I have a US permanent residency interview but my pport from El Salvador has expired, do I need to renew it?

At your interview you may be asked for a photo identity and the pport is a good identity to produce. You also need your pport anyway if you want to travel. So I feel it is best you keep it up to date. You could apply for a renewal and even if you do not get it by the time of your interview at least you can carry your expired pport and proof that you have applied for a renewal.