Coolant leaking from the thermostat housing is a common problem. Most often it is the gasket around the coolant thermostat that has lost its seal. However, the gasket is not available without purchasing with the thermostat as a complete unit. Because the coolant thermostat housing is often deformed from repeated heat cycles, it is prudent to also replace the housing at the same time as the thermostat.
Tools needed: 10mm socket, 8mm socket, 3mm Hex, T-30 Torx, extensions, ratchet, pliers, flat blade screwdriver, coolant receptacle.
Optional: ramps or hydraulic jack & stands.
Depending on how low the vehicle is, or how shallow the receptacle for capturing spilling coolant, ramps or raising the vehicle may be the first step. I've raised the front of the vehicle and placed on stands in this procedure.
Open the boot and disconnect the battery. This will prevent an unwanted electrical spike when disconnecting the ECU later.
Before closing the hatch door, place something in the latch opening to prevent the door from locking with the battery off-line.
Raise the bonnet, remove the IC cover (T30), and release the throttle body tube and intake snorkel clamps with a flat blade screwdriver. Insert the screwdriver blade and turn sideways to unfasten.
Unscrew the 8mm self tapping screws from the air box cover and remove.
Cover the throttle body hose to prevent debris from entering. Pull the air filter. Disconnect the intake snorkel from the radiator support, pull rearward to dislodge the tab on the right, twist the snorkel to the left to disengage its tab in the air box base.
Continue twisting the snorkel tube while rotating rearward, and remove.
Remove the cover on the ECU, use compressed air to blow away dust and sand that may drop in the ECU when the connectors are removed.
The harness connectors are disengaged by pulling the locking brackets. Do not attempt to force the connectors off be pulling upward, the connectors will eject when the brackets are unlocked.
Unscrew the 10mm bolt securing the air box base in front of the ECU.
Remove the air box base by pulling upward and to the front, there are two mounting grommets in the rear attached to the cowl, from which the base must release from.
The thermostat housing is located on the side of the cylinder head. There is a bracket attached, from which the O2 connector, brake boost tube, upstream MAP sensor, and harness section, have to be removed first. The upstream MAP sensor is secured with 3mm Hex bolts.
With those items out of the way, it is easy to see residue from leaking coolant around the thermostat housing. Before removing the thermostat housing, place a catch pan under that area of the engine.
Begin removing the 3, 10mm bolts securing the thermostat housing to the head.
Remove the hoses attached to the housing; hose gripping pliers help with removal.
For hoses that have tenaciously fused themselves to the housing, insert a flat blade screwdriver to release the bond, and allow coolant to lubricate the hose further.
The leaking thermostat can now be removed.
Crusty deposits can be cleaned off with a plastic spatula.
The area around this thermostat opening had some nasty burs. I used 2000 grit wet/dry sandpaper to smooth the edges and gasket contact surfaces, then degreased with brake cleaner. This surface condition and level of deposits is out of the ordinary.
Put the new thermostat in the new housing, the air bleed hole is positioned at the top.
Slide the coolant hoses on the housing, move the clamps to their original position.
Before placing the housing against the head, be sure the thermostat gasket is seated within the housing groove. Pinching the gasket will negate all the work you just performed.
Start with the housing's lower bolt, and screw most of the way in. This will help stabilize the housing while attaching the bracket to the outside. Torque the thermostat housing to the head at 9 ft-lb.
Reattach everything in reverse order. When pushing the air box base reward, feel with the fingers to be sure the grommets are mounting correctly.
When reconnecting the ECU harness connectors, place the female connector over the male, push the bracket in to lock in place.
Use pliers to close the band clasps.
Both BMW and MINI antifreeze are fee of Nitrites and Phosphates which can encourage precipitate from the coolant mixture, and both are concentrates, which are mixed with distilled water before pouring into the coolant expansion tank. Follow manufacturer's recommendations for mixture ratio.
Pour coolant mixture into the top of the expansion tank, after the coolant level has stabilized, open the bleeder screw located in front of the intake manifold. When fluid escapes, close the screw.
Add coolant to the expansion tank to a level between the MIN & MAX marks. Operate the vehicle until it reaches operating temperature, allow to cool completely, open the bleeder screw again till fluid escapes then close. Adjust coolant level in the expansion tank if needed. Check fluid level before the next couple cold starts and adjust again if needed. Look for evidence of continued leaking.