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.

JUANPCM

I wish i was getting 15 in the city, from what ive noticed, i do around 20 highway, but right at the edge, barely 20.. what im doing is everytime i fill the car i reset trip A, thats how ive been doing it, last time, it wasn't almost empty, but i was, im going to get some gas, the trip was 79miles, thats was with no highway, thats when i decided to take it to the last guys, they charge like dealer.. so i didnt want to do it...

I have an OBD, but a cheap one, w/o Bluetooth, I can buy one to use the app you told me, can you recommend a good BT OBD? I was reading reviews to get a new one, one thing that it might be related and haven't been able to fix it high idle, its around 850, 900.. there was an app that would let to reset the idle, i have tried several times with the accel pedal sequence and i think im doing it correctly but never fix it, so i found there was this app, but needed an specific OBD that has some stuff that others dont have? thats why im asking if you can recommend a good..

The air filter, spark plugs and MAFs where always checked when i took it to the mechanics, i did it too and also cleaned the throttle bodies, and i dont think there are any obstructions, at leas not in the intake, i will have to check the exhaust to see if i can find something.

It rolls easy in neutral, ive rotated the 4 tires when the car is up, so i dont think its the brakes, tried that too, doesnt seem to have any resistance, both front bearing are new, less that 6 months... might have to check the rear ones but im almost sure they are fine

JUANPCM

I guess i am using the car, like i was explaining before, i was just using the Trip A to check how many miles i get from a full tank.. thanks, i will do that and then report the number.

ILM-NC G37S

iTrader: (3)

That idle is way too high. It should run at 650 +/-25rpm depending on temperature and engine load. The NISSCAN products- WINDOWS PC or Android app- should be able to adjust that idle. I believe @Krzysztof47 used that program to adjust his idle.

HOWEVER, and this is important: Does your car surge when braking to a stop? ie, when coming to a full stop as the car downshifts to a lower gear does your car "jump" forward?

JUANPCM

Yes and that also driving me crazy, it feels annoying when idle or when i stop at red lights, ive tried everything i found over the internet, and yes, thats the app, but apparently needs some specific OBDs, do you have some info which one will work? a bit expensive but i guess its worth it.. and about the surge, my car regularly doesn't do that, but i definitely have felt what you are saying, and if im not mistaken only when cold, but will pay attention to it on my way home after work, but for sure its not something it does all the time, dont remember the last time it happened.

Btw, my tank its almost at 1/2 so ill drive it a bit and re fill it and will let you know the numbers..

ILM-NC G37S

iTrader: (3)

I might have the answer to your problem. As you have a 2008, there was a technical bulletin regarding this erratic idle/ surging issue. Attached to this post is a copy of ITB07-048b and I am willing to wager that your ECU needs to be reflashed, per the ITB. Right now your engine is lugging due to the higher idle which s not only bad for the powertrain but likely the reason for your abysmal fuel consumption. I feel this is the answer you have been looking for. No guarantees, but...

In my case, the idle would be fine one minute then suddenly jump to 900-1100rpm and would not settle down until I restarted the car only for the cycle to repeat again. Once the dealer performed the reflash I have not had one single idle issue since and it has been at least 10 years or longer.

Something to consider...

JUANPCM

Thanks a lot, at this point i am willing to try everything, actually, posting here was the last thing before actually taking it to the dealer... and hopefully thats the reason, it would be amazing, since everything looks good for every mechanic so far... now that you mention the powertrain, idk if it is related, but im having a very rough shift from 2 to 3 and a bit from 3 to 4, ive been shifting manually at very low rpm so it doesnt hit so hard, sometimes it hit very hard, and thats also while cold, 5 minute of driving and it start shifting fine, ive read some of the forums here people talking about it, but i didn't dig much into it, if im not wrong also reflashing the ECU was the solution, i dont remember..

mines been almost always stable lately at around 900, but like you mention in your car sometimes it jumps to like 1100, and varies, only once saw it around 650 and just for a bit...

So ive read about these technical bulletins, but never seen anything related to my car, it doesnt matter that my car is so old now, they should fix it for me for free? should i just call the dealer and ask them about it?

Btw, I know exactly how many miles ive driven since my last fill, using google maps, and it matches the car trip, 70 miles, and its already half tank, i will still fill it and do the math..

ILM-NC G37S

iTrader: (3)

Print a copy of the bulletin and bring it with you to the dealer- or any certified Nissan repair shop that has the CONSULT tool and access to the ECU file database. It does not matter how old the car is it can still be serviced. I had a copy.on my iPad all the same.

You will likely have to insist they reflash per the bulletin. Initially mine did not want to do the TSB as they wanted to replace a dozen things. I was adamant and they eventually relented and did the TSB. Been fine ever since.

The dealer will probably charge you for one (1) hour of labor at whatever their prevailing rate may be. Do NOT let them convince you to go down a diagnostic rabbit hole. Insist you want the TSB done, period. If they balk go elsewhere.

JUANPCM

Yeah i know they will try to do more to charge more, thanks, ill insist on only doing the TSB

JUANPCM

So I called 2 infiniti dealers yesterday, 1 didnt answer and the other one didnt know what i was talking about after talking to a few people, at the end they transfer me to the manager but went to voicemail, left one, they guy didnt call back, so i think ill print it and go there this weekend... anyway, i did the math and so my car is doing 12.5mpg? am i doing it right?

Edit: I forgot to say, that 40 of those 80 miles where highway miles

ILM-NC G37S

iTrader: (3)

Yes, you are calculating it correctly. But that is still an abysmal number (12mpg).

The service department "may not" immediately know what you're talking about as these cars are 17-years-old as is the technical bulletin itself. Bring a hard copy and go talk to a "senior" service advisor or, better yet, the service dep't. manager. Most "advisors" these days are glorified salespeople who do not know a lug wrench from a espresso machine.

JUANPCM

And if i had to calculate, im pretty sure it should be around 9mpg on the city.. anyway, i will try that this weekend and hopefully that at least help or fix the problem, thanks again, i will let you know when i have some new info.