JUANPCM

Hi guys, Ive have already asked about this, but nobody help me, its driving me crazy, G37 coupe 2008 with a K&N CAI and had to replace the the Cats and decided to go with FI HFC, but the gas was already a problem before the cats, I didn't notice when this started, its been several months like this, but i don't even do 100m per tank in the city, with a lot of expressway ive seen like 160 per tank, after resetting the fuel info and driving to work i saw like 8 miles per gallon, it is usually at around 15 but that's because i drive a lot of on the expressway, when i fill the tank, its usually says around 250 miles left to empty, took it to my mechanic like 3 times, couldn't find anything, i tried a couple more but the same, last week took it to a Infiniti specialist, its full of modified GTRs and Gs so i figured they could find something, Guys said nothing wrong and since i have an aftermarket CAI he thinks the only thing that could fix it is to tune it and that is $1000, ive searched and read and it seem that you don't need a tune just for a CAI, can anyone give me an idea of what to do? oh 1 think that its been happening lately, its when i turn on the car, the fuel gauge doesn't move, the next time i turn on the car works normal, its happened 3 o 4 times recently but i think that's unrelated. Btw, i don't have the oem intake cause it was broken so i cant test with oems. Thanks.

beefychicken

​​​​​​​CAI and HFC's shouldnt have a major impact on MPG. I get 15 city, 20ish highway, but i casued that with similar bolt ons and a tune.

First question that comes to mind is if you are tracking your miles per gallon manually or using the cars computer? From what you noted about the gas gauge sticking, i would avoid depending on the car to calculate MPG and would just track it manually on a couple of fill ups.

Other thoughts assuming the mileage you reported is actual and not a computing error generated by the car:

Try monitoring the engine fuel trims using the torque pro app (or something similar). With your mods, they should run +/- 10% off of zero at the most, probably more like +/- 5%.

Are the air filters clean? any intake or exhaust obstructions?

If your motor is motoring fine, then look into the possibility of a slipping transmission or excessive rolling resisitance (stuck brake pads, horribly worn bearings, something like that - but that sort of stuff would show up as a lot heat and noise around one or more of the wheels).

ILM-NC G37S

iTrader: (3)

Are you relying solely on the cars "idiot computer" to determine MPG or are you using the proven, reliable pen and paper method?

There has to be something drastically wrong with your car to be getting those numbers- if they are representations of actual mpg.

For comparison, I'm running R2C intakes with Fast Intentions RHFC's- NO TUNE- and I average 400-450 miles per tank with 40/60 city/hwy.

Fill the tank, reset odometer, drive it until you get to 1/4-1/2 then fill up again. Divide miles driven by number of gallons needed to fill then report that number.