Coolant leak and overheating

For a coolant leak you need to check all the hoses and fittings by radiator, heater core, expansion tank for leaks and/or integrity. Also make sure that your water pump is not leaking along with radiator cap and thermostat body. Overheat condition has no effect on battery - if it does not hld charge it is because it is chemically worn out and needs replacement. Until you correct water loss, have extra water in car at all times and run your heater full blast - windows open!!!!

Best test,,NAPA sells a combustion leak tester(balkamp #700-1006) A tube of liquid that will change color if combustion gas is in the cooling system.Thinking you may be right about a head gasket problem or possibly a crack in the head.
It comes with instructions and is cheaper than having a shop guess at the trouble.

I am not aying that this is what I think it is but I found this on your year BMW

Some BMW, Volkswagen and Audi cars have a common problem that the water pump impeller becomes dislodged from the input shaft of the water pump leaving the water pump looking ok from the outside, but failed on the inside. If your car experiences this problem it will over heat rapidly much like a thermostat failure. For exact specific repair procedures for this repair visit our online repair manual page.




Failed BMW, VW, Audi Water Pump Troubleshooting Procedure

• Check Water Pump Operation - To troubleshoot this problem first drain about a half a gallon out of the cooling system, next remove the thermostat from the engine. With the thermostat out start the engine (stand back) coolant should be forced out of the thermostat port, if not the impeller has dislodged itself from the water pump input shaft and needs to be replaced. To replace the water pump finish draining the remainder of engine coolant, remove the serpentine belt and coolant hoses. Next remove the water pump mounting bolts and remove the water pump. Clean gasket surfaces and reinstall new water pump and reassemble. Refill with coolant and recheck system, top off engine coolant after it has re-cooled.

exhaust gas coming thru the resiovoir, sounds like a head gasket

We have had some similar problems with some BMW's,the last one we replaced the expansion tank.Before we attached the upper radiator hose to the radiator we filled the engine with antifreeze through the hose.Install the hose and fill your expansion tank to the proper level and run the engine to operating temperature.

if you replaced the thermostat thats actually connected to the engine and not the thermostat thats underneath the expansion tank/reservoir, I would recommend filling the expansion tank to the top remove bleeder screw and allow the vehicle to sit keeping coolant filled to the top of reservoir, once you feel there is no more air in system reinstall bleeder screw and correct fluid level in expansion tank. There are a couple different tools that are used to properly bleed the system. one tool you can pull a vacuum on the system and when the vacuum is released coolant is pulled back into system. At the same time you could have a blown head gasket. There is also a tool out there to check for combustion gases in your cooling system which would be a signs of a blown head gasket. Another recommendation is to pick up a cooling system pressure tester (can be picked up as a loan a tool @ autozone) usually pressurizing the system will help to remove air pockets

replace your coolant reservior cap; the rubber ring has become brittle and "steams" the coolant out as you drive

have you replaced the bleeder screws? and the expansion tank cap they both have O-rings that compress and will fail over time also check the hose at the top of the exp tank it also has o-rings inside it

The water (coolant) in the expansion tank will rise and fall with the engine temperature. What the expansion tank does is collect and return coolant to and from the engine. When the engine warms up the coolant gets hot, builds up pressure and opens up the radiator cap. The coolant then goes into the expansion tank. Now when the engine cools down the pressure drops in the cooling system and the pressure drop (vacuum) pulls the coolant back into the engine via the radiator cap. The radiator cap allows the cooling system to build up pressure and by doing so increases the boiling point of the coolant, but when that pressure exceeds the caps rating the cap opens and the coolant goes to the expansion valve. The cap has another part to it that when the engine cools down and a partial vacuum is created in the cooling system a "valve" in the radiator cap opens and allows the coolant to be drawn back into the engine. I would look at your radiator cap to see if any gunk or build up is on it, and check the rubber gaskets for cracks. It's easiest just to replace the cap because they are inexpensive and easy to replace (2-10 dollars). The expansion tank should have two hoses on it. The one on the bottom comes from the radiator and the one on the top (possibly part of filler cap) runs down and is open to the ground. That way if it is overfilled or becomes overfilled it will slowly leak onto the ground. When and if you change your radiator cap, make sure the engine is cooled down, remove cap and start engine and turn heater to full blast, full heat. Leave the cap off and let it run until engine warmed up. This should burp out any air pockets that may have happened when coolant was changed. Also top off the coolant in the radiator while it is running. Hope this helps and good luck

I wonder why the overheating caused you to change the engine instead of checking with the cooling system. Some time back my 323i overheated and coolant expansion tank burst open, but that probably due to ignoring the coolant level low light for a couple of days. I replaced the coolant expansion tank, thermostat and few hoses connected with the tank and it worked fine for me.
Try to check you coolant level - may be you should have replaced your water pump and not the engine.