My wife's 2006 R50 has a significant coolant leak. Shop quoted $2400 for radiator and thermostat replacement and some other stuff. I figured I'd give it a shot myself. But when I jacked it up the leak seemed to be coming from nowhere near either part.
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This is the area directly above the leak. Pic taken from under the car, facing toward front of car. The arrows point to the two areas that have visible blue coolant. The one on the right did not seem to have come from anywhere else, i.e. at first look it seems to have been leaking from that seam.

 

I should preface by saying; unfortunately, I don't have an answer.
However, Given signs of where the coolant is leaking down onto the gearbox/axle - I might venture a guess that it maybe a leaking waterpump gasket or thermostat housing (???).
I also, recently, posted about having a massive amount of coolant leaking from the right front side of my R53. I hadn't overfilled the reservoir and could never find the source of where the coolant/leak was coming from. It was weird. The leak was only momentarily for a few days, finally subsided and completely stopped after about a week or so. What was strange is that; My reservoir barely dropped below the top of the full mark during the time of that incident.
In my case, I might suspect that the coolant may have been leaking/purging from the small weep hole on the waterpump (?) and trickling it's way down to the front of the car.
I hope someone else here, can perhaps, add their own personal experience or input in effort to help you figure it out before spending any unnecessary time and money at the mechanics shop/dealership.

 

The thermostat housing sits above the gearbox. It's likely either the thermostat housing is cracked or the gasket has gone bad.
I advise removing the airbox and checking the top side of the gearbox.

 

if it's not been done you might go for a full cooling system refresh. go to realoem.com, find the part numbers and refresh. if you plan to keep the car its worth it. should not be anywhere near the price the shop wanted to charge

 

Thanks all. Currently in the process of replacing radiator, thermostat/housing/gasket, and all hoses I angrily mutilated in removing them.