Nissan Repair: 2000.altima sucking alot of gas, fuel pressure regulator, nissan altima

Question
Hi Von, i have a problem i own a 2000 Nissan Altima Gxe It' has 102.000 miles on it now, but ever since i done the last tune up and stuff it sucking a lot of gas and when i make sharp turns get on freeways ramps and stuff my fuel gage needle goes down a bit and burns off some gas, like a said i did a full tune up on it and also change the o2 sensor plus clean the fuel injectors(which came with the turn up) i don't know what to do now is there anything that is wrong because it wasn't eating so much fuel like it does now,please help my car is becoming a gas gussler and gas is not cheap.
P.s theres also a rattling sound coming from the bottom or from the engine when i'm driving forword i took it my mecannic and he told me it was the type of gas i'm putting and to not use 87 octane fuels, i change the fuel grade to 89 and it still making that rattling noise can u please tell me what can it be ?

Answer
Ansony,

Have the timing checked to make sure it is set correctly.  Spark knock is not good for and engine and if it making the noise use premium until you get the timing retarded.  One the fuel consumption I would be checking for a leaky injector or two.  Have the fuel pressure checked with the key in the run mode and then with the key off and see if the injectors leak down too soon.  Another source of the problem could be a defective fuel pressure regulator keeping the fuel pressure too high and that would cause the high fuel consumption too.