Hey guys,

Have a major mpg drop, trying to see what I may have missed. I have an 06 G35x sedan with a coupe mid-pipe, stock muffler. A month or so ago, my guy that I go to mentioned that the buzzing noise I kept hearing from my hose clamp laden (6 total to keep the ratting heat shields in check ) OEM y-pipe was the flex pipes on their way out. Instead of replacing just the flex pipes I found a very lightly used y-pipe off an 07 g35x sedan. Knowing that the G37 ypipe looks similar but has smaller intakes, I made sure to measure the intakes and outlet of this new ypipe. Intakes were 2.0” even and the outlet was 2.5”- perfect. I got it installed, everything was smooth and then I noticed that the city mpg was taking a massive hit. I figured maybe the ECU needed to relearn and gave it a few weeks- still nothing. Mpg is down to 12-13city from about 16.8-17mpg. In the same time frame, I also got new tires- Pilot A/S 3 on the stock 17’s but I’ve been running those at slightly higher pressures so I don’t think that’s the problem. Any thoughts?

 

Thoughts anyone? Easiest solution would have been to put the old one back on and see if that changed anything. Unfortunately, I let the shop junk it thinking this one was perfect and I didnt need any more extra "back up" parts in my basement.

 

i didnt read the thread, just the title. but if you are untuned you are likely getting worse gas mileage.

get a dyno tune done.

 

Thanks Andrew but the car's bone stock for the most part. The y-pipe replacement was to replace one that was on the way out, not for any power gains.

 

you have done a power adder. at the very least, do the "riverdance on the gas pedals" to reset the ecu and allow it to learn the new values.

it also may be a heavier right foot. another possibility is the higher flow of the y pipe has moved your power band up out of a range you are used to, causing more throttle input without realizing it.

either way, a tune is not a bad idea, but kind of a waste with no other mods. i think the ECU reset may do it for you.

 

Thanks EPik. Riverdance- I like it, very appropriate reference.. I actually did that a few days ago, waiting to see what I get for the current tank and maybe do one more fill up and get an an average. If that doesnt change, im thinking I may put on my winter tires and see if I can eliminate the tires as a possible issue.

 

Not sure if you've figured this out yet but let's begin with using hose clamps to hold on your converter covers, have muffler shop weld them back on! Reminds me of my first car I used coat hangers (wire) to hold up my exhaust pipe. Chances are very good since installing the different y-pipe it's making a different more enjoyable sound and your right foot is reacting to it...right foot/brain, get it? Resetting the ECU is nothing more than removing the ground wire on your battery for 15 minutes...no more!
Gary

 

I like to just hit the brake pedal to drain the residual voltage instantly instead of waiting 15 minutes...12 hours... whatever people say. Then follow up with a throttle pedal, throttle closed position, and idle air value relearn procedure.

Good luck OP

 

Leaving your battery disconnected for much more than an hour you're taking a chance of putting your ECU in a coma...12 hours for sure!
Gary

 

You just convinced me to snow off my car tonight after work and drive it for the night. It is two hours away... But now I want to keep the battery charged up for the remainder of the winter...

 

Do you regularly take MPG readings? Your response tells me you probably don't, putting it back on would seem like a waste of time if you do. BTW, I have the same model as you and I get about the same MPG. Your not that far off really.