What's wrong with my MPG?!
 

Yet again I need your help, heroes of my350Z forums.
I've noticed I have ridiculous gas mileage considering driving conditions.

My mileage is :

17.5L/100km
13.44 MPG (US)
16.6 MPG (UK)

My driving conditions are :

80% city driving (no heavy traffic, jams are rare in my driving hours)
20% semi-highway

My driving style :

I drive like a grandma. Here's why. I got my clutch and SMF resurfaced and fixed and I was told to not push it for about 200 miles (or ride a clutch just a tiny bit every stop and go). That's why I drove slowly during these miles, so 17,5L per 100km just stumped me. So:

- Idling for one minute before driving (when engine cold)
- 95% - gear shifts at 3k-3.5k RPM, cruising at 2k RPM
- 5% - hard acceleration on 2nd and 3rd gear (not above 6k RPM)

My Z :
2006 Rev-Up
-Invidia Gemini True Dual Exhaust
-Resonated test pipes
-Plenum spacer
-Bigger upper plenum
-ACT Streetlite Clutch
-ACT Streetlite Flywheel
-Whiteline diff bushings

Tranny, diff oil - stock
Engine oil - 5W50 (I think it's rated differently here in Poland than in US)

Fuel : 98 or 100 Octane Unleaded Premium (I don't know the rating in US)

Honestly. What the hell?

I have similar mods on my HR and I get 19mpg (US), I'm 80% highway though.
Don't think there is anything wrong with it, are you tuned?

By tuning you mean flashing my ECU with new soft?

Have you (or had someone) pull codes?

Myriad things it could be from bat cats to random misfires. Only proper way to do this is to take advantage of the Z's (and just about every car since 1996) diagnostics. No one here can tell you what the problem is without putting hands on it.

OBD2 codes or Consult codes? I have OBDlink and Torque. It doesn't show any codes except "cat efficency below treshold" which I cancel.

OBD-II. And that ignored P0420 DTC "could be" your main problem. A plugged or damaged cat can drop mileage by a significant amount.

In your specific case, it's also possible that one of your O2 sensors might have become disconnected or damaged if/when the tech dropped the tranny for your clutch work and is causing a catalyst fault. Just a guess but a possibility.

General sidenote/rant: People, OBD scanners have a purpose.... NOT just to turn off CELs. Oh lawdy.... :icon14:

I don't think sensors are damaged. They were mounted during tranny job. I think they forgot to install foulers which I asked them to do. I use obd2 mainly to scan and detect problems and do measurements. What's your rant about? Genuine question

Honestly, after I did CAI, spacer and HiFlows my car did the same PLUS ck engine light. Reflashed with UpRev but no dyno. Noticeable improvement in acceleration and gas usage went back close to normal.
Yes, it costs $ but at this rate, it'll be faster and pay for itself in a year of mpg.
Good luck.

OK, maybe this belongs in the "Click n' Clack" thread but....

My rant is not personal so don't take it that way. I didn't mean it to point this out to you specifically (hence my use of the word "general".)

It's just that I see so many threads on this (and every other) car forum that ask "What's my problem?" without people going through the basic, logical steps of troubleshooting, starting with using the incredible diagnostics systems/tools that are provided by every manufacturer of cars since 1996 (and earlier if you include the prehistoric OBD I.)

DTCs or diagnostic trouble codes will point the tech/home mechanic at least in the right direction in at least 75% of the time they're found.

In my capacity as a moderator, I review just about every new thread that comes along and it gets a little old when I see this time and time again so I get a little annoyed at the lack of basic troubleshooting skills (or laziness?) by people bent on "adding this or that" performance parts but have absolutely zero skills in the basics, which is the foundation of doing it right.

Don't get me wrong, I am NOT discouraging asking questions as I get the people who say they have no mechanical skills and are honest about it. And that's a good portion of why forums exist in the first place. But is it too much to ask for people to do at least basic research either here or by consulting da Goog first?

So, in the end, I'll repeat, don't take this personally, but I have no truck for people who believe an OBD scanner is there to mask a problem by resetting the DTC/CEL and thinking it's "all good". SOMETHING triggered that DTC so fix the problem, don't hide it ferhevvinsakes!

[ End rant, off soapbox ]

So, you say that flash is a must? Unfortunately things here in Poland are way different than in US. Performance parts are way more expensive, you earn thrice as much as us and parts availability is incomparable. We don't have Osiris here. I don't know what soft they use here but it costs 400-600 bucks for initial flash. Remember the salary issue, so multiply it by 3. But I digress, I already talked about flash this morning. One mechanic specialized in Z's said the gain would be marginal (up to 5hp), other one said that with the mods it's up to 20hp which I see as worth the price.

if a tune is what you are seeking. Z1 motorsports here in US does mail in ECU flash. it's not the optimal tune..but it is do-able. I do not have this mail in flash but i'm sure others can vouch for it. you mail in ur ECU..they send you it back tuned with "universal" performance mods. you car won't touch the dyno at all. with all your aftermarket parts your car might be running very lean.

things you should check that involves mpg performance
tire pressure
new/clean intake filter
clean MAF sensor
foul spark plugs

Crap. I wqs in the middle of am elaborate reply but it disappeared :/ In short : I never ask questions on mb without research and analysis and I know how frustrated can you be.

Thanks.
All these things were checked long ago. Tire pressure checked, filter - 2k km and checked for debris, maf was cleaned some time ago, plugs are as new as air filter. I checked for air leakage/vacuum leakage with carb cleaner. No change at idle at all.

Regarding flash. Do I send them physical ECU? If so it will cost a lot more than doing it locally

Just to add on to the general issue that Mic was ranting about, I am also dismayed by the number of Z owners who ignore their CELs and repeatedly turn them off with their code readers. Your Z's sophisticated diagnostic system cannot help you if you continue ignoring it. If you fix the problem, the system will detect the change and turn the light off!

I've cleared them only because I've consulted two mechanics about this code and I got an answer that "it does nothing to how the engine runs"

And that's the superior intelligence of the Nissan implementation of the OBD II system. Unlike my SL55 that throws a code/CEL and doesn't understand when the fault is fixed... REQUIRING a specialized reader to re-set the dang dash light warning.

Oh right, it's a f***ing Mercedes.

"Drive to your nearest Mercedes Benz Service Center". (Not kidding, newer models illuminate such a dash message.)

I pity any MBZ owner who doesn't have a clue and does as the car instructs, er, wait, demands of its owner. :icon8:

I am not saying it's a must BUT if you expect maximum performance and gas mileage out of your Z then it IS a must.
This VQ was designed and the ecu controller was factory programmed for the parameters of air-flow with factory hardware. Changing the air box, adding a plenum spacer, HiFlow cats and whatever else is not going going to optimize this engine because the ecu controller has not been told about these changes and is not looking for them.
I like the comment from above, I take my car to Z1. I think they are the best on East coast of USA. Email them, tell them your mods, and ask for a quote for a "reflash".
GL!

https://www.z1motorsports.com/

But isn't ECU adapting itself to current airflow?
Ok, maybe they'll send me the soft but how will I flash it?

I've averaged 14.2 MPG since the day I bought the car new. I do about 90% of my driving in a hilly city.

On the highway, I get about 28 MPG.

That's just the kind of car this is.

Most of people report better mileage that's why I asked.
I've resetted my mileage metter and started to drive more aggresively. To my surprised it's about 16L/100KM which is a bit better.