I have a 03 montero limited with the 3.8 I had the timing belt changed at a local shop, and now the engine has a bad case of detination even on 93 octane gas. it has always run on 87 octane before. Does anyone have any ideas what the machanic altered to cause, and how to fix it????

 

sounds like the timing belt is off tdc

 

mxmikie is right.
When I bought my 03 LTD at 60k from a dealer on ebay, I had them do the belt and plugs before I picked it up. After 40k miles, it started to detonate (sometimes bad enough cyl4 would misfire) and I always use 93 octane. I tore it down myself and discovered the crank sprocket was a tooth off tdc (top dead center). Unlike the cam sprockets, the crank sprocket is small so 1 tooth could be as much as 10-15 degrees. If the belt is not put on in the correct order, tension (right) side first it could pull the crank clockwise when the belt tensioner is set and end up being a tooth off.

Take it back to the mechanic. They need to take the belt off, realign the timing marks on both cams and crankshaft, and put the belt back on (again, tension side first!).
This job requires special tools and really should be done only by a dealer who has them.

Ps. you really should use 93 octane all the time. Even though it seems to run fine with 87, these engines are tuned to use premium fuel and you'll get better mileage and maximum performance.