How can you tell the radiator is running out of coolant? If it's because the Temp Sending Unit gauge is showing the engine is running hot, and you have been driving it a while like this, you may be beyond the initial cause for the leak and may now have a secondary leak, like a bad head gasket. IF this is not the case, then 1) Check for obvious signs of radiator fluid leaks - like the radiator cap atop the radiator (look for signs of dried antifreeze that has run down the radiator); check the connections of the radiator hoses (upper & lower) at BOTH the radiator<=>water pump, and radiator<=>upper intake manifold, and then check the overflow tank for signs of leaking. When the car heats up, a small amount of antifreeze (radiator fluid) will move to the overflow tank. After the car is turn off, the engine cools and antifreeze is sucked back into the radiator (this happens normally every time you take your car for a drive and the engine warms up). Next you can pressure check the coolant system. The pressure check will reveal an external leak or can be used in conjunction with confirmation of the looks of your spark plugs to confirm the leak location (the cylinder that is taking in the coolant will have heavy white deposits on the spark plug). If you have these deposits, take it to a mechanic for an estimate on repair (which will likely involve getting a new head-gasket). Good Luck.
Thanks. What about the expansion tank coolant it does not get sucked in the radiator, it always full all the time?
If the tank appears to be overfull that may indicate a problem with head gasket(combustion gases going into the coolant and forces it back to tank) or if it has a pressure cap on the radiator it could have gone bad and just vents the coolant back to tank. The pressue cap should be tested when a pressure test is done. You could try a new cap first since that should be low in cost.