I've owned my 645 since 2007 and love it. More often though I'm getting a low coolant level alert. I'll cap it off and go for another few thousand miles and it comes back on. I've got 167K miles on the car and drive about 600 miles per week. I'm wondering if I've got a head gasket leak based on no fluids on the ground showing any leaks? Not sure if there is any aftermarket "stop leak" that can be used for the 4.4L motor. Any suggestions? Thanks