I have a 2011 S40 T5 and I've been losing coolant at a steady rate but there was no obvious coolant leak. Today I put in on car stands and removed the plastic underbody protector and started the engine. I did this several times and each time observed the same thing:

1. Initially, there is no leak as the engine warms up.
2. When the engine coolant temp gets to the 200-210 range (Fahrenheit), the coolant begins to drip. It starts off slow with a dribble coming down the seal where a plastic box (?) is bolted to the transmission. Over a couple of minutes, the drip becomes a constant light flow of coolant. It appears to be coming from above the transmission in the area of the thermostat, but I couldn't get a visual. Coolant is then flowing down multiple places, down the outside of the transmission dipstick and splattering onto the coolant hose that runs to the thermostat (at the T-joint) and other places.
3. The radiator fan kicks in once the coolant temperature hits about 220. Then as the temperature goes down, the leak completely stops. I can then run the car (in idle) for 20 minutes, with the car cycling the radiator fan on/off, and coolant temperature ranging from 197-220, without any leaks at all.

I haven't found anyone online reporting this scenario. I'm taking it to my mechanic as soon as they have time, but I'm hoping to get some insight into what might be the issue here. It doesn't seem like hoses as the leak stops. It seems like I need a new thermostat, but why wouldn't the leak be constant (either from cold start to it opens up, or from when it opens up until the car is turned off). This situation: that we have no leak, then a leak, and then back to no leak, is puzzling. Can anyone help with an explanation? Thanks in advance!

 

could be one of two things - when the thermostat is closed the coolant is diverted away from the radiator as a path but does circulate within the block and heater. Once the thermostat is opened the coolant is allowed to flow through the radiator (and its related hoses etc). Sounds like there's something going on within the radiator or connecting hoses. The second thing to note is when the coolant warms up enough to open the thermostat (typically at 180 to 190F), it also expands and the cooling system becomes pressurized so its possible your system leak may be pressure related. Either way your shop will likely do a test which forces pressure into the system to see where the leak is.