Ok ill try to explain this the best I can while keeping it short


My rx8 does not get over 22mpg no matter how i drive, a steady 45mg without stopping with cruise control on, its the best I can get. I use Torque MPG display which I have tuned to be very precise, I tested it many times by comparing my gas fill ups with its readings and its never more then 1mpg off.

So anyways I been driving the car too easy without reving high lately from trying to get best mpg i can and I been noticing that its been taking a bit longer to crank the engine, most likely from carbon build up because I have not been red lining it at all and keeping the rpm way too low. I decided to go on a 50 mile trip and just let it loose, rev the hell out of it to clean it up, so i figured i would get something like 13mpg on this trip but to my surprise I actually got 22mpg!?! this was all done on back roads, red lining 1st and 2nd at almost every red light.

I thought torque was acting screwy and giving wrong mpg readings but after refilling my tank I noticed torque mpg reading were bang on... So after the fillup I decided to go on a 45mph cruise down the same back roads and I was actually getting 26mpg after 10 miles of driving easy with cruise control on! thats by far the best mileage I ever got with my rx8 so far owning it 3 months.. So I decided to do some city driving with some hard accelerating and again to my surprise my mpg was way higher then im use to seeing in the same situations, i got 23mpg in city driving with lots of hard accelerating. im used to seeing 16-17mpg with the same driving habbits before.

So I guess what im wondering is, can the front o2 sensor cause this without throwing a cel? Im thinking that driving the car hard helped heat up and clean the front o2 sensor which started giving accurate readings. I always kept a good eye on my AFR when cruising and before it would always be around 15.0 - 15.15, but after reving the engine hard and cleaning things up it was always closer to 14.7 - 14.8, is that a good sign?

All i know is the car felt a lot better then im used to it feeling, accelerated way smoother and easier and just FELT RIGHT

I really want to get a new front o2 sensor but they are damn expensive, I wouldn't mind a second opinion, or maybe some way to test my current sensor. Maybe I could make a data log with torque to show whats going on?


2004 rx8 MT 88k miles, 1 CEL for rear o2 sensor, ordered it and still waiting for it to come in, cleaned maf and ess, brand new plugs wires and coils installed 3k ago

 

When was the last time you replaced the Plugs, Wires, and Coils?
Do you run the car hard every now and then? (once a day redline, or weekly mountain passes?)
When was the last time you changed the oil?
The average mpg for the rx8 is 16-23ish.
Worst I have gotten was 13mpg.
Best I have gotten was 26 mpg.

Some where on the forum is a method to test the O2 sensor functionality, search around.

 

the ECU actually runs off both sensors, so i suppose its possible for the front 02 to be bad, but for the ECU not to catch it the rear would also have to be slow/bad too.

these cars seem to like being driven pretty hard, there is the obvious cycling of all the intake valves, but the metering pump is open a lot too

 

plugs wires and coils were changed 3k ago, i red line it about once a day, some days i dont though, oil was changed 2k ago, maf and ess were also cleaned, engine has always felt nice and smooth to me

Im really good at driving a car to get its best mpg and since i was getting 22mpg at my absolute best I just felt there was something a little off, also if I decided to floor it a few times (like 3 or 4 times) and only to 6 or 7krpm in first and second gear it would have brought my overall mpg down to 17 mpg on a 50 mile trip which is disgusting, it caused me to never want to rev high for any other reason but to clean the carbon out because of how much gas it guzzled when I did so..... I have a feeling if the average rx8 driver on these forums drove my car they would get 15mpg at best lol, but yesterday when everything was working perfectly I was able to do the same 50 mile trip with same habbits but WAY more hard accelerating and got 22.5mpg.... Also I would always avoid the highway because anything over 60mph would get me 20mpg or less, so I also tried some highway cruising last night at 65mph and I averaged 23.5mpg! these numbers just seem right, the MPG rating when it was released was 18-24mpg and I was actually able to get 24mpg on highway doing 60mph.

You see this is such an amazing car that I fell in love with in the short time i've owned it but the only thing that pissed me off was the extremely bad mpg I get without driving hard, and when driving hard you could literally see the gas needle going down. It felt so good driving it the way I was last night and getting the good mpg results I popped a stiffy . It seems the rx8 will absolutely guzzle the gas when something isnt working properly, like maybe a malfunctioning front o2 sensor, slightly clogged cat, or possibly even a rear o2 sensor

So it seems it would be one of three things, front o2 sensor, rear o2 sensor or the CAT. One of them seems to start actually working after a long trip of hard accelerating. I already ordered a new rear o2 sensor yesterday since I had a p0037 CEL since owning the car and im going to put a midpipe on. If I don't notice a difference after doing that then I think I should get a new front o2 sensor.


I will post back in the future when I have the time to get all three of these changed and see which one makes the difference.

 

Cheapest place I found for for the front 02 was amazon. I just replaced mine, they sell the same one as advance auto, but for almost $100 less.

good luck

 

Air pressure in tires?

Are your wheels dirty?

At that speed (45 mph), on a level road, in 6th gear, I get 28'ish. I got less when my brakes were dragging and my tires were low.

 

you know that brake dragging thing is interesting. on the list is to clean and grease the caliper slide pins. one of my cars has the original brakes on it, @73k!

 

Wow that is a great deal compaired to everywhere else thanks!, is the NTK the OEM front sensor that comes with the car? how about the rear is the OEM NTK?


"Air pressure in tires?

Are your wheels dirty?

At that speed (45 mph), on a level road, in 6th gear, I get 28'ish. I got less when my brakes were dragging and my tires were low."

Air pressure is good, 32 psi all around

are my wheels dirty???? I dont know what you mean exactly but my wheels are clean and the brakes look pretty clean but they do squeel a bit when i use the brakes. I know for sure it not to do with my tires, but I will jack my car up on the weekend and feel how the wheels spin by hand.


So on my way to work today I did a test, drove the car easy and let it fully warm up, checked my instant mpg on flat road at 45mpg and it was around 23-24mpg as im used to seeing, so I did a couple redlines in first and second gear and presto! instant mpg shoots to 28 at 45mph.

So i was paying a lot of attention to my AFR before and after revving hard, before revving hard it would be 15.0 - 15.2 during 45mpg cruise, after revving hard it went to 14.6 - 14.8 during a 45mph cruise. I also noticed that when I let off the gas and left it in gear it would slow down WAY slower and smoother then it used to, it used to be very jerky when i let off the gas in gear and came to a stop pretty quickly.

does that mean its most likely my front o2 sensor thats causing my MPG issue?

 

The one in the link is the front sensor that I bought, and it is the OE sensor from what I understand. Looked just like the one came out of my car too. I'm not sure about the rear one, I would just go to Advance Auto's site and look it all up. That's how I found out what the correct sensor was. It says "upstream", "downstream", etc on the advance site. Then I just searched around using the brand & part number and saved ~$90. You could do the same for the rear, probably cheaper on amazon as well.

Don't know what to tell you on whether it's the issue or not, sounds like something is outta-wack....and it couldn't hurt anything lol.

 

the Rx8 is Japanese, so the O2 sensors are going to be NTK or Denso

 

Dirty wheels was to see if the brakes were dragging. You will wear the pads if they drag, that will cover the wheels with brake dust. Sorry for the cryptic question.

See this thread on O2.
https://www.rx8club.com/trouble-shoo...please-243462/

It appears that if it is not working well, it can really hose things.