About a month ago I replaced the belt tensioner assembly, installed a 17% pulley and the correct shorter belt in my 2006 MCS. I started it up and everything seemed fine with one hole showing on the tensioner and the car has overall worked great this last month. I happen to check the tensioner today and with the car running I cant even see the last tensioner hole.

So which may be at fault, the tensioner or the belt stretching? Perhaps there is another issue? The belt and tensioner are under warranty so I need to see which one to exchange. I'm leaning towards the belt.

Any ideas?