3900 idler pully replacement
 

This may vary on your vehicle as this was done on my 08 impala with the 3900, it should be the same or similar as your 06-07 monte carlo

I replaced the lower one, locally best price I found was $66 but Rockauto.com had AcDelco for $34 so I went that route, this could be used for any of the 3 idler pulleys I could probably get a cheaper one but if I can replace it with AcDelco I will
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Tools needed
3/8 ratchet
10mm wrench
13mm socket
Piece of pipe (not necessary but helps)

Step1
Pop the hood
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Step 2
Remove resivour and set aside
2 10mm nuts
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Step 3
Lossen up your belt and pull it off the alt
Use your 3/8 ratchet and inset it into the tension or to do so
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I use a pipe for a little more leverage
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Step4
Locate the one you will be replacing
Upper smooth one
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Upper ribbed one
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Lower smooth (the one I replaced)
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Step 5
unbolt old pully
Mine was a 15mm
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Step 6 install new pully
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Step 7
Reinstall belt
(See step 3 and reverse)

Step 8 reinstall coolant resivour
(see Step 2 and reverse)

Step 9 shut the hood

Step 10 pat yourself on the back for a job well done :thumbsup: