i want to know when is the timing belt need to be changed
The 3.5 V6 has a timing chain and not a belt so no special service required.
Nissan has used a cam-chain on all their 4 cylinders and V6s for years. The cam-chain is under the front engine cover and runs in oil at all times. Theoretically, they should last the life of the engine ... but Nissan has had a few issues with their cam-chains and the tensioners/chain guides on a few of their engines.
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