DIY Disclaimer: I hold no responsibility should anything goes wrong when you decided to follow my guide. It’s strictly meant as a reference for knowledge sharing purposes.
2007 BMW E90 N46B20
Mileage: 108k km
Been losing 500ml coolant weekly.
Photo taken from beneath the power steering pump.
I have no idea where the leak came from due to limited visual access but I can confirm it is from one of the water pump components. And losing fluid at 500ml a week, I can’t be keep topping up and dilute my coolant mix which is not healthy for the engine.
So I decided to take the plunge by buying all the components for that location (just in case), including one of the short hose.
Item 1: Water pump: 11 517 511 221
Item 2: Thermostat: 11 537 510 959
Item 4: Thermostat Housing: 11 517 572 859:
Item 7: Connector: 11 517 542 128
.. and Item 1: Water Hose: 11 537 572 159
The spark plugs are for different project. 
The rubber O-ring came with the water pump are not utilized because the rectangle one already available in the new thermostat housing while the round shape O-rings are for the old connector which was designed differently. New connector as you can see from the photo above at 4 o’clock location is entirely coated with rubber.
Workshop guide:
1. Our E90 N46B20 power steering pulley actually do not have the 3 holes to reach for the 2 water pump fastening bolts. Hence the only way is to remove the power steering pulley. Slacken the 3 x T40 bolts on the power steering pulley before remove the V-belt. The tension provided by the belt help securing the pulley from spinning freely. The pulley is very light and made of plastic material.
2. Use a size 16mm wrench to tension up the V-belt tensioner and release the V-belt.
3. Remove the power steering pulley.
4. Slacken the 2 x T40 bolts fastening the water pump to the power steering pump. These bolts are very tight. My Torx key set failed me because it bent when I apply force:
I had to order this:
5. Now the tricky part. The 4 x T45 bolts is hard to reach. Luckily they are not extremely tight compared to the above. The “L” shape Torx key excels. The T45 bits I bought is mostly hopeless here due to the narrow access.
Below is how I slacken (NOT RELEASE) the T45 bolts and if you are confident, you can release the water hose connections as it will give you a bit of extra room:
6. Water hose disconnection. Start from the top. Lift up the metal clip (refer bottom picture, from point A to point B) and wiggle your way out, 3 hoses connected to the thermostat.
7. Disconnect the thermostat wire located at the bottom of the thermostat.
Got the following from Pelicanparts. Similar connector. Just press the metal clip to the green arrow’s direction and pull the plug off.
8. Now you can release the 4 x T45 bolts. The water pump connector may still be hanging to the engine and require some prying to get it out.
9. After both the power steering pump and water pump is out, then only I remove the 2 x T40 that fasten the power steering pump and water pump together. Water pump can then be taken out. Viola!
Removal took less than 45 minutes. Installation is not easy. Remember to fasten both power steering pump and water pump together before attempting the 4 x T45 power steering pump mount. Do not forget the connector at the water pump. Very tricky to put it back in because it keeps jumping out.
All in, I spent 4.5 hours to complete the job, inclusive of beer break in between.. Started 8:30pm, completes 1am.
More photos for references:
Power steering plastic pulley can be open using the T40 Torx key. Notice the V-belt still in place.
After taken out the water pump:
Check out the thermostat top water hose. Size 10mm bolt. Also you will notice the water pump connector below the exhaust manifold.. took me a while to get it out. Actually mustn’t apply force as it gets tighter. Just need to repeatedly tap it to get it out. Ya, I replace the connector too.
The old water hose, lucky I replaced it. Ya, I applied gasket maker here too :
Another view at the stubborn connector:
Bottom view:
Back view:
Check out all the screws and bolts taken out. How do you like my metal screw pan? I believe we bought it from IKEA toy section when my children were young. It was on the way out to the donation bin when I rescued it and turn it into a screw pan by adding few magnets at the bottom. Viola! No more losing screws..
Old versus new. I apply gasket maker whenever possible. In fact, I’m suspecting my leaks are from these areas and not the water pump.
This is how I open the thermostat, using a 1/4” bit. 7 of these screws. 3 to secure the thermostat to it upper housing and 4 to secure the water pump.
2 bottles of 1.5L coolant with 6L of distill water. Still plenty left.
That’s all for now! Thank you for reading and please provide feedback or query if any.. 
