My belt was making noises so I changed it several days ago. But I loosed the idler pulley too much, actually I took it off. So after I installed the belt, I can hear some noises and I guess it is from the idler pulley. Now things become worse. Sometimes the idle speed goes up from normal 650 to about 1,200 or even 1,500, then it may go down again. Is it because of the idler pulley or something else?

 

why did you loosen the idler pulley?...that should have stayed stationary...

do you mean the tensioner pulley?

 

You must loose the idler pulley so you can move it by turning the tensioner. My problem is I took it off. I should just loose it a little.

 

The fact that you simply "removed it" doesn't mess it up as long as you reassembled it back correctly and retightened the locking nut again after adjustment.

 

I guess I reassembled it correctly. I have seen several people complained about their idler pulley here. But I really want to know why my idle speed changes.

 

Check your belt tension. If your new belt isn't tight enough, it won't put the same amount of drag on the engine as your previous properly adjusted belt. It'll also squeal, especially when cold (first starting up). If it is too tight, It'll cause too much drag, which the engine may try to compensate fore and therefore make your idle erratic. This is also a good way to wear down important bearings, like those in your alternator and compressor. Probably isn't too good for your crank bearings, either. I'd rather have my belt too loose then too tight. You can snug it up just to the point that the belt doesn't squeal.
As for the idler pulley, as long as you put it back together right (washers in right places), it should be good. Watch it while the engine is idling and check if it is wobbling and/or making a weird noise. If not, you're good to go. If it is, I'd take it back apart and reassemble correctly.

Dave