Hey everyone,

I�m having an issue with my 03� g35 coupe- my fuel economy in the city is disgustingly bad.

I get about 3.3km/l (250 km�s a tank) or 7.7mpg in the city but, on the highway my fuel economy is amazing. I have been looking everywhere for a solution but from what I heard.. this is literally IMPOSSIBLE. From what i�ve heard my car should basically be a forest fire but it runs great (other than the horrible fuel econ).

I need some suggestions from you guys. The only issue that I know of is the knock sensor(P0327) and I will be fixing that ASAP but I refuse to believe it is hurting my fuel economy THAT BAD.

PLEASE HELP

 

Get everything fixed that you know is wrong with it before you start trying to solve other problems. You never know if one issue is causing another one.

 

Okay, immediately after making this post I actually have been having problems with the car. I�m having to replace my crankshaft position sensor at the moment. My issue is that the car will choke up if I press the gas pedal too fast. It seems more like a fuel issue to me than a sensor, but code P0335 popped up as soon as I started having this issue.

I will take your advice and get my knock sensor replaced when I can but it seems like a long process� should I replace anything else while the engine is torn apart?

Thanks for taking the time to reply, i�ve only had the car for 3 months and i�m trying to bring her back to running good.

I�ll attach a video below that shows the car halting every time I accelerate. I apologize for the poor quality.

 

Throttle bodies are known to get carbon build up and not open/ close properly, try cleaning ut but u have to do a relearn aswell so please youtube how to do a relearn u can do the relearn your self

 

hey everyone, it�s been a while since my last reply but my crankshaft position sensor is now fixed. The knock sensor still needs replacing but i�m working on that.

My fuel economy continues to be horrid. I am going to fill up with chevron 94 and add some fuel injector cleaner as a start.

Anymore advice/tips to diagnose this issue?

 

You should always be using high octane fuel. If you were using regular 87, that's part of the problem.

Given that you say the knock sensor is bad, at idle/low rpms, your A/F mix is probably way off feeding too much fuel. When you get going, things even out a bit and you get better mileage.

But do remember that for these cars, they hate to idle and they suck fuel at idle. Which is what one spends a lot of time doing in the city. Still, 7mpg is too low. You should be around 13-15.

Most pertinently, poor gas mileage with G35s is not an issue, it's a feature. These are not economy cars, they are tuned to get the maximum power out of six naturally-aspirated cylinders.

Get that knock sensor fixed. Also, make sure your plugs are correct and not worn out.

Good luck.

 

Hey G2B35AGN,

Thanks for the help, I always fill with 91 Octane. I have never used anything lower than that.

Still no new revelations about my issue. I just filled up with 94 Octane and added some Sea Foam fuel injector cleaner to see what that does to my fuel economy. I will post back with some analysis when my tanks used up.

I wanted to mention that all my spark plugs and coils were just replaced by the previous owner before I purchased it.

Haha I know they aren�t fuel efficient cars but something is definitely going on here. I am determined to find a solution so anyone else that has this issue can fix it properly.

I will get the knock sensor fixed soon.
ONE LAST QUESTION - my g35 burns some oil, is it possible for this to be the reason my fuel economy is so poor? does the fuel mixing with oil really impact the engine efficiency that much? No one seems to have the same issue as me and I know the VQ�s are famous for burning oil.

Thanks guys.

 

That depends, how much oil is it burning? Take a look at your PCV valve to make sure its not stuck open, which will suck oil vapor into the engine. Its best to replace it, but in the mean time, if it doesn't rattle, you can hit it with some MAF cleaner to clear out some of the gunk until you can replace it. Its best to use OEM, as I've come across PCV valves with different strength springs, which can cause them to function improperly.

 

thanks srv2nv,

the part seems relatively cheap, should i just go ahead and replace the PCV anyways? How can you tell if it�s stuck open/closed?

 

You can test it by removing it and giving it a shake. If it doesnt rattle, it needs to be replaced.

 

***UPDATE*** - ***FIXED***

Hey guys thanks for everyones help. great community out here.

My issue has been solved and my gas economy is SUBSTANTIALLY better. i have gotten about 210km�s on half a tank using 94 octane.

If ANYONE is encountering this issue in the future and needs help, here is what I have to comment on:

1) Sensors:
!!ALWAYS USE OEM SENSORS!!
When I bought my car the last owners replaced all sensors but obviously cheaped out� do NOT do this.

I have had to replace my Crankshaft, Camshaft Bank 1, Camshaft Bank 2, and MAF sensor now because of a lazy job (almost $600 worth of parts). I suggest using the brand HITACHI. I have had ZERO issues with the new sensors yet and will keep this thread updated if i do.



WHAT I BELIEVE WAS MY ISSUE:

my Mass Air Flow (MAF) sensor was either cheap or broken OR I had a vacuum leak in my intake/throttle body.

If you are experiencing a poor fuel economy and know that the sensor has been replaced� make sure it is a certified OEM MAF sensor (HITACHI). The car was NOT throwing any codes for this issue [*note*: until i removed my intake to replace the first camshaft position sensor (bank 2), a code popped up for my MAF sensor. so I quickly replaced it].

OR triple check that everything is vacuum tight on your intake/throttle body. A good seal is important.



SIDE NOTE:

If you are experiencing engine misfires under heavy load (e.g. high rpms or heavy accelerating) it is most likely the crankshaft/camshaft sensors being faulty. Start there and diagnose the issue. No one seems to be talking about this in any other thread for some reason.

I will reattach the video for anyone that is confused. watch the video to see if your issue is the same as mine. The best way to describe this issue is a loss of power and the car jolting. the slip light and vdc light both turn on when its misfiring.

I hope this may help anyone having a similar issue to mine. Thank you!

 

Thanks for coming back and letting us know how you fixed the issue. Glad to hear its running good for you again as well.

 

I had Hitachi CAM sensors on my VQ35HR and they caused hell and a half, check out my thread on that here: Long and rough warm or hot restarts
The VQ35HR prefers Denso over anything and my advice to everyone is to buy sensors from the dealer and not from eBay or you'll end up like me..
As for MPG, i get around 10-12 in the city and MAYBE 17 on the highway on my 5AT 3.53 gearing. If i grandma it, it should be a lot better but i have a truckers foot and don't care about MPGs as this is not a hyundai..