cr0y

car: 1999 Z28 m6 61k miles

I am getting around 13 mpg city not driving hard at all (just commuting to work) I usually hit 4th but rarely 5th. I am planning on pulling the plugs and seeing if they say anything, but could I just be running rich? I notice my borlas pop every once and a while which would be a symptom of running rich.

Any advise would be awesome

thanks

MeanWS6

could be a variety of things. o2's, spring issue, bent valves

GuitsBoy

iTrader: (4)

When I think of commuting, I think of a moderate highway cruise around 70 mph in 5th or 6th gear, with a little residential/city driving on each end. I get right around 20-22 MPG with 4.10s, dead o2 sensors and SES light blaring, even with the 10% ethenol fuel around here.

If youre only driving a short distance to work, the engine may not even reach full operating temp and that may be why you get such lousy gas milage. Also, simply running a car in 5th as opposed to fourth gear makes the car travel 33% more miles per the same number of revolutions / RPM. It could simply be that youre driving in the city where these engines really suck down the fuel. Once youre cruisingon the hiway, milage should come back up for you.

777

iTrader: (21)

wow i only get around 15mpg and drive WAY more conservativly thatn yall do...i do have a large cam though so i guess that's why...i'm in 5th at 30mph and in 6th at 40...however, this is all city...and a lot of stop and go traffic...why would you ever drive in 5th at 70mph ?

777

iTrader: (21)

what kind of gas mileage do you get with the 408?

cr0y

spring issues and bent valves sound terrifying being that i just bought this car

777

iTrader: (21)

i highly doubt you have spring issues or bent valves...it's prolly just your driving style

GuitsBoy

iTrader: (4)

SES light not on, right? MAF plugged in right? Try normal stuff like air filter, fuel filter, plugs, fuel injector cleaner, sea foam, possibly some new front o2 sensors as well. That should tune you up as well as you can be tuned up.

cr0y

Just out of curiosity. How does the cars computer know when certain things are wrong such as a bad spark plug, bent valves, etc?

GuitsBoy

iTrader: (4)

It cant tell with any certainty what component is bad. It can however tell what cylinder the problem exists in due to minute variations in crankshaft speed.

DWillTA

iTrader: (3)

I cruise on the Interstate at 80 - 85 mph in 5th gear at around 3000 rpm and still get 18 - 20 mpg all around driving. I live in West Virginia. This is not a very 6th gear friendly state. I do get better mileage when just cruising for fun at 50 mph in 4th but I can get at worst 17 running 75 - 80 mph on Interstate in 5th & a short city run getting to and after 15 - 20 miles on the Interstate. I see anywhere from 200 to 260 on a tank of gas. It takes 12.5 to 13.5 gal. to fill up. I run Sunoco 94 or Exxon 93 depending on where I am when I need fuel.

General Jack

iTrader: (4)

Could very well be your o2 sensors. Mine went bad and I lost ~50hp and ~2 mpg. New O2s will cost you about $160 from autozone. PM me if you need a part number, although I know you can find it here with the search function.

cyphur

iTrader: (10)

**** that.....i spend as much time as i can in 5th and 6th. no way in hell you'll find me in 5th gear on the highway unless i'm passing or racing at high speeds(rare).