Long story short: replaced a faulty thermostat housing in my girlfriend's '09 Mini Cooper S to correct a P0597 fault. Topped off the reservoir with distilled water and ran the engine in park for a few minutes with the thermostat bleeder screw open - coolant level dropped to normal. Closed the bleeder screw and took the car for a 15 minute drive with no issues. Was feeling pretty good.

Parked, checked the coolant level and noticed the reservoir was empty - all that movement must have gotten more air out of the system. With the engine off and warm, I added more distilled water into the reservoir until it reached the MAX fill line. Then I heard some ticking noises and the reservoir started bubbling. Looked under the car and a fair amount of coolant was dripping out from the passenger side of the engine where the belts & accessories are located. I couldn't see where it was coming from, but enough dripped out to empty the reservoir again. Suddenly realized I may have just screwed up big time and Googling has led me to believe that adding room-temp water to a hot engine may have cracked the block or head.

Obviously this isn't a common problem but if anyone here has input / suggestions please help. As it stands I'm planning to have the car towed to a shop for a proper diagnosis, but I think I may have just made a $3,000 mistake.

 

Welcome aboard...before you jump into assumptions, jack up the car and look around the area where it was leaking. There is a big chance you broke the water pipe connecting the thermostat to the pump.

Secondly, why are you using water and not coolant???

 

Jacked up the car and looked - there doesn't seem to be anything dripping from around the water pump or the plastic pipe, but there's a heavy drip coming from where the block meets the oil pan near the passenger side rear corner. I pulled the dipstick and there doesn't appear to be coolant mixing with the oil, but at this point I'm going to assume the block is cracked near the base. Will have an independent shop confirm this week.

Assuming the block is cracked, is there any possibility it could be welded or otherwise repaired? My girlfriend was planning on getting rid of the car soon - the engine is making a knocking noise that might be the "death rattle" and she has constant problems with the ABS / oil pressure / TCS systems and she doesn't want to invest much in it.

 

That area is where the water pump resides. Jack up the car again, fill it with water and see where is comes out from.

 

What MiniToBe said and No, if it's cracked you'll need a new block. If the engine was making a knocking noise and the oil pressure light was on, the car should of been brought in to a shop right away. Sounds like it was starved of oil, there's a reason you change oil early and check it often with the 2nd gen cars.