You can use water for the short term. Besides removing heat, the coolant also lubricates the water pump and prevents corrosion. If you are going to add water to drive around and check for leaks and fix it I think it will be fine for a few months. There are some that have used water for over a year and nothing happened, then again that is not enough time to see actual wear on the water pump or the impact on corrosion.

Or you can go to Harbor Freight and get a pressure tester for 70.00 or so with tax after a 20%coupon that is found everywhere. I did that to find out my leak in my radiator and replaced the radiator...and ummm...several other parts that worked perfectly fine until I tried to remove them.

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2004 E60 545i Sport 6MT. Black/Black Build Date 6-2004 102k miles