I have been driving a 120y. Recently, the radiator water seem to decreasing fast (about.. mmm... a liter a week) the car had already gone radiator change with 3 core c22 vanette bout 6 month ago. The temperature increase to half (meter) in traffic jamm. It looks normal.

I'm pretty sure that the head gasket were still in good shape as there were no strange things happen in the engine operation.
There were no leaks elsewhere.
But.. where does the water gone to?
p/s: Is it the thermostat?

Well, water can't just disappear. It can only leak a) on to the ground or into your bellhousing (if the rear welch plug is stuffed) b) into your crankcase and oil (causing milky oil) or c) into your cylinders where it's being burned off, causing a constantly steaming exhaust.

Check your block carefully around the welch plugs looking for any signs of a leak. I'll bet it'll either be those or you've got a head gasket leak.

Cheers,

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Chris [;-{)>

You should also look at attaching an old coke bottle to the overflow on the radiator cap. That way you can rule that out as a cause.

I think it sounds like headgasket, maybe a cracked head. Remove the spark plugs one at a time (with fluel and spark disconnected) and look to see if water is in the cylinders when the engine is hot. Crank it over, you'll see the rusty of coolant coloured water come out the spark plug hole.

This test will be less effective when the engine is cold.

Chris

Also check the thermostat for leaks.

In my old A12 it leaked like a siv.

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Datsun Sunny
Z18 Turbo
Stock...ish

i had the same prob with my 120 i put an new rad, thermo housing and water pump eventually i discovered a hairline crack in my timing cover so i araldited it up and it was ok i still leaked outta d overflow hose a bit

i just finnished rebuilding a 120, had the same problem and couldn't find it's cause. on puting it back together i picked up the water pump and it had a small leak from the hole that it has for when it fails. i've never had it do this before (small leak) as it usually just pours out