1999 Infiniti i30(posted here because the same drivetrain and more hits) auto transmission. My mpg is horrible. first 18.x 16.x now 14.5. This is high performance v8 category. There are a few things that I will try but I can't see it helping much. 1st I will empty my trunk as I probably have approx 40-50 lbs of things. Next, I will fix the slow leak in my passenger rear tire. 3rd, I will try driving like an old lady and finally I will replace all ignition coils (random misfire noticeably only when in drive and stopped). I am still surprised that it want to 14 though.

I only use 91 (California's highest) octane, NGK spark plugs have less than 1000 miles, One front O2 sensor is new, Egr cleaned and no codes. Smog was passed with flying colors as shown below. What can I try?

 

well i get 14mpg and i drive in NYC its my daily driver

 

you more than likely have a bad coil. i had the same problem, changed cylindar # 5 coil pack and it went away.you may want to clean your IACV too

 

did you actually scan for codes or is there just no check engine light?

 

Ouch. with a ghetto 00vi. no egr/evap or iacv, a failed maf and bad *** exhaust leak I still got ~19mpg on my last tank o.O

 

Two months ago, I got codes for both front heated oxygen sensors. I had 12-13mpg mix then.

Replaced one (B2S1), cleaned all the connectors related to oxygen sensors, and checked for open or short circuits. Took me a month or so trying to deal with the problem. No codes now. Got 25mpg HWY for the last tank. Still going to purchse some new spark plugs.....

 

I never drive this car without a scanner. 0 codes.


I emptied my trunk and now I need to readjust my headlights, BADLY.
I can see the height difference. LOL

When I get my coils I will report.

 

I get 12mpg in town and still don't know why. a friend with a 98 never gets below 18mpg in town and his car has 50k more that mine ?)

 

I only have 80k miles. What pisses me off is that my brother's 97 Buick reagal GS (Supercharged 3.8l engine) gets 19 mpg.

 

How are you calculating MPG? Reading the gallons used on the pump when you fill up or just assuming E or 1/4 = x amount of fuel?

Looking at your emissions test, the car is running great. Do you do mostly city driving?
Take it on a long trip, see what your MPG is. Looking at the emissions, you really should be up in the mid to upper 20's with a/f ratios like that. It's not running rich at all.
If it was misfiring, you definately would have failed the test.

 

Iam using the trip counter and the amount of gas the vehicle takes to get full. thhat way, there is no guess work.

 

Still haven't gotten the coil packs in the mail yet but finally got to fill it up and got 20.64 mpg so I am heading in the right direction.

Who would have thought that removing 60+ lbs and fully inflating tires will result in a 6 mpg gain. This is 30% highway.

 

Improperly inflated tires can put a dagger in your mileage. It is very important to keep them filled. I'm not surprised at all about your increase given what you did. Congrats!

 

A bad KS, coil, etc. can hurt mpg's. It did for me. Went from 15mpg to 21mpg. When I'm on the highway I let the car cruise at higher than average speeds so I know it can get better mileage if I drove slow as hell all the time.