Mildphil

So I do mostly city driving, but the traffic is rarely heavy and I only have a few lights to my destination every day, nothing too bad, usually going between 20-45 mph, but mostly 40+. I just refilled, took 9.8 gallons to fill my car up to full (from a quarter tank) and my trip meter says 144 miles(I reset at my last fill up), which would mean I am averaging around 14.5 mpg city...is this bad or what? I also have no mods on the car at all, bone stock.

LS6427

iTrader: (2)

Might be low miles on the car......but its still OLD as dirt....like mine.

14.5 mpg city isn't too bad.....but it could be a little better.

-I would Clean the MAF......
-Run a full can of SeaFoam through 1/8 tank of gas. (or as low as you can get it, then pour the whole can in and run it to almost empty.)
-Fuel filter change
-Spark plug change

To get max performance and mpg.......new front 02 sensors will make a difference. Even with no code (SES light) they could be starting to run rich, which is what they do when they start to go downhill before they throw a code. Get a scan to see how they are switching....to see if they are working perfectly.

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Carguy8888

iTrader: (1)

I am in the same boat and I did everything LS6 427 mentioned except the o2s yet (106k ordering tomorrow). I can almost watch my gas needle move w/ only 342 gears.
I have spoken with LS6 427 & read many of your posts, you are very knoledgeable. Would a cat going bad affect gas mileage? I have a piece rattling near my collector.

Mildphil

Alright I will have to look into this, because it not my driving habits, I have an M6 and rarely drive hard, I'm never over 2500 RPM when shifting and usually shift at around 2200. Also I have had the car since may, and it was sitting for one year here (Florida) and then for a year before( in Michigan). Car still had Michigan plates and stickers. So I'm guessing from just siting around all that while, some of this stuff could have very well gone bad or needs a cleaning. And would I do the scan with something like an OBD hand tool?

Mildphil

It would hurt mpg since airflow might be obstructed, but you would also feel a change in performance. When you get your new O2s in (where did you get them btw?) keep us updated on if you notice any differences in mileage.

wssix99

iTrader: (5)

I'm not worried about your mileage. In urban city driving, I get 10 mpg in the same car that can get 29 mpg in slow highway driving.

City driving can vary so widely, I'd suggest taking a highway drive at a slow constant speed and then measuring the mileage.

LS6427

iTrader: (2)

The bad cat just throws a code to tell you the cat is dirty/clogged....but I don't think it changes anything as far as efficiency or performance. Actually, most people, including myself, have had my rear 02 sensors tuned out so they are turned off.

The TWO front 02 sensors are the ones that do all the metering and send Air/Fuel info to the PCM, so then the PCM can tell the fuel injectors how much fuel to spray......

I recently had a bad 02 sensor on one side.....front passengers side 02 sensor. Car ran strong and normal but I was using a ton of gas....as in 90-100 miles per FULL tank. I went to Lashway Motorsports and Josh scanned it for me....it was dumping 80% more fuel than it should be.....or something like that is what he said.

I replaced both front 02 sensors....I was back to 230 miles per FULL tank. So sometimes a bad or failing 02 sensor will dump massive amounts of fuel but the engine will seemingly run great.

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LS6427

iTrader: (2)

You need to have it scanned with an OBDII scanner while the engine is running and up to operating temp....and someone who knows how to read 02 sensors and how they are switching.......

Just to give you an example....when I had my 427ci stroker....if I drove very moderately with no aggressiveness.....I would get between 200-230 miles from a tank of gas. Meaning about 13.5 gallons, because of course I wouldn't run it dry before hitting a gas station. That's about 16 mpg from a BIG stroker engine.

My new 390ci at 10.8:1 compression.....gets a solid 230-240 miles per tank. So that's about 17.4 mpg........and that's with occasional WOT runs.....and some spirited driving.

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DANOZ28

iTrader: (1)

14.5 is low , my worst is 18 with an A4 in mid summer heat. start with air filter , spark plugs & plug wires. then check / clean maf. after that i'd get a tune. my normal milage is 20 and if highway only 24. with m6 you should be 20-22 city & 25 hwy. (imo).

redtan

iTrader: (5)

No way in heck is an fbody, even stock, getting 22mph city. Unless it has injectors unplugged.

Mildphil

Well at least my air filter is brand new, old one wasn't too dirty really but I replaced it anyway a few months ago.

Mildphil

Good idea, Ill fill up, take a drive around the highway, then I will come back and fill up to see my mpg.

wssix99

iTrader: (5)

Or it isn't a real city. lol

Carguy8888

iTrader: (1)

Amazon Denso 234-4617 Oxygen Sensor $43.06 ea, I read mixed things on the Bosch sensors. They will be installed in two weeks. I will update but the car is a nice weather weekend car.
I also bought two new collector gaskets to take care of that problem.

Carguy8888

iTrader: (1)

You described my exact problem. I did have someone scan them and they were switching. My guess is they are tired & switching slooooo (106k)? Do they slow down after time? I read you like Denso's 234-4617 which are on the way.

DANOZ28

iTrader: (1)

well i have a friend with an 2002 35th ann SS m6 i've riden in that has never got worse than 20 and he lives in mpls. my A4 never worse than 18!

wssix99

iTrader: (5)

MSP is closer to a suburb than a dense urban city with long roads and infrequent stops. (Unless you are downtown.) In denser urban environments, 18 mpg is not attainable.

ghardester

I drive mostly 2 lane rural roads and get about 20-22 mpg, a little traffic, not really cruising. When I take it out on the more open road where I can sustain highway speeds of 60-70 then mpg goes up to about 25, sometimes a little better on a long trip.

If your driving is really city driving then I don't see 15 mpg being out of line. Of course there are a few things you can try, but these are not city cars.

http://www.tcm.com/this-month/articl...-Blacktop.html

M4N14C

iTrader: (3)

16mpg here. New plugs, new wires, seafoamed twice, MAF cleaned, fuel filter changed, new O2's. All that's left is to get a tune, hope that bumps me up to 20....

wssix99

iTrader: (5)

Yes. The sensors get contaminated over time. That contamination causes the sensor to work less efficiently and mileage suffers. If you are interested in more, this has some great articles on how that all works:
http://www.wellsve.com/sft503/counterp_v2_i3_1998.pdf

Then... you have to figure out how much is that mpg worth? Suppose a new sensor will get you an extra 2mpg. At $4 a gallon and an average performance of 20 mpg, and $100 for a new set of sensors - the sensors won't pay off for 5000 miles.

... Wow... I'm getting really old. The last time I did that math, sensors were 2x the cost they are now and gas was 1/3 the cost.

... I need to go out and buy some new sensors for my car!