I just bought a '98 Bonne SE with the 3800 Series II engine. New serpentine belt, tension pulley and water pump. The Check Engine light came on and I assumed it was an O2 sensor since it is still running smoothly. I also have what sounds like a rubbing or whirring noise coming from the belt-side of the engine. After a quick check, a mechanic says that the there is a "lean" code and an O2 sensor code, although I didn't get the code numbers. He also says the noise is actually a vacuum leak, but he didn't have time to dig into the source of the leak yet.

Does anyone have any experiences on what may be the source of the leak before we tear the engine down to find it?

Thanks,

Check all your vacuum lines for cracks, wear or deterioration. One way to find the source of the leak includes spraying carb cleaner near a suspected vacuum line problem. If the engine rev'* when you spray it, that would be the source of your leak. Chasing down a vacuum leaks does not mean tearing down the engine yet...you just need to put some time into it.