Although the reasons why the cooling system was air can be many and not necessarily what would be so scary it was. Here is how to clean it up. There is a special bolt in the radiator on top of BMW 318i, and on all other BMW E36 of skiving air from the radiator. It should Unscrew, if topped up the fluid in the overflow bottle after a lot leaked. If you add a little liquid, i.e. if at least the third part of the expansion tank was still full, you can not Unscrew the bolt at all.
Now the whole procedure of taking away the air from the cooling system BMW E36:
1. Open the lid of the expansion tank (pic.#8).
2. Gently loosen or completely remove the bolt release air(pic.#7) from the radiator (located near the expansion tank, the top of the radiator)
3. Turn on the oven to maximum temperature and maximum airflow
4. To start the car engine
5. Fill the expansion tank liquid, while unscrewed from under the bolt of the radiator will not go liquid with no air.
6. Screw the bolt venting of the radiator completely.
7. Refill the tank to a liquid level and turn off the engine.