What do you guys think could be causing my 2005 350Z with just 43K miles to loose performance. The MPG gauge found inside the driving compartment has being progressively decreasing for the past two months. For the past two months the number have come down from 19.7 to 18.9 MPG with about 0.1 MPG drops every couple of weeks approximately.
Any ideas or areas that I should be looking at, any help will be very appreciative, members of my350Z forum.

 

could be many different thing changes in outside temp ..dirty air filter bad o2 sensor bad spark-plugs i would check those first

 

It would be absolutely impossible to get the exact same MPG and minor variations like you're seeing are completely normal.

You're over-thinking this dude

 

Do you reset the MPG counter every time you get gas?

 

Are you using 91 octane?

 

The driver is responsible.

 

I'd recommend you keep track on you MPG's manually. Reset your trip counter after you fill up - next time you get gas, take the miles traveled on the trip (one tank of gas) and divide by the quantity of gas put back into the car. Make sure you refill with a full tank though.

 

I should mention that my car has being tuned a couple of times. The last time I remember to pay attetion to the MPG gauge after the second tuned, the number was at over 20. When I first got the car back in 05 the MPG number were pretty consistent, no major drops.

>> I haven't had any ECU code due to a faulty o2 sensor. Spark plug have being replaced a couple of times, Iridium, OEM and Air filter is brand new.
>> I haven't reset the MPG counter, I don't know how to do it.
>> I always use Super Premium gas (93, I stand corrected, thanks Flo-ridaZ33) on my Z nothing less.
>> I don't drive my Z hard a lot, I'm a pretty conservative driver.

There is no doubt in my mind that my car has loss performance lately. I'm starting to wonder if it could be an injector problem or a fuel filter problem.

 

Doesn't Texas have 93 octane fuel?

 

Reset the MPG counter , run thru a couple tanks of gas andget back to us.

Reset it every tank of gas. Very simple, the button under the button used to toggle over to it. Push it when the MPG counter is displayed.

 

They might have switched to ethanol blends in your area recently. That will decrease your MPG and F with your tune.

 

Which brand is not running Ethanol in their gas that you guys know off. That will be something that I have to consider next time I filled up.

 

Hey just one more thing to check I had heard that the president passed a bill allowing the gas companies to start using 15% blends of ethanol now. Make sure thats not there because like jmr0311 said ethanol will decrease your mpgs.

 

This ethanol blend issue is irritating me even more so since I'm almost certain the last filled up I did on my Z, the one thing that stuck to my head was the label on the gas pump that was saying "15% Ethanol Blend".

I've being noticing that my Z lost of performance has being more apparent since I relocated to Irving, Texas. I wonder if the Ethanol factor on the gas in the area can be attributed to the sudden lost of performance on my Z.

I heard that only the premium gas contains ethanol, is that correct guys? Also, I'm pretty sure gas companies and gas retailers are legally obligated to disclose to the customers if their gas contains ethanol.

What do you guys think I could add to the gas tank to help with the 1/2 tank of gas that I still have of this ethanol crap that I put on by accident.

 

All the pumps that I've ever seen say they have up to 15% ethanol but I doesn't specify how much exactly and all the pumps say it... 87,92,93 octanes.

 

I had to Dyno tune's done on my Z using the Osiris and both of them were done in Georgia.

I need to take my car to the Dyno to do a couple of runs and see what I'm getting now and compare it to the charts I have from those two previous Dyno runs.

 

are your tires inflated properly? this can dramatically decrease your mileage. ive seen 6 mpg drops from extremely underinflated tires.

 

Roughly 1mpg drop is within acceptable tolerance. Depending on the conditions and your driving habits, you can easily see bigger variations in the economy numbers. I think you're being a bit over-reactive. Personally, I see 2mpg+ swings between fillups, depending on how I drive on a given week and if it's more highway driving vs. more back roads. Some points to keep in mind:

1. driver is a major factor. Your economy is greatly dependent on how hard you press with your right foot.

2. This is the time of the year where the air is getting colder. What that means is that air is denser. This affects the car is several ways. The car experiences great air resistance while driving which cost you, your tire inflation drops which also costs you (if you haven't been on top of it), and most importantly, your engine is ingesting more oxygen per unit of volume which means that affects your fueling. More oxygen means more fuel used which means dipping mpg. I always see economy drop in winter months which is likely due to another reason. See #3.

3. Ethanol. I believe the ethanol content increases during winter months to help stabilize the fuel when the temps get cold. Frankly, ethanol is great for tuning. Why? Because it is a superb knock suppressant. If you have means of altering your fueling and timing, you want ethanol. Alcohol raises effective octane of the fuel and provides additional stability which means you can bump your timing and lean out your mix for even more power.

4. You've been tuned. It's been a few months and the honeymoon is over. The loss of power you're perceiving may just be you getting used to the car. This effect has been well documented on this forum for years.

In the end, if you're concerned about alcohol content in your fuel and how it is affecting your fueling, take a look at your long term fuel correction. If you're getting large amount of ethanol, your trims will be big on the positive side meaning that the computer has been adjusting to the alcohol by adding more fuel based on the learned feedback from the O2 sensor. This will also be a good indicator where your gas has been going. Since you're not throwing a code, it means that the ECU has not exhausted its capacity to adjust. I believe the ECU can add up to extra 20% based on learned fuel trims. After that, it starts complaining and may throw a code. It is important to keep in mind that the ECU is fully capable of adjusting dynamically to the fuel differences on its own and keep the ship running in top shape. You shouldn't have to change anything there unless you're running on a standalone solution.

Do some data logging and you should be able to tell if you're having fueling issues. Honestly, I don't understand why people rant about ethanol, keeping the political aspect out of it. Just based on performance, yeah, your economy drops but you now have much better fuel for making power. More alcohol is good for power because of it's good stability. Yeah, your mpg drops but you gotta pay to play.