Every new engine came with a thermostat, so if it's not there, then somebody must have removed it at sometime.

Nobody has "removed" the thermostat, as it has never been there. I think i'll get a new unit and install it. Thanks anyway

From your description, your engine is seriously overcooling & I would suggest that somebody has removed the thermostat completely, or it is stuck in the wide open position. Either one is bad.

If you purchase a nice new one, along with a gasket & install it, there is a good chance that your engine will run just fine.

It doesn't matter if your radiator is cool .... the engine thermostat will keep the engine at the right temperature. Even if the radiator is cool.

So check the temperature gauge on the dash. If it is between the left and right lines, it is OK.

180 F, 82 C
<-- wiki, search for Thermostat

you need to have the thermostat to keep the coolant in contact with your bores and other engine internals long enough for effective heat transfer. Your temp gauge will not be giving you an accurate reading of the temp of your engine. The coolant will run cooler, but the temp of bores and such will be higher. If you dont want to fit the thermostat, at least cut one up and just put the backing plate in to give you some restriction to flow.

using a B11 sunny electric fan switch with a relay, i had the bung from a sunny radiator put in my radiator when it was recored at the beginning of my overhaul.

i think i was being too clever and got a 160F thermostat for the engine and it runs 'cool' all the time... in my setup a 180F is probably actually a good thing... others may want a 170F or a 160F if you live at the equator...

other things affect engine temp, like oil coolers too..

I have a Datsun Pulsar Thermo switch on mine. They came out with a A14 engine here in South Africa but the engine was mounted to pull the front wheels with a front wheel drive gearbox. The stats of the switch says that it kicks on at 85C and off again at 81C.
So I would say this is the optimum temp for a A series engine.

Put in a thermostat as soon as possible. Running a engine too cold will cause premature wear in the engine because it is not running at it's designed temp.
Almost like a runner that does'nt stretch and warm up his muscles before a competition.He will pull a muscle.

75-80 degrees C get a high capacity radiator and a genuine nissan thermostat youll never have another prob

Can't tell you about the motor itself.. But your heater would be a bit... Cool... I would imagine

I have fitted an electric fan to replace the stock pulley radiator fan. It remains on whenever i turn the key. It doesnt have any thermostat or shut-off system. The cooling for the water is extremely good that even if i drive for more than 100km, i just have to leave the fan for abt 30 seconds & i can dip my finger into the radiator tank. The temp gauge barely touches the bottom line.

Can somebody please tell me how can running below the optimum temperature affect the performance and fuel consumption and perhaps other issues? thanks.