- To gain access to the pulley and release the tension on the belt, see AGgranger's write up on serpentine belt replacement. (MINI Belt (de)tensioner tool, philips head screwdriver, small Allen wrench for holding tensioner in place)
- To remove pulley bolt: 13 mm ratcheting wrench (see photo below)
- New idler pulley - MINI part number 11-28-0-946-004. (Check www.realoem.com to be sure you have the right part for your year; the part number quoted here is for a Sep '03 build Cooper S.)
The symptom I had was a high-pitched rattling sound whose frequency and loudness increased with engine RPM. It was very noticeable especially when driving by a wall that reflected the sound back.
I pinpointed the source as the idler pulley (also known as the "stationary pulley" or as MINI calls it, the "adjusting pulley") after removing the passenger side wheel and wheel well liner and using a rubber tube "stethoscope".
The idler pulley doesn't run anything like the supercharger pulley or the AC pulley. It is simply put in the path of the serpentine belt to make it more serpentine (!), which provides more contact area on the other pulleys that have to run something.
Here is a good intro video explaining the role of the serpentine belt and the pulleys and the potential problems that may follow a failing pulley:
The steps are:
1. Gain access to the pulley and relieve the tension on the serpentine belt. AGranger has a good guide for this HERE. Follow the instructions through step 5 in that guide.
2. Below is a photo of what you should see. The idler pulley is in the center, just to the right of the large (rusty) crank pulley. It is removed by simply unscrewing the central hex-head bolt with a 13 mm wrench.
I used one of those ratchet-type wrenches with the bendable end (see photo below). The space is tight, so it might be very difficult with any other kind of wrench.
To get to the bolt and have room to twist it, I put my arm/hand (in white gloves) around/behind the wheel strut. There is just enough room to push the wrench down and release the bolt then ratchet it slowly off (see picture below). The pulley should come off with the bolt.
3. Here is a photo of the old/bad pulley, front and back. Spinning the old pulley made a rattling/clicking sound, whereas the new pulley was totally silent and clearly had lubricant inside which dampened the spinning.
4. Put in/screw in the new pulley, making sure the belt is properly placed around it. (When you remove the old pulley, the belt will pretty much stay in place, making the placement of the new one easy.)
I asked our local MINI shop for the proper torque for the pulley bolt and the guy said "about 20 foot-lbs." There is really no space to get the torque wrench in there, so you will have to estimate. You do not want to over tighten the bolt because you are screwing into the aluminum of the engine block and do not want to strip it! Fortunately, the tight quarters of the operation make it difficult to do so.
The cost of the pulley at our dealer with our club discount was $36, which is a good deal. The lowest price I found on the Internet was $28 + $10 shipping at Pelicanparts.
After replacing the pulley, reverse steps to re-assemble.
I hope this is helpful to someone else