Does a timing chain stretch?
Not only will the chain loosen up, but the gears will wear down, causing even more slack in the chain- how many miles are on the car- is it making noise? If the mechanic is correct, it should be done before it breaks- this is a semi-expensive job, so you can always get a second opinion-
Have you thought about a second opinion? I've had a few Chevy engines with steel sprockets and chains and they'd flop around, but never had any problems with them. On your Nissan it apparently has three different tensioners inside for the three chains it has. Here's an Amazon link for the Timing Kit. You'll see the parts list near the bottom of the page. http://www.amazon.com/Evergreen-TK3034WP-Timing-Chain-Water/dp/B00LLQGKZG/ref=sr_1_fkmr0_3?ie=UTF8&qid=1418833338&sr=8-3-fkmr0&keywords=2004+NISSAN+QUEST+Timing+Set&pebp=1418833357467#productDetails
The timing chain guides break on that engine and cause a loose chain. You will want to replace all of them including the chains if you are going to get it fixed.
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