I have a 325I 2003. I wanted to put coolant in my car and I started putting it in the reservoir. But I wasn't sure if that is where i put it so I stopped. Will it hurt if I put a little in the reservoir and if the needle in the radiator is already top-off should I wait till it need some. A friend of mine did some work to the car and he said the water was really low. So for a temporary fix he put water. But told me that I needed coolant.
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The first thing to do is repair any cooling system leak that you have. (Only on a cold engine). The filler cap is on an expansion tank, which is on the right hand side of the radiator as you look at it from the front of the car. You add a mixture of antifreeze and water, at a 50-50 mix, into the tank until the float in the tank rises to the top. Replace the filler cap firmly. Run the engine until it is warm. Next to the filler cap is a cross headed plastic screw. Loosen this screw slowly until you hear air escaping. Do not unscrew this screw fully out. After the air has all escaped, coolant will start to come out of the screw. Tighten the screw back up. The cooling system is now full. A final check would be a road test to make sure the temperature gauge, heater, and cooling fans are all working correctly.
Since your friend added a significant amount of water to the system it has diluted the mixture of coolant to water. Instead of just adding coolant you need to drain the system and replace it with the correct ratio of water/coolant.
As Allan hinted at, the only way the coolant would be low to begin with is a leak somewhere. Unless that is what you friend was repairing you still have a long term problem that will need to be addressed before it causes your engine to overheat.