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Message: <blockquote data-quote="jarance" data-source="post: 440597" data-attributes="member: 21">While the engine is warming up, did you see any water overflowing from the radiator inlet where the cap is to be?When water is overflowing from the cooling system, what would think is replacing the water? Air !! Your mechanic is inducing air pocket in the cooling system. I have mentioned many time in my postings that the pressure test is NOT a sure way of finding leaks. This is based on my 12 years driving a BMW. Here is a tip on how to check for leak.1. Filled the radiator with water. 2. Open the bleeder to bleed the air and keep filling until all the air bubbles is purge out.3. Start the engine and rev up the engine speed.4. Keep filling the radiator and check the bleeder cap for air bubbles. If there is none, close the bleeder cap.5. Keep filling the water until overflow. (Basically not allow any air in the radiator)6. After the engine is warm up for about 10 minutes, off the engine. Wait 1 minute and check for water drop let from the bottom of the car.7. Trace the source of the water droplet.The above test is basically heat up the water and expanding it. If there is no air trapped in the cooling system, the cooling system will be pressurise by the expanding water. If there is any leak it will be seen by the formation of water droplet.Remember to drain the excess water after the above test.</blockquote>