Would appreciate some advice here. Mine is a 2007 BMW 523i. A few days ago the warning light came on with "engine oil level too low" so on Sunday I topped up about 2L (maybe a bit more) of Shell Helix 5w40. Today when my wife drove it home the car started to shake badly, and after parking there was a lot of yellow/brownish liquid on the floor. I opened the bonnet and saw what appears to be coolant oozing out of the expansion tank. The tank cap still looked intact, though. The tank is a genuine BMW one and was replaced a few months ago only. As the car was still hot I didn't dare to open the expansion tank cap.
Could this be due to engine oil overfill? But why would that cause coolant overflowing? Or the expansion tank or somewhere in the cooling system leaked (it seemed to be slowly leaking so I topped up with distilled water some time ago)? Could that cause the car to shake?
No engine overheat warning came up, though.
Any advice will be appreciated!