Hi everyone,

I have a major issue with my G... Let me list the specifics before I address my situation:

• 2004 G35 Coupe Automatic
• ~111,660 miles (179,700 kms)
• 2005 19" Forged/Staggered wheels
• 100% stock

I've owned this car for a little over 2 years. I've been filing up with 91 Shell. Usually fill up with 2-3 ticks away from E, never run until the light comes on. I change my own oil with Castrol Edge Fully Synthetic 5W-30 and K&N Oil Filter. I just changed my spark plugs last week with NGK G-Power.

Before changing my spark plugs, I had very bad hesitation. I'd floor the gas and it would take quite sometime for it all to register. The loss of power/hesitation has now been solved, or so I think. Slightly tapping the gas makes the car fly.

I'll attach a picture of my old plugs to show how bad they were... Now to my situation. I changed my spark plugs due to having HORRIBLE gas milage. I'd be getting around 120 miles (190 kms) on a FULL tank of 91... The harder I drove, the less. Keep in mind, I would never floor it. I'd keep my RPM's under 3,000, but still horrible mileage.

Now, after changing the plugs, I filled up and have driven about 75 miles (120 kms) on this tank and I'm about 3-4 ticks away from half, so looks like the new plugs helped a bit, but I really don't think I'll even hit close to 215 miles (350 kms) on this tank...

I don't have any fuel leaks. I have no codes on my dash. I'm at the pump way too often. It's about $65 for a full tank. I can't even enjoy my car by ripping it a few times in worry that I'll go through the tank too fast.

This is my last ditch effort to fix my horrible gas mileage. I understand these cars aren't Honda Civics when it comes to gas, but not even hitting 200 miles on a full tank is insane. My old plugs did have quite a bit of carbon buildup. I'm going to try some fuel additive to maybe clean out my fuel injectors, exhaust, etc. Hopefully it helps...

I cleaned my MAF and TB a little while prior to changing my plugs. I did get a little better response and just slight increase in gas mileage. Maybe 2-3 miles, if that.

Please, someone let me know what they think it could be. I feel as though I may just be running too rich... I really don't know since I have no codes, so I'm assuming my O2's are okay. I really hope someone can help me out.

Thanks in advance.

 

Do you have a scanner that can read live data? I'd start there and look at your fuel trim levels.

How does it idle? When i had real bad gas mileage it was due to a vacuum leak. I did get a 171 code thougj for running too lean. You could be running lean or rich and it's not at the threshold to set a code. I think it's +-25 to get the cel to come on.

 

No, I don't have a live scanner. I'm going to take it to a shop and have them pull me data.

Idle? Seems quite steady. It idles at the first tick. I believe that's 100, don't remember off the top my head. By +/-25 are you talking about O2 sensor readings? Sorry, not too familiar with all that.

Also, bro do you think running some additives may help? The previous owner ran regular gas. Ever since I've had it, I've been running 91. Did you see the plugs? They were pretty built up with carbon. I feel like a very strong cleaner *may* work.

Let me know what you think. Thanks in advance, brother.

 

Lift your car up at each corner and try to spin each wheel by hand freely. It's possible that you have some stuck or dragging brake components or something wrong with the differential that's giving you extra rolling resistance.

Does your car roll ok in neutral if you let it coast? You could have a worn pilot bearing (6mt only) or some bearing in the tranny too.

Does the car roll backwards on a slight uphill with it in neutral or with your foot on the clutch and in gear (if 6mt)?

How does your exhaust sound? It's also possible that you have a clogged or partially clogged catalytic converter... usually this will make the exhaust sound really muffled and strange and the car will have low power and poor economy.

 

It sounds like you have a vacuum leak somewhere since you can't floor the throttle without hesitation, I would take apart the plastic stuff on the intake tract and THOROUGHLY inspect for cracks, especially around the corrugated flexible section, also check your PCV valves and the hoses coming off them, they should be tight on the fitting and not able to turn easily, PCV valves should rattle when you shake them. Also check that little plug at the front of the upper intake manifold to make sure it's not cracked either.

Does your engine have an oil consumption problem? If so, how much does it drink.
Does your engine have an exhaust leak?
Did you check your air filter?
Did you do the ECU reset after you unplugged the throttle body / MAF ?

 

You should buy your own scanner it will pay for itself after it's first use. It should idle around 650+-50. The CEL will only come on if your fuel trims are adding or removing 25% of fuel. It uses short term fuel trim and long fuel trim. Short term is instant corrections and long term are the baseline settings. So, if you have a vacuum leak that only requires an extra 15% of fuel it won't trigger a CEL. But, your car will automatically be using extra fuel.

The A/F, O2, MAF, and bunch of other systems all play together. That's why it's a good idea to see your fuel trim readings. Then from there: replace air filter, clean MAF, etc...

I'd run some BG44K in the tank; I like their product. I've only ran 89 in my car for the past 11 years and haven't had any problems.

A sticking caliper is also a good place to check like partyman suggests. The rear calipers always freeze up and I've replaced four of them I think. That will kill your gas mileage, pads, and rotors.

 

I checked to see if there was resistance on any wheel, nothing.

Yes, no issues with neutral, but then again, I have automatic.

I think you may have pointed me into the right direction. Yes, my exhaust sounds amazing, just as the G's exhaust are supposed to, BUT since I bought it, I always felt like it wasn't as nice as most of the other G's I've heard. Given, I haven't heard how my exhaust sounds from the outside, only as I drive it, but even then, it doesn't sound as crisp. Previous owner put regular gas, could that have clogged it up? For the meantime, could I run some strong cleaner through my gas tank to hopefully clean carbon and any other stuff found in my car?

If the additive doesn't help, what would you suggest? Should I look for an OEM y-pipe or get something aftermarket that doesn't affect the original sound of the exhaust?

Please let me know. Thank you

 

Thank you, but so far, it seems as though the loss of power/hesitation was due to the overdue spark plugs. Now, as soon as I even touch the gas, my car flys. I was getting out of my driveway and slightly tapped my gas which made me fishtail, so I'm assuming plugs solved that issue. The only issue that's still there is gas mileage. It seems to have gotten a little better, but there's something else going on for sure. I'll be able to tell once I finish this tank of gas.

No oil consumption.

Exhaust leak *could* be possible, but I'm pointing more towards clogged cats.

Air filter was upgraded to drop-in K&N not too long ago.

Yes, reset everything after. Idle, ECU, etc.

 

I'll go to my local parts store to pick up a scanner. I'll report back to you with the readings.

Yes, heard lots about BG44K. I'll see if I can find it locally, if not, I'll run another product that's apparently really good as per some users on this forum.

Hmm, sounds possible. Wouldn't I feel drag if my calipers were sticking? Or is it so slight not to feel drag, but heavily affect gas mileage?

 

Make sure you get something that can read live data. You don't want something to just clear codes you can rent them or buy one. I bought an Autel diaglink for 115 bucks and then for an extra 15 dollars you can get the Nissan/Infiniti package. Allows you to view all the modules and what not. i can use this scanner on all my cars for basic ODB or buy a vehicle specific package for extra functions.

I also have a Chinese Consult III I paid almost 400 bucks for, which I break out if I need to do some real things the dealer can (except code keys and what not).

I've always had to buy BG44K online and usually you can get it cheaper.

I just replaced a caliper and didn't really notice anything until the shop failed it for inspection. I'm more reactive to my car though. When I took it off the pads were real low and I couldn't even separate the bracket from the guide pins. That's what you get with Cardone (reman brand) crap though....

I've replaced my y-pipe twice as well. First time I bought a Megan Racing one and it rotted out within two years. I just had to replace it again due to a failed inspection, I bought a cheapo from RockAuto.

Easiest way I found exhaust leaks is to stick SeaFoam through a vacuum line. It will be real smoky, but it will come out of places that exhaust gases shouldn't normally.

If you have an IR gun you can take temperature readings before and after the CAT. I think that should help you figured out if one of them is clogged.

Main thing with using lower gas is pre-detonation and if you start getting pings, misfires, or knock sensors going crazy. I would then look for new gas or switch up stations, but I haven't had any issues in 11 years of running straight 87. Now, if I raced my car and drive it extremely hard than I would probably use higher octane fuel, but for daily driving and what not; I don't think it's a big deal at least in my experience.