The idler pulley on your Ford Focus is designed to aid in the routing of your serpentine drive belt. In order for the belt to do its job and transfer the torque from the crank pulley to the engine accessories, it must be routed a certain way; the idler pulley simply aids in the routing path. If you find the pulley is damaged or has a bad bearing, you will need to replace it immediately. Ignoring a bad idler pulley can lead to a seized idler pulley, which then brings with it a broken belt and a stranded Focus.
Raise the bonnet and locate the belt tensioner. The tensioner is an arm with a pulley at the end of it which holds tension on the serpentine belt.
Slip the belt tensioner tool into the tensioner and pull the tool to the side, releasing the tension from the belt. Once done, reach down with your free hand and slip the belt off the tensioner pulley and release the tensioner back into place.
Locate the idler pulley and remove the bolt from the pulley’s centre holding it to the engine. The bolt can be removed with the socket set. Be careful not to drop the bolt or its washer down into the engine bay.
Pull the old pulley off by hand and insert the new one. Slide the bolt and washer back into the centre of the pulley if your replacement pulley does not already have a bolt and washer.
Tighten the bolt back down in reverse of removal.
Remove the pressure from the belt tensioner with the tensioner tool in the same manner you employed in Step 2 and slide the belt back over the idler and tensioner with your free hand. Release the tensioner and remove the tensioner tool when the belt is back on.
Make sure to verify the belt is properly routed by comparing the routing to the belt routing diagram printed on a white sticker under the bonnet. Do not start the engine until the belt is routed properly.