The idler pulley on a 2003 Nissan Sentra 1.8 is changed by removing the engine accessories, removing the drive belts, and loosening the retaining bolt. The pulley can then be removed from the vehicle and a new one bolted into place.
First you would need to take the serpentine belt off of the engine, and then remove the bolt in the center of the idler pulley. I have included an instructional vieo of how this is done.
Locate the idler pulley and release the tension. Remove old belt and replace it with the new one (there should be a sticker on the underside of the hood or on the radiator housing showing the path for the belt) and re-tighten the idler pulley.
Take the belt off, take the bolt off of the idler pulley, take the pulley off, put a new one on replace the bolt, and put the belt back on
take the serpentine belt loose and there is 1 bolt holding the complete belt tensioner onto the engine, take the bolt out and it's off.
help
We recently replaced the serpentine belt on our 2003 Chevrolet Tracker. We were able to place a wrench (I believe it was 9/16") on the tensioner pulley and apply pressure so the pulley released tension on the belt. This however did no allow enough slack to install the new belt so we removed the idler pulley at the top center of the engine. With the idler pulley removed we were able to route the belt and with the tension released on the tensioner pulley, routed the belt under the idler pulley and reinstalled the idler pulley, then released the tensioner pulley and the belt was installed. Be sure to note the exact route of the old belt before removal so the new belt follows the same path. Hope this helps.
The serpentine belt on a 2003 Toyota Rav4 can be loosened by turning the retaining bolt on the idler pulley. The belt can then be shifted inward to loosen.
your questoin is not specific, there are many pulley's
About $40 at Walmart.
It's fairly complicated. The sensor is located near the bottom to the engine behind the crankshaft pulley and timing belt cover. Not a job for an inexperienced do-it yourself mechanic.
About 6 ltrs