Who knows what Nissan put on their early QG batch production. Nissan Sunny 130Y around 8km/L considered best at it's time. Early QG18DE should higher than 8km/L. As I said before, many Pre-FL Sentra owner complain about the same things especially VTC functionality inside which is different from QG16DE since QG18DE using N-CVT which is already used by Nissan since 1987. If N-CVT is the cause, we can't do nothing except put a new version ECU's controlled VTC same as FL1 & FL2 or new VTC engine. One of my Pre-FL friends already put SR16VE (VVL) to replace QG16DE (VTC) inside.    The problem usually happens on Pre-FL model since it is rojak with Nissan Pulsar.

Or you can try send your car to mechanic to tuning Cam head inside the engine matching with fuel save you desire, but it will affect your car acceleration. Since all Sentra are DOHC, tuning should be no problem for most experienced workshop. If you send to TCM they only tuning using computerize. Try going for manual tuning for better result, after all your car warranty already ended after 3 years purchase, so nothing to lose.