So I got the car and the carfax didn't show any history of the timing belt being replaced. As I had a Cavalier years ago that was never the same after the timing chain went bad and screwed the engine, I decided to change it before it potentially needed it. As I drive away, I get about 2 miles down the road and the car just stops accelerating..

Call the mechanic up and it turns out that my idler pulley had lived it's life (original part) - the plastic was melted and the bearings are fused. Serpentine belt which I hadn't really checked earlier now looks like crap. They were able to change out the pulley and get me home, but there are a few side effects:

1. Idling now is a bit rough with an occasional 'rumbling' noise. Either way it isn't as smooth as before and I feel it may be due to the belt sustaining more damage when it slipped off and got spun around a few times.. they claim it isn't the pulley but it was 9pm in Africa so a used one was all I was able to get. It may be worth noting that the "rumble" goes away if placing the car in park.

2. There is now a noise that wasn't there before. It's not constant by any means, but it is new. Its almost like a loud hum for a few seconds, then gone. Also belt? I cannot seem to reproduce the issue at will.

3. The one positive side effect is that my mileage (which was low before and I'd attributed it to the upstream o2 sensor needing replacement) has gone up by nearly 32% in the ten miles I've driven it since the changing of the pulley. Obviously something is working right and I'm enjoying the profound mpg increase.. is there a chance that replacement of the belt and the O2 sensor will further improve things?

I go in to change the belt tomorrow morning. I would like to know if it makes sense that the noise/rough idle is caused by the belt, and potentially what else may be something to check given the idler went out before I end up stranded again .. lol

Appreciate it!