I bought 09 mini cooper s a month ago and the shop replaced the timing chain and currently I replaced the thermostat and coolant temperature sensor. the car had problem with start up but it runs fine after it started up. the weird thing is after a few minutes drive and leave it cool until next morning and popped the hood and open the coolant reservoir, the pressure is really high so even if I open the cap really slowly, coolant exploded out of reservoir. Today I pulled out spark plugs and those from cylinder 2-4 was little wet and found out it was little coolant on it so I noticed that since coolant or oil in it, the car is hard to start up. (right?) and I checked the oil cap and I saw little milky fluid (like oil and coolant mixed) on it. while I was pulling out valve cover, one of bolt was broken in half... so I couldn't even replace the valve cover gasket....and I noticed that there were some milk fluid on breather hose & hose from valve cover on driver's side.... I feel like I'm pretty f***ked up... lol. Is it head gasket problem or oil cooler? I wish It could be oil cooler. How do i have to get the broken bolt out of valve cover..?
if it's head gasket problem, anybody used a head gasket sealer from Walmart or Autozone? did it really work?

 

Oh man that is it not a good day at all. It does appear your head gasket has failed, allowing the oil and coolant to mix. DO NOT use the “mechanic in a tin” solutions. Those concoctions have a use but not this time. The head will have to come off to make the repair. The broken 5mm bolt for the valve cover shouldn’t be too hard. I’d expect an ez out would be suitable to spin it out. You could also try a drill bit with a reverse direction. It may catch it enough to spin it out. I’d doubt it’s actually frozen in there.

 

Impact screwdriver will probably do it too if it is more frozen. You just must be careful not to hit it like a gorilla or enough to damage the threads. You usually just need to make a flat slot for the screwdriver with a dremel tool. Sorry for all the work you will have to do.

 

To get the bolt out you might consider drilling the center of the bolt minimally and inserting a screw extractor commonly found at auto supply retailers. It would appear from your description the coolant system is getting pressurized from combustion/exhaust. You might try to diagnose just doing a cold and then warm compression test. Anything +/- 7% give or take variance from cylinder to cylinder would tend to indicate a head gasket or other compromise between the cooling system and the combustion chamber.

 

Thanks for your advise. I really don't hope it's head gasket since I didn't have any problem or overheat before I replaced the thermostat.. I replaced thermostat purchased from ebay and coolant temp sensor went out a few mins after I started up.. so I bought only coolant temp sensor from pelicanparts and replaced it and temperature worked fine. If thermostat is defective then it could mass up head gasket or something else?

 

There is talk of the oil filter housing allowing coolant and oil to mix...it has an o-ring type gasket underneath that can fail. I would start there. If that doesn't work, you are looking at the head gasket. I haven't done one these cars, but if you are mechanically inclined you can do it. Just may have to find a few special tools.

 

I just had to replace the oil cooler on my '08 MCS. Oil in water but not water in oil. Fixed my problem. The white foam on the oil cap MIGHT be from not running it long enough to heat up the oil and evaporate the normal condensation. But that is a big MIGHT.