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One morning, I started our 2008 Mini Cooper S. Minutes later, an indicator light attracted my attention to tell me that the engine was overheating. I immediately turned off the car and checked under the hood. Not hot, but the electric fan was blowing on full in an attempt to alleviate the problem of being "too hot." I started the car up later in the day, and the same problem occurred, but this time the MIL illuminated. Suspecting a sensor issue rather than any true overheating, I checked the MIL and found that it threw out the Faulty Coolant Sensor code. Good. That would explain the weird behavior where the fan is running to cool the radiator when there's no way the engine has had time to heat up the coolant. I then got the Mini into "diagnostic" mode so that I could read the temperature.
ASIDE: Diagnostic mode in the Mini can be accessed by holding down the right-most button on the tach for 10-15 seconds. The tach display will eventually read out a menu number 1, which displays your VIN number. Sum up the digits in your VIN (e.g., 11111 = 5), and remember them. Hit the right-most button many times until you get into menu 19, which is the code lock, type in the sum of your VIN that you previously memorized. Hold down the right-most button again to get back to menu 19, and then click that button some more to cycle around to menu 7. Wait for one second, and then your coolant sensor temp will be shown. You can click some more into menu 7 to find the air intake temperature as well.
My Mini was reading gibberish, some sort of ASCII character that I didn't recognize. After hitting the gas a few times, the reading shot up in a matter of seconds to 145 C and the overheat warning came on. Yup, faulty sensor, but what to do about it?
Here's the tricky part. On the 2008 Mini (R56 engine), the coolant sensor sits on the thermostat housing. The housing is on the right side of the engine, but it is buried under air intakes, vacuum lines, and electrical wiring harnesses. It takes some work to get to it. I figured, if I was going to do that work, then I may as well replace the thermostat housing, just to get ahead of any potential thermostat failures that would have me right back in the engine compartment. On most cars, that's a bit silly, because a thermostat is a cheap replacement. However, on the R56 Mini, the thermostat is molded into the thermostat housing. That's right, they come as a team, you replace the housing with the thermostat. As such, it seemed like it would save me time in the long run to replace everything.
Getting to the housing is tricky. All told, I spent a solid 5 hours on this project spread over two days.Disconnect the battery: 10 mm socket to remove the terminal, battery located in the upper left of the engine compartment under a flap.
Drain the coolant: I put the car on jack stands, then got under the front to look at the coolant lines. A little on the driver's side of the car, there is a pipe that acts as a union between two coolant lines, I disconnected one of those and drained the coolant. I did this with the expansion tank (white bottle in the lower-right of image) open.
Remove ducting: I removed the air intakes. One goes from the air filter box to the turbocharger, and I disconnected that first, disconnecting electronics for MAF as well. I removed the air-filter box just to have access to more of the compartment, and then removed the "snorkel," the air intake that runs from the air-filter box out near the headlight. There are two vacuum lines that run up from the back of the engine compartment, one goes to the block and one to the turbocharger.
Loosen wiring: There is a wiring "aggregator" that rests atop the thermostat housing (black plastic), I popped all of the connections that helped hold that in place, including the three ECU connectors. This allowed the "aggregator" to at least move around enough that I could work.
Remove plumbing: The thermostat housing has 6 coolant lines emanating from it. Most of these can be loosened while the housing is on the engine. I used a pair of vice grips to pinch the hose connectors, but there are specialized tools for the job or you can use channel locks (reminds me, I need to get some channel locks). I could relatively easily remove the coolant lines in the front that go to the radiator, and the ones in the back that go to the heat exchanger (heat in the cabin). There is a small line going to the turbocharger, and I needed to have the housing off to get the right angle to remove that plumbing. There is one that actually slides around the back of the engine (I am serious), and that one can't be removed until you unbolt the housing, but there is a clip that needs to be removed; you have to crane your neck around from the side to see it, and you pretty much have to work it by feel, just pull straight-ish up.
Remove the housing: Use a 10 mm socket to remove 3 bolts that attach the housing to the block. Pull it straight out from the block, and the connection formerly with the clip (around the back of the engine) will come undone. The housing, once off the engine is the craziest car part I have ever dealt with. I am no mechanic by trade, but this engineering marvel looks like it could also be used by the Machines in the Matrix Movies as part of their human growth towers. The offending coolant sensor is on top of the housing. Mine had a clip, and therefore could have been removed and replaced, but my searches on the Internet suggested that some housings have pre-molded coolant sensors. My replacement housing also had a removable sensor, though.
Install new housing: Installation is reverse of removal. I sprayed the outside parts of the new thermostat housing with some dry Teflon lubricant to make it easier to get the coolant hoses on the new housing. Tighten the bolts carefully, get each one started before applying any real force on the gasket (which is pretty well designed).
Reinstall other stuff: Put your wiring back in order, reconnect the vacuum lines, and reassemble the air intake duct. Make sure to re-attach the hose/pipe/hose union on the underside of the car (I used this to drain the coolant). Put the battery back together.
Add new fluid, bleed the system of air: Adding new fluid is different than any other car I have had. There is no radiator cap to work with, you can't just open up the radiator and pour stuff into it. Instead, you have to fill from the expansion tank, which maybe has a 1-2 L capacity. The system uses more like 4 L of coolant, so you have to be able to get fluid into the expansion tank and then move it around. I filled the expansion tank, and then squeezed a couple of the hoses that run from the tank to the radiator. This made a lot of bubbles, but clearly moved fluid from the tank elsewhere in the system. I did this a few times, until, frankly, I got tired of doing CPR on the coolant system. I started the car up and watched the fluid go down; I had the expansion tank open, topping it up as it got lower. I did this a couple of times, until I wasn't getting any more fluid into the system. That's not because the system couldn't take more coolant, I think it's because the engine wasn't hot, and therefore the thermostat hadn't popped open to allow coolant to flow through the housing to the other reaches of the system. Now I closed the expansion tank, and let the car heat up. I watched the coolant temperature using the "diagnostic" mode. After replacing the housing, the coolant temperature read a comfortable 18 C, and it heated slowly from there as the engine warmed up. Once the engine was warm, the thermostat opened, and coolant began flowing, and some air bubbles leaked out of the expansion tank. For the final part, I bled the system. There is a bleeder valve on top of the thermostat housing, and you can access it with a long screwdriver. The bleeder valve is located just to the right of the engine block, between the block and a bracket. Once warm, I opened the valve 3-4 full turns, and a hiss of air/steam/etc. escaped the valve. Once coolant leaked out, I closed the valve back down.
Test drive: take the thing out for a drive, it was well earned. Watch the coolant temperature, and then park the car and let it cool. Keep an eye on the coolant for the next few drive cycles to ensure that you don't run low as the coolant seeps into the nooks and crannies of the coolant system.
Get a drink: I didn't do this step, because it was getting late, but I really should have settled down with a well-deserved libation and called it a day for auto repair.
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