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tensioner and idler pulley

What's the rule of thumb for tensioner and idler pulley replacement?. Consider that they are relatively inexpensive and can be a weekend DIY hour or two, should they be replace as a maintenance item?

Thanks.
 

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These are wear and tear items but there are no rules about what mileage to change them. If you are near 100k miles, check these pulleys including the water pump. After removing the belt, check if pulleys are "freewheeling" or noisy and replace them if they are. Belt is also cheap and should be replaced at the same time. First video if the brand new idler pulley. The pulleys should not spin freely more than 2-3 seconds and should give you at least some resistance. Same applies to belt tensioner. Second video is a freewheeling belt tensioner pulley which was used for 100k miles. Notice the wobbling?

ML320_W63 brand new idler pulley - YouTube

ML320_W163 old belt tensioner - YouTube
 

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That video you linked...sounds EXACTLY like the sound from the front of my motor. Thank you!! Now off to find the DIY.
 
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