Hey guys I got a little bit of a problem, well basically I’m getting really poor gas mileage in the city. Now the car is a 2003 Z06 with only 12K miles on it, and the only mods done so far are the HID lighting and Cags eliminator and oh yeah also the CLB was installed. I just had it smogged about a month ago and it passed flawlessly.

And a little more info is during freeway speeds the car can easily get 30 mpg’s and I can squeeze out atleast 400 miles on a full tank, but the problems lies when I’m driving around town which is where I do most of my driving. I am only averaging around 10 mpg in the city and that is babying the car. The DIC when set on Instantaneous reads any where from 5 mpg to 10 mpgs while rpms are held steady. I know that these cars aren’t the best in the city but I still figured I could get closer to 15+ mpg if I baby the car but that’s not happening.

There are no codes and no history codes in the computer. I checked the short term fuel trim and it was reading -3 to -9% which means its running lean. Long Term fuel trim is also also in the minus. The funny thing is I get the worse gas mileage when I’m driving slow, I mean around anywhere from 5 mph to 20 mph I’m getting 10 or less mpgs, but when I’m driving around 40+ mph at the same rpms my MPGs go up dramatically. And also I noticed that while driving at slower speeds the mpgs vary drastically, and what I mean by that is by pressing the gas pedal just a little more while cruising the mpgs go down severely, while still roughly at the same RPMS.

I checked the O2 sensor by hooking up my scanner and it shows that its switching but my scanner is slow and the voltage out puts is delayed on my readout so I can’t get a good reading of the sensors actually switching.

I bought the car in Nevada and now it resides in California and I only fill 91 shell gas. I’m thinking my pre 02 sensors are biased lean but then why do I get such good gas mileage during freeway speeds. The car almost seems like its running wide open throttle at slower speeds and my MPG are showing it. And trust me I not driving this car like its stolen, I always shift before 3000rpms and really try to conserve on fuel.

If anyone has any ideas or might be able to point me in the right direction I would greatly appreciate it, if not then I’m just going to swap the pre O2 sensors for new ones and see how it helps.

Thanks again guys for all the help and sorry for the long post.

 

idling along at very low speed with the AC blasting away will not give great mileage. Only venue worse is the drag strip.

With that said, I'd suggest doing a few precise mpg checks the old-fashioned manual way. Fill the tank tight. Drive until its very low. Refill the tank tight and compute the mileage. You may find that you're doing better than you now think. That was always the case in my 02Z.

Ranger

 

Hey Ranger, thanks for the input it is greatly appreciated. But I have done the mileage calculation the old school way by filling up and then setting trip to 0 and doing the math and it still stands behind what I’m saying. With the calculation I’m getting now, I only averaging around 170 miles on a full tank with about 75% city and 25% freeway. And I rarely use my AC, so I don’t think that’s causing the poor fuel mileage. Again thanks for the help though.

 

How is the performance of the car when you hammer it through the gears? Any hesitation...idication of engine miss as in a plug wire loose.

Have you checked for codes?

Ranger

 

try getting to higher gears a lot sooner, might help some. but there isnt a whole lot u can do when ur stuck in traffic.

 

hey ranger the car pulls like a bat out of he!!, and drives like a champ. I haven't felt any hesitation and the engine runs smooth with no signs of missing, and there are no codes or even history codes. The tail pipes do get pretty dark whick I know is an indication of running rich.

Hey Vic, thanks for the advice, I actually am trying to upsift faster and it helps some but not a lot.

 

With no codes and a smooth running motor across the rpm range, then it's (1) the driving venue or (2) If there's a recent decrease in mpg in the same venue, a change in the formulation of the fuel you're running is a candidate to check.

Ranger

 

hey Ranger I know its weird because there is no codes and nothing to really go off of except for the black exhaust. I've been driving the car pretty tamed and really don't get on it at all, so I'm pretty sure its not the driving venue, and I've only been putting Shell gas since I bought the car and that’s what I heard is recommended on the forum. I'm just going to order some new 02 sensors and see how it goes from there, otherwise I'm out of ideas and will just have to except that this car gets really and I mean really poor mileage around town.

Thanks for all the help guys.

 

On my commute to work; 1.5 miles each way with 5 stop lights I always seem to hit I have been getting around 15-16mpg. I still have the skip shift and use it, 1-4 at like 2K rpm, then to 6th as the speed demands.

 

Try keeping the revs below 1500 rpm in town, and let us know what happens.

 

Black soot on the exhaust tips has been characteristic of all my Z06s where ever they are driven, street, in traffic, all highway, drag strip. Soot is always there.

Ranger

 

Hey TT thats what I'm talking about, your getting closer to what I want and you hit lights all the way through. And just a bit of info on my driving style: I generally shift from 1st to 3rd because I want to keep the RMPs down. Thanks for your help, it conforms more of what I should expect for city driving with this vehicle.

Hey Warp Factor, I actually try to keep the RPMs as low as possible and driving around at 1500rpms isn't a far stretch from how I'm driving now. I've noticed that even at 1500rpms I still get around 6-8mpgs while driving at low speeds. Thanks for the help.

 

Your mileage is about what I get around town but, my car is pretty modded. If you have already tried using a different top tier fuel source; my next question would be when is the last time you changed the spark plugs and or had a tune up? You might also check the air filter and for any air leaks in the air coupler etc.

 

My 04 Z gets about the same as yours. I've resigned myself to the fact that this is just the nature of the beast, but man it's worth it!

 

If the O2's think the car is lean, then they would be artificially richening it up, causing the poor city mileage. ( mine has run 16.4 over the last several thousand city miles so I am sure somethings wrong with yours). Either the O2's are borderline, or perhaps an exhaust air leak is pulling extra air before the O2's. MAybe even a defective check valve in the air injection system can let in extra O2, causing the sensors to sense lean, and richen up the mix. That may be why it runs extra responsive in town, a bit more fuel than emissions and economy require. That may also go away at highway speeds, due to changes in backpressure/vacuum in the exhaust. Just a thought.

PS The instant fuel readings at low speeds always reads low, cause you are not moving fast enough to get the fuel used vrs distance covered ratio right, such as the 0 mpg at a stoplight. Its the same with all the instant mileage cars I've driven, low mpg until at least 30 or so.

 

Sounds familiar. Do you reset your DIC when you refill your tank? I was averaging 14 mpg around town. I put headers on and it dropped to 12.8 mpg. I did have my foot in it a lot, especially since the headers sound sooooooo good. After reading another thread about Z06 gas milage, I descided to try not winding it up so much. So for the past couple weeks i've been shifting at 3k or so rpm, and going easy on the go fast pedal. So my cureent average mpg is 16.2 with very little highway driving.

 

Cammed and headers I average about 19.5 with a 75% city/ 25% hwy driving.

I've learned to shift earlier and keep it under 2500 rpm unless the need arises

 

For around town, shifting at 2000rpms or less feels perfectly reasonable to me. Using instant, any gear lower than 5th seems to be horrible mileage. And 6th is the only one that yields impressive mileage.

 

I can't bring myself to shift at that low RPM's .....but I'll try it.

 

Hey LT1GMC, that’s what I’m talking about. I really do baby this car when driving on the streets and try to shift before 2000rpms all the time and it still doesn’t make much of a difference. I have a feeling that something is wrong because the fuel trims are on the lean side so something must be causing that. I have a feeling that the O2 sensors are running biased lean but then again a exhaust leak or even those other possibilities sound good to check out. I will let you guys know how it turns out.

And thanks for the info on the DIC not being very accurate at lower speeds, I will keep that in mind while trying to diagnose this problem.

On a side note, does everybody’s tailpipes get black after say 50 miles of driving, and I don’t mean pitch black but a noticeable difference and I mean noticeable. And how many miles are you guys getting out of a quarter tank, I’m around 50 miles with strictly street driving and babying the car 90% of the way.



Hey Jay, I do reset my DIC everytime I fill up and 12.8mpgs sounds about the same as what I'm getting and my car is stock and I drive on the side of conserving fuel.


And this usually wouldn’t be a big deal for me because I did buy this car to enjoy but with the way gas prices are rising I wouldn’t mind squeezing every last mile out.

Thanks for all the help guys.