Just letting you know that a friend of mine works on a lot of Chrysler 3.7L engines including the Liberty. He says the serpentine belt tensioner spring (not the pulley itself) seems to be consistently breaking at around 60,000 miles. He suggests to replace it at 50,000 miles. With 56,000 miles on our 2006 Liberty 3.7L, I replaced my belt tensioner as a precaution. It turns out one of the two pulleys on it was about to fail anyway. I bought the replacement at Autozone of which the one pulley was the wrong diameter. He got another pulley of the right diameter off the shelf and all was well. I replaced the serpentine belt too. The tensioner assembly looks like this. One pulley is stationary, the other pivots off the spring.