Alright, I have a 2004 Mazda 3i with 107K that seized up on me. Here's what happened...

I noticed my gas mileage declined about 4-5 mpg's...I also noticed that I was nearing an oil change, so it was changed, but the mileage didn't come back...I was driving on the highway, doing about 65-70, and noticed there was a whirring sound I had not heard before, so I feathered the throttle a bit to see if the whir would respond to engine RPM, but it didn't noticeably...the sound went away on its own after about 5 minutes...reached my destination, pulled to a stop and smelled burned rubber so I popped the hood...to my dismay the effing serpentine belt was halfway off the tensioner, I was able to put it back on the tensioner pulley, started her up and the ****** just slipped off...i left it off, got a tow to get it to the mechanic, I stop by and they tell me the motor is seized up...they did say that the belt was torn AND the tensioner pulley was broken? How the eff?...and they don't know why, authorized them to pull off the oil pan and see what they could find...results tomorrow...so to my point, question rather...

a busted tensioner pulley should not cause a motor to seize up, correct? i'm not too privy on causation, but that doesn't make sense.

What does cause a tensioner pulley to break? and what can cause an engine seize? No oil, too much water (which would imply a bad seal somewhere?), or a foreign object inside the motor? What am I missing?

Effed and looking for a solution...

 

i've seen a few posts where the tensioner pulley will crack - like in half, like the middle bearing was too large.
The pulley in itself should not cause any engine issues, but it does drive the water pump, that would cause issues. Though this would have registered on the temp gauge, and you'd need to run the engine a bit for it to overheat. I assume you pay attention ot the gauges since you noticed the couple MPG drop in efficiency.

There's a second 'stretch' belt that's on the lower pulleys, I don't know what that drives, (AC maybe) but I doubt it's that critical to the engine operation either, and it shouldn't have interfered or been effected by the serpentine belt.

Good luck, and let us know what happens.