Hello all
I have an Audi A4 1998 model. For the last 2 days the car is getting overheated. The temperature gauge is crossing the middle point and going to extreme right. I have checked the expansion tank and filled in the coolant to minimum level.

When the car is not running or on, there seems to be a leak and coolant is draining under the hood. However, after refilling the coolant, I drove for a little while and I checked under hood. There is no leak this time. But there is lot of smoke coming from the hood. As soon as I stop the car there is lot of smoke coming from the hood.

My mechanic told me that the thermostat is not working. Can someone help me understand why there is lot of smoke coming from the hood? Also is there a simple way to tell if the t-stat is faulty and needs replacement?

Thanks
KB