So I filled up last night, calculated my MPG......14, last tank was 15.1MPG. This is a 50/50 mix of city and highway driving. Normal 3k-3.5k shifting, with the occasional 15sec spirited run.

Mods are:
-Green filter
-HKS turboback
-Tune

I am running a test pipe but the O2 sensor is plugged in and no check engine light is on. Even when I beat the mess out of my 350z, the worst I got was 17, usually I averaged 20-21. One thing that I did notice is that my idle is right around 1k RPM. Every once in a while, it will blip down to 800ish, for a 1/2 second, then back to 1k. It will do it 4-5 times then stop. I have no clue what it could be. I bought the car used, it had DEVO Tuning stickers on it and by looking around I know it had a BOV at one point, at least a cat-back, and it appears to have suspension mods done at one point. Suggestions as to why my MPG is in the toilet and why it seems my idle is on the high side?


***Update 1/28/2008***
Figured I would give an update to help narrow down problems or come up with other options.

-I have Works throttle body seals coming tomorrow.
-I was tuned by inco9nito99 for my mods
-Car just turned 27k miles
-I know for a fact now the clutch was done recently
-I just went through a tank of Shell 93, 75% highway 25% city staying under 5 psi of boost.............18mpg. An improvement, but still not where it should be.
-Checked to make sure hose clamps (worm style) are tight.

I have yet to:
-Change fuel filter (If needed)
-Check/change plugs. I have BPR7s. Should be ok for my mods right? Boost is around 22-23psi, I have seen it spike to 25psi once.

Short of fuel physically leaking somewhere(none that I can see), I'm starting to draw a blank here.

 

Are you tuned for the TBE?

If not that could be the issue.

 

My car idles just below 1k, maybe 900rpm. As far as the gas mileage, your never going to get the mileage you did in your 350 as it was not turbocharged. Did you get gas at a different station than normal? I've had tanks where I've averaged 15.5mpg, but most of the time I get around 17-18mpg.

 

As far as I know, the car is completely stock except for my mods. Didn't think adding a better flowing exhaust could actually hurt mileage. As far as filling up, when calculating my mileage I always fill up at the same station and it has 93, not 91 or 92, if that matters.

 

A tune should up your mileage to 22-24 mpg city/highway

 

yea...i would DEFINITELY pick 93 octane over 19 octane

 

What, I typed 91.....

 

Shouldn't I wait until I get my FMIC, fuel pump, cams?

 

get a tune... for the tbe and supporting mods

 

Maybe ur using cheap 93 octane gas from those cheap gas stations. I used to use Mobil gas station, I'd get around 260 miles per tank. I tried a off-name gas station... i was making 240. Then i saw some Mobil gas station being refilled with a gas tanker that did not say MOBIL. So i quit using that. Im stuck with Shell and Sunoco 93 octane gas. ~270 mpg. Stock IX MR.

 

I have never seen gas mileage that bad (except at the track) ! Might try getting the CEL fix for the O2 sensor. It is a fitting that pulls the sensor out of the middle of the exhaust so that it doesn't see as much of the non-catalyzed exhaust....and +1 for tune.

I usually get 19mpg, and my right foot never behaves.

 

CEL fix? I don't have a CEL showing, do i still need that little spacer?

 

Buy a GT30/35R.

Your highway gas mileage will DOUBLE.

The stock turbo, and likely combined with a boost leak or off tune is likely your problem. I got rid of the stock turbo. And I get double the gas mileage. I'm in the higher 20's now. If I stick right to 5th gear and 60-65mph and stay out of it. Over 30.

 

I don't think you'll need a tune for fuel Pump and FMIC. But for the cams you'll definently need it. I would actually wait for the parts to be in and then get a tune. That way you don't need to waste time and money.

 

Exactly why I am trying to sell my Miata. What supporting mods would I need to have in order to run a GT30/35R besides the following?
-LICP&UICP with larger FMIC(maybe AMS)
-Crower Cams
-Wal 255
-Helix DP, test pipe and HKS cat back
-I believe the car came with an Exedy stage 2 clutch or something similar
-Obviously a tune

Do I need to upgrade the bottom end and valvetrain as well? I'd like to go with a bigger turbo, but I don't see me being able to build either the top end or the bottom end anytime soon