Hey Everyone, looking for some expertise on the issue for my R56 MCS. I haven't had a problem with the cooling system at all until now. I have replaced the expansion tank once previously when the OEM was leaking at the seam. Replaced it with a non-OEM part from Napa parts as it was the easiest to obtain and same day available at the time. This was several years ago now. I didn't have another issue with it. Stopped at a gas station to fill the tank this afternoon and saw coolant leaking from the front of the car. Popped the hood and sure enough the coolant was leaking from underneath the cap visibly and nowhere else with the expansion tank now full. Drove it home quickly as I was close to get it in the garage fast.

Other recent history is the car was not running until I had the JBE replaced at my indie shop in November 2019 and DIY'd the water pipe and thermostat replacement right before having the JBE switched out. Had no issues doing the work myself and the car was running fine since then and before this issue arose today. Since getting the car running again post JBE replacement and changing the thermostat/water pipe I have kept a close eye on the coolant level to make sure I sealed and did everything right. Kept checking for leaks and never saw one. All was good and the car has been running fine. Coolant level remained consistent and even marked the fill line with a sharpie to make sure I was at the same level. Since November 2019, I have driven the car several times a week locally without a problem until the shelter-at-home orders. Since staying and working at home, I have turned the car on a couple times a week without driving it. I keep it in my garage. Today I ran a couple errands that were essential and the last stop was putting gas in the tank. And then this happened. I arrived at home and parked in the garage, didn't hear the fan and though no leaks in the driveway, began leaking when I shut the engine off in the garage. Popped the hood again after a few minutes and saw the expansion tank only had the near bottom with some coolant left. I'd say it leaked a good half gallon of coolant. I'm not sure if the coolant was boiling, it didn't seem to be bubbling or hot to the touch. I could be wrong. I checked the oil cap and it's not milky. Checked the oil level stick and it's fine also. I had the oil service done with the JBE replacement. I didn't hear the fan go off at all so I'm thinking something is up with that in causing the issue. Not sure if having it sit for nearly a month is also causing this problem. Last time I drove it out was back in mid-March.

Any guidance or suggestions as to what to do next? What to look for? After getting the car up and running great this is definitely a sad day for me since I was motoring just happily until today.

Thanks in advance for any help with this issue.

 

I would say this is the exact order I would troubleshoot it after ruling out any leaks.

https://www.autozone.com/diy/coolant...ir-to-overflow

When you go the new expansion tank, did you reuse the lid by chance?

 

Truth I don't remember if I kept the original cap or went with the "new" non-OEM cap from Napa. I'm 90% sure I rolled with the "new" one.