Hi all! I'm a long time lurker, first time poster. I've got a 2007 R56 Justa that I've been trying to diagnose cooling system woes on with no luck. Long story short, I bought my MINI from a private seller with 140k on the odometer with a known coolant leak. At the time, I found that the expansion tank cap didn't have an O-ring, so I got a O-ring kit as a short term fix, and ordered a new cap just in case. I've also since replaced the thermostat/housing and expansion tank as a precaution. There are no leaks at the thermostat or any of the hose links that I can tell... so here we go:

This morning, I went to check my fluids, as per Saturday protocol, and I noticed my expansion tank was overfull when dead cold. I siphoned the excess out, re-bled the system (had to add almost as much as I took out!) and took it for a warm up run. The expansion tank was overfull again. I waited for it to cool slightly and cracked the bleeder on the thermostat housing, and the levels went back to normal. So I bled the system again thinking there was an airlock, and it looked to me like coolant is being pumped in just fine. Warmed it up again and just watched the expansion tank levels. They rose, again, to almost the filler neck. The hoses all seem to be properly pressurized when you squeeze them, but I don't want to blow the seams on my new expansion tank. I'm currently waiting for the engine to cool down again to see if levels drop. I haven't gotten any overheating warnings, although my coolant temp sensor throws a "low read" code which will cause my fan to run for a good 5-10 minutes after shutting down until I reset the CEL.

So I guess my question is: is this okay, or is there something else nefarious going on that I need to be aware of?

 

I just performed a complete cooling system refresh on our '07 justa. Following the Bentley service manual, I filled the cooling system, reinstalled the cap, and started the engine. I let the engine warm up to temperature and then cool back down. Once it was cooled down completely, I filled up the coolant recovery tank to the "add" level line and installed the cap. Start the engine and then repeat the warm up and cool down cycle. Repeat until the coolant level is steady at the "add" level line. It took about three cycles, but I never touched the bleed valve on the thermostat housing.

OBTW, you can't blow the seams on the coolant recovery tank, cooling system pressure is 13-15 PSI, and the cap will relieve excess pressure by dumping excess coolant overboard.

 

+1
also, you need to turn the heater at high temp and full fan so coolant can circulate through the heater core.

 

ALSO, it is important with these pressurized systems to make sure the expansion tank cap is on as tightly as it can be. That was the cause of the overfull coolant tank- pressurized system pushing fluid to the lowest point of pressure, in my case, a normal-snug but not completely tight tank cap. Hopefully someone can learn from my fail, thanks y'all!

 

So after you tighten it up, all the overfill problems went away? Glda you've sorted it out. This is good lesson learned. I'll have to put that into my book. LOL