Nissan Repair: Radiator Water is always low, nissan pickup, exhaust gas

Question
thank you for the reply, i will do a check for leaks. but what about the carbon monoxide test, where i can find it and how much it costs? thanks.
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-----Question-----
everytime I check radiator water, i find it low by little bit but there is no over heat in the car. I can't see where is the problem. Nissan Pickup 1990.
-----Answer-----
Bashir,

Check to make sure you do not have a small leak at the thermostat or water pump.  It can be so small it does not drip on the ground when parked.  But when driving it will drip and lower your coolant.  Also, if you have a really small headgasket leak it will do the same thing.  Check for leaks first.  You can do a carbon monoxide test on the radiator to see if exhaust gas is present.  If it is you probably have a headgasket that is blown.

Answer
Bashir,

You could do it yourself with a kit for about $25 bucks.  Or get a shop to do it for you.  Basically, they sell a cone and some colored fluid.  You put the cone on the radiator, where the cap goes, and then you pour the fluid in the top and then, using the bulb they provide with the kit, you suck air from the radiator through the fluid.  If it changes color you have a blown headgasket.  If the fluid remains the same color and does not change the headgasket is good.