The head gasket is gone or the block/head is cracked. Water is mixing with oil. If you remove the valve cover and the head or the intake manifold you may notice a fluid of a deep battleship gray color. This is due to grinding metal fillament mixing with the water and lubricant due to failed lubrication because chances are that you are losing oil if the block is cracked and engine coolant is mixing with water and oil or the head gasket, or the manifold seal is broken and may need to be replaced. An oil leak doesn't always mean that the block is cracked-- your problem could just be a broken head or manifold gasket. This is easy enough to replace but requires extensive dismantling of the top of the motor.
When a car is hot, keep the car running and add water to the radiator. When a car is cold, you can simply put the water into the radiator.
Just flushing the radiator, no but to flush the system, yes.
maybe my radiator is defective
There are several causes that will reduce the water level in your car radiator. The most common cause is evaporation. Engine overheating will also reduce the radiator water level.
Thermostat and radiator
first turn on the car and add the water to the radiator while the engine is running-- this allows the water pump to work and force the water to all the thirsty parts this will help the radiator and the crankcase fill much faster
you mean like overheating? could be a number of problems, radiator leak, radiator broke, radiator fan, leaky hose, broken water pump, broken head or head gasket, or the car running to lean.
If your radiator is leaking then you will need to replace it. Additionally, if you find that your car is constantly running hot you will need a new radiator.
You should see the coolant circulating in the radiator when the car is running
After running your car for 20 minutes or so ,and there is coolant in the radiator, you will get no heat.
This is likely a stuck thermostat.
It can. You should leave the engine running while you add water to the radiator. If you add water with the engine turned off, but while the engine is overheated, you might damage the engine or the radiator.