Something is definitely wrong with my car my navi shows 11.5 mpg all city almost 80 miles and already a half tank

 

in the beginning during the first 10k miles i used to get 240 miles to the tank mainly city miles...13.8 is not good..but doesnt sound tooo toooo for an awd g35 mainly driving city. who knows how your driving is lol

 

13.8 MPG is unacceptable mileage for this car in ANY configuration unless i'ts crazy heavily modded, there's something wrong with the calculation of that MPG, there's something farily wrong with the car, or a combination of any of the above.

Anything under 17-18 MPG you should be digging pretty hard into what's wrong with the car IMO

 

I get 17.5 mph when I am not being a dolt. Stock 04 sedan rwd. Auto. Only non stock is a 5/16 md plennum spacer and coupe airbox. New plugs at same time. Mileage was much worse before. Here is the kicker- I live in an area that has nothing but dino crap below us. S#>^ load of oil yet I pay $1.30 a liter (x 3.79 in a US gallon) equals $4.93 a gallon. Granted it is 94 octane but the 91 is only 3 cents a liter cheaper. Hows that for gas mileage? I should be driving a Smart For Two, but I aint smart smart enough for one!
Also gave a restored 65 GTO that gets about 6MPH. No chance saving fuel with her.

 

I was averaging 21mpg a year ago (04 Sedan), less than 16mpg today.

All filters new
New tires at the correct TP
Run Lucas Fuel System Cleaner every other oil change
Only use 91 octane
Reset the ECU several times
Idle speed perfect
No black smoke at all
Car runs as good as it did when it was new (I am the original owner)

During the winter months, I tried to chalk the sudden drop off to dreaded 'winter gas', even though the previous 6 winters were in the 20+ mpg range.

I am the same person, driving exactly the same as I have driven the previous 7 years.

I have been reading about Ethanol in gas and the negative side effects, but a 20% drop seems too extreme to be Ethanol induced (this stuffed would be banned if it caused this kind of reduction in MPG). One can make the argument that gas makes a difference (I use Costco because I save at least 17 cents per gallon over Shell, Exxon, Chevron, etc.), but 20% again seems to be a stretch for this theory.

I am down to sensors: EGR, MAF and Oxygen(4).

The dealer insists that if these sensors fail, they invoke CEL lights....I want to think that because they are sensors, there must be a gray area between being functional and CEL based failure. I also find it interesting that many others that are having similar problems are in the same 90k+ mileage range....coincidence?

The charge to diagnose all 3 is $120.

I'm thinking about taking this path soon as the 5mpg drop is not a welcome addition when gas is $4.20+ per gallon.

What's really odd is that around the same time, my wife's Honda Pilot dropped off 3-4mpg as well.

I am convinced that Southern CA is a test bed for pushing the limits of low quality fuel in the name of environmental protection, but that's the subject for another day.

 

Once again, check for vacuum leaks.

Simple thing that NO one seems to know how to do, or think about. It's also the #1 cause for low performance and/or crappy mileage, and many other issues.

 

I think I'm in the same boat as this dude..my mpg SUCKS big time.. I think I will get new plugs pretty soon to see if this helps my mpg..

 

Most of you with "better" mpg seem to have 06 or prior models. 07 after the body redesign and a change in displacement(I think), mpg got worse IMO. I am getting 15 mpg or lower driving in the city. I do get close to 25 mpg on the hwy which is reasonable, but city driving does not give me any hope of getting better than what I am getting. I am used to it.

 

Makes sense, but wouldn't these manifest themselves in the form of rough idle and erratic shift behavior?

Will dealer computer diagnostics detect a vacuum leak?

I do have a 5/16 spacer that I installed over a year ago,and I have noticed a few others that have the same spacer complaining as well.

Definitely a possibility.

 

One of the easiest ways to find vacuum leaks is to get some starting fluid and spray each line while the car is running. If the rpm's go up the you found your leaky hose. At least that's what worked on my old 300zx with vacuum lines running all over the engine bay.

 

G35Driver > General Discussion > G35 Sedan V35 2003-06

I would hope people posting in this section have 03-06 models.


Vacuum leaks can cause a ton of issues, but you can have a vacuum leak and have an engine that runs perfectly fine, just gets bad mileage. This is because the engine is pulling in EXTRA un-metered air. It is n't expecting this air, and once the O2 sensors read the mixture the ECU sees that it's getting extra air, so it adds fuel. The ECU thinks the MAF sensor is reading wrong, so it's trying to 'correct for it'.
The thing is, vacuum leaks are intermittent a lot of the time, and you don't have a vacuum leak at higher Throttle positions, so this means that the computer is constantly changing the mixture, at the end of the day, it results in more fuel than is needed because the whole 'recipe' is wrong.

And No the Computer won't show a vacuum leak, that is a manual check. The computer will show a change in fuel trims, etc, but that's complex if you don't understand it.

This will work but is HIGHLY un-advised. Starting fluid is CRAZY flamable (ETHER), spraying it on your engine is just a bad, bad idea.

There's a couple ways to check it safely, the safest and best way is using a Smoke machine.

Probably the second best and more "DIY" way is Propane, though it sounds dangerous, it's WAY safer than any Starting fluid, Carb cleaner, etc, etc.


If you put a piece of hose over the end you can route the hose into the engine bay.

Tip: If you have a scantool, you can watch the "fuel trims" while doing this. If the SHORT term fuel trim spikes all of a sudden when you're checking then you know for sure you have found the leak (area pulling the propane in).

There's a lot of ways to check Vacuum leaks, some better than others, and honestly, I haven't found one that is 'great' if you don't have a way to watch the Short term fuel trims or the AFR (wideband + gauge). j

 

Don't have any exact readings on mpg for my 04, but a 6 mile drive with I would say a 70% hwy, 30% stop and go traffic takes out about 1/8th tank of gas. That's normal gas, I recently put in mid-grade(89 octane) and currently tracking it.

 

Good write-up TunerMax. I'm guessing that this is not something the dealer would do for you...

Has anyone successfully found vacuum leaks on our cars using the Propane or another method ?

Seems like this would be a good DIY.

Where on the car are the vacuum hoses you should check initially?

 

Remove the engine cover.



The main thing in the center is the Intake manifold. The throttle body is the 'start' of the intake system where you'll be checking. Basically, anything after the Throttle body needs to be checked. The most common spots are the vacuum lines, the black ones that run fromt he Manifold, back of the throttle body, etc, etc. Follow them all around.

Also check all around the intake manifold gaskets, send propane down in the middle underneath the upper intake manifold as well. Should REALLY check this if the Intake manifold has been off, spacer, etc.

 

Checked the bolts on the plenum last night and they were loose (I could unscrew with my fingers exerting little effort)

I expect they may be the culprit. I went through and followed the tightening sequence here:http://www.myg35mods.com/tutorials/spacer/index.html

But did not have a torque wrench. I will pick one up at lunch today and re-torque all the bolts (18), then continue with the vacuum check if this does not resolve the problem.

Interesting how these bolts on the plenum worked themselves loose. I wonder if this if others with the same 5/16 spacer realize this could be an issue as well.

Thanks again TunerMax.