Hey everyone,
My 1997 Infiniti I30 is giving me some trouble that I'd like some help with. I got the car from a dealer auction last spring (my uncle has a dealership and I was an assistant) for the purpose of my college beater. Since then, I've put new rear struts on the car, had cat-back exhaust replaced due to rust, new alternator, tune up (NGK factory spec plugs, fuel filter, air filter, PCV valce), new knock sensor, cleaned EGR tube, MAF, IACV, and throttle body, and replaced the front primary O2 sensor (didn't realize it had 2 when I ordered, didn't have a CEL but wanted to be sure they were fresh). Only other codes are rear O2 and speed sensor (I firmly believe this is due to joins on back of cluster coming unsoldered due to intermittent cruise control, all gauges and odometer function properly). I almost always run premium, with an occasional tank of regular or midgrade.

On a 150 mile highway trip today, 70-75 mpg with almost constant speed, I average 22.5 mpg on regular fuel (approximated 60 degrees and rain). This was measure by starting with a full tank, driving the 150, then filling it up and doing the proper math. It certainly seems like I should be getting better than this, I'd like to be seeing at least 27 mpg highway. I had similar mileage last year, as bad as 12-14 mpg city (that was in the nasty winter MI had thought). The car seems to run smooth and drive well. Car has automatic tranny. Is there any common maintenance item I missed, and are there any other things to check based on your guys and gals experience? I'm kind of at my wits end trying to figure this out. Love the car to death, but want better mileage after all I've done to bring her up to snuff. Thank you!

 

Personally I've seen 1.5 to 2.5 MPG increases from replacing lazy O2 sensors. If you have an O2 sensor throwing a code, I doubt the engine is able to trim out an optimal fuel mix for the bank where the sensor is failing.

Here's an interesting video about O2 sensors and MPG below.

http://www.youtube.com/watch?v=Oxb3a5KXBaY

Unfortunately, I don't have any experience with a failing speed sensor. If I had the problem I would start by removing, cleaning, and inspecting the sensor and related electrical plug connections. I'm sure other members will chime in with recommendations.

 

You've done the O2 which is good.

Try TCW3;

https://maxima.org/forums/fluids-lub...itive-new.html

Also, make sure your tire pressures are good, too. Do you have OEM 15" rims? Bigger rims will hurt mileage as well. Your highway mpgs should be about 28-29.