Hi guys,
I'll try to keep this short. My 1997 I30 wasn't getting the MPG I expected the past few months (24 hey at 65-70). I then decided to calculate my strictly city mileage since it didn't seem good. Well, after filling a tank and using my odometer, I calculate just over 10mpg. Now I have fun with the car sometimes, but certainly no worse than average. Now, it is cold here in Michigan (with winter blend gas) and my alternator went out two weeks ago, so the car sat idling longer than normal when I would stop places so I wouldn't need a jump (now fixed), but this still seems way too low. I only use premium, and car has new NGK plugs, fuel filter, PCV valve, air filter, knock sensor, and throttle body was cleaned up with rag and seafoam (not perfect, but better). I also seafoamed the car through the intake.

Now onto codes. The car had an EGR code (probably clogged tube), speed sensor and serial communication link codes (I believe these are relate to loose connection in gauge cluster since cruise works intermittently, but speedo and odometer work fine), and rear O2 sensor. The CEL went off last week for some reason (perhaps when alternator was replaced), and now only shows the speed sensor, but I obviously suspect others will come back.

What should I do next? I don't want to keep throwing unnecessary money at the car. My first hunch is that my primary O2s are on there way out, even though they haven't thrown a code yet (especially since back on is bad, fronts can't be too far behind). On my Torque app, they do fluctuate voltage between about 0.2 and 0.8, but at idle seem to stay constant around 0.7. Any other suggestions? Thanks, and stay warm you northern folks!

Eric from Michigan

 

When's the last time spark plugs were changed? Auto or 5 speed I30? Bad Primary o2 sensors will definitely result in bad mpg. I'm not sure about the egr though as my maxima doesn't have one.

 

There it is. Replace your primary O2s. You could get away with replacing just the front one. The new, cleaner primary O2s will probably make the secondary O2 codes go away, too.

 

^^^^yep yep or if all else fails you can use an antifouler or o2 simulator on the secondary sensors and trick the ecu that way you'll never have to change them!