adapter that screws on in place of the radiator cap. Something that looks like a bicycle pump with a pressure gauge attached to it is used to "pump up" pressure to simulate pressure generated when the cooling system is at operating temperature, the leak is then found sometimes it is obviously leaking from a coolant hose or cracked housing, failed gasket or water pump, sometimes the engines cooling system needs to be inspected from below or inspected using a mirror and flash light. If the coolant loss is still evasive dye can be added to the cooling system that may indicate the source of the leak. A mysterious coolant loss may even be a sign of a head gasket that is not sealing well.

are you having to add coolant? if not check the thermostat.