I don't have a link to a DIY for this part, but I have just fixed a small leak from the transmission thermostat on my 2005 X5 w/ 4.4i engine. My leak appeared on the plastic part that the transmission thermostat is attached too, but I went ahead and replaced both just to be sure that it was fixed.

If you have a basic understanding of cars and some mechanical skills you should be able to complete this repair yourself, no problem.

Here are the basic steps:

1) Raise the front of the car on jack stands or ramps.

2) Remove the bottom plastic splash shield from the front of your X. There are about 10-12 bolts holding it to the front bumper and undercarriage of the X. With this out of the way you should be able to see the plate holding the transmission thermostat. The metal box looking thing below the thermostat is the transmission fluid cooler.

3) drain the coolant from the bottom of the radiator (blue drain plug on bottom of radiator). Replace the drain plug with a new one. (BMW part no: 17117571988)

4) Check to make sure that the leak is coming from the transmission thermostat and not from something else in this area. On mine, the leak was from a small crack on the mounting plate that holds the trans thermostat and cooler. If you aren't sure, replace both.

5) Remove the hoses attached to the transmission thermostat. There should be a top and bottom hose, and two side hoses, if I remember correctly. You may know how to work with BMW hose connections...use a small screw driver to pull the metal clip up fromt the hose end, and pull the hose off of the connector.

6) The transmission thermostat is mounted to the plastic housing by a single bolt (torx head, I think). Once the hoses are off it should be easy to see the screw holding it on. Remove the fastener and the transmission thermostat should pull right off. Replace the rubber xrings on the mounting plate before installing the thermostat back on.

Transmission thermostat: BMW part no: 17107559966
Trans. thermostat xring: BMW part no: 17101439139 (2 needed)

(The only thing that may need to be replaced to fix your leak may just be the Xrings. You may want to just replace them and see if that fixes your leak. The Thermostat may still be good, unless you see the plastic cracking or something.)

While you are in there, I would suggest replacing the Xrings sealing the transmission cooler (BMW part no: 17101439140 (2 needed)). That way both potential leaks are fixed.

7) Reinstall the transmission thermostat to the plastic housing. Screw in the anchoring screw, and re-attach the hoses connected to the thing.

8) Fill and bleed the cooling system with BMW coolant. See other threads on this site for the exact procedures for filling and bleeding this system on your ride.

Or you could do the full cooling system replacement, all hoses, expansion tank, water pump, and thermostat. You will save yourself a lot of headaches down the road by replacing all of these things now instead of piece by piece.

Your best friends: realoem.com and ecstuning.com

Good luck and hope this helps.

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2005 X5 4.4i black on black
2003 325Ci black convertible