INFERNY

Lately I've been noticing that my mpg seems to be dropping. Right now I average about 9-10 miles per gallon in my '01 Z28 M6 and most of my driving is spent driving around 40 mph in 4th with an occasional pull (My daily commute is only about 5 miles round trip). I checked my FIPK intake and it was clean. I also checked for any leaks under the car and I didn't see anything. So what else should I check? And what else could be causing the bad mpg?

Darth_V8r

iTrader: (4)

My guess would be oxygen sensors going bad. They tend to dump fuel when they go bad because they read false lean.

OSU Football Fan

iTrader: (23)

Try dropping the car into 5th gear going 40. I cruise in 5th gear at every speed from 35-55 and I average around 23-24 MPG in an M6 Z28.

It could also be a dirty MAF, fouled spark plugs, broken plug wires, bad O2 sensors, a vacuum leak etc. How long has it been since you gave your car a tune-up? Does your car have any CE or SES codes popped? Are you running a lot of mods without a tune?

Need4Camaro

iTrader: (1)

I personally think your commute is too short. You're likely doing all street driving and I am betting the car doesn't even get a chance to leave the ECM's premapped fuel table through at-least half of your commute (which is 2.5 miles each way). It uses a pre-mapped fuel table from the PCM which causes it to run a bit rich in effort to warm the motor up and heat up the O2's quickly. The O2 sensors are not used during the initial startup / warm-up phase so you burn ALOT more fuel during the warmup phase until the O2's begin switching.

Take it on a longer highway drive and record your MPG, see if it comes up then.

wssix99

iTrader: (5)

This is pretty normal for street driving and is similar to what I get. As suggested above, 5th gear will help. A full emissions tune up, will also get you a few MPG. (Carbon Cleaning, new O2 sensors, cleaning the MAF, make sure the air filter is clean, new spark plugs.)

^ The catch is that all this will probably cost you a few hundred dollars. It takes a while to recoup the benefits in gasoline, but it will happen eventually.

Need4Camaro

iTrader: (1)

I get about 19 MPG on the street in my A4 LS1. I've changed plugs, wires, 1 front O2. I also replaced the MAF, LID and TB but I didn't see any difference (because I can't stay off of it everytime the light changes green).

For the OP I really believe his issue is his commute. Its not long enough for his car to completely warm up and change fuel maps and cars run pig rich during the initial cold start. I bet he would get entirely different numbers if he drove atleast 10 miles one way

wssix99

iTrader: (5)

Rigth. There are lots of variables. I've gotten anywhere from 9mpg (real city driving in the Big City) to 29 mpg on the highway after a full emissions cleaning and tune.

INFERNY

Yeah I do all city driving. California traffic sucks; I guess I didn't realize that my MPG would've dropped that much. The thing is if I had a dirty MAF or fouled plugs wouldn't there be other symptoms besides bad MPG?? So far the bad MPG is the only issue I have been able to spot so far.

INFERNY

I have only owned the car for about 3 weeks and there hasn't been any check engine lights so far. I haven't checked the OBD yet for any codes. In addition, I have no idea if it has a custom tune or not, but the mods installed by the previous owner that I know of so far are:
-SLP LM1 catback
-JBA headers
-K&N FIPK intake

Need4Camaro

iTrader: (1)

The MAF doesnt necessarily have to be "bad" to decrease your fuel mileage, it will still "work" just not optimum so yes it may not be throwing a misfire but still can hurt MPG. I still am 90% sure it's your commute though, the distance is way too short for the car to warm up and enter the proper fuel table.

wssix99

iTrader: (5)

Fouled plugs or fouled MAF - yes. Dirty plugs or dirty MAF - no.


How can you possibly know what "bad MPG" is then??? As I mentioned above, the range you are giving is normal for what I'd expect on a car with some build up dong city driving.

You probably have normal MPG - but the only way to confirm that is to find some open freeway, fill up the gas tank, and then do a mileage run at 55 mph for 30-60 miles. (You must drive like a grandma. When these cars were designed, the national speed limit was 55 mph. If you go much faster than that, your mileage drops significantly.)

Need4Camaro

iTrader: (1)

I never really thought of this, but surely there is some grace for the M6 cars in 6th gear and also the A4 cars with 2.73's, the Overdrive on both is a bit overkill. Way back (and this didnt happen very often) I had a 01 Z28 with 2.73's and I was able to pull of 30 MPG at 80 MPH. I think that was more-less a freak occurance because that very rarely if ever happened. I averaged about 25ish MPG in that car though doing normally 65 - 70 MPH. My 99 Z28 though, Im almost certain has 3.23's but I never really bothered to confirm. Max I've gotten was 27 MPG on the highway doing an average of about 75 MPH which I think is still pretty dang good for a decade old V8 that has over 200k miles on it. AVERAGELY though the car gets around 23ish MPG combined city and street and I have no complaints with that...its actually getting about what my 2015 V6 gets but...I honestly dont really consider the 5th gen V6 an economy car either.

wssix99

iTrader: (5)

Yes. The speed limit was 55 AND gas was very "expensive." (Had you told me in 1999 that I'd be paying $4 a gallon today... I probably would have purchased a Honda Civic...)


I'd believe it. The A4's always had better mileage and the best I've done on my M6 was the long highway run at 29 mpg.


Wow. You must be really well maintained. I can get 26-to-27-ish after a carbon cleaning at 75 mph+.


I don't know about the V6 but I definitely saw the V8 as an economy car and still do! (Especially compared to what is being sold today.) The economy of the LS1 was a big selling point - especially in the age just before cylinder deactivation. (Back in the day, I remember it being competitive to a Honda Accord on the highway.) Compared to what we can buy today, these cars are feather-light and can get great fuel economy, as a result. (When they aren't driven stop-and-go in a city!)

DANOZ28

iTrader: (1)

because my 4 popper avg 30.5 im sorry i'll never believe the v8 folks saying they got 30 mpg (unless coasting down pikes peak) best i got was stock 25.5. currently after my stall i get 18. before stall avg 20.5. i road tripped in a stock 2002 35th ann m6 and we got 25 mostly hwy. hope that helps give you a range of what you should get.

wssix99

iTrader: (5)

That's what I was saying with regard to these cars have always gotten great mileage compared to those "economy" cars. How much does your little 4 popper piggy weigh???

Geee. I wonder why your mileage is so poor compared to a stock setup?

SoFla01SSLookinstok

I get better MPG than that with my stroker. Running around town in 4th. Maybe start with a tune-up if you don't know the maintenance history. Lots of other good ideas.

Need4Camaro

iTrader: (1)

Yeah but you see your 4 Banger is AVERAGING 30.5 MPG on a consistent basis as in, between street and highway driving. All in all every day and every drive you're AVERAGING 30.5 MPG. There's a difference between averaging 30 MPG and "obtaining" 30 MPG. My 99 Z28 will never ever average 30 MPG city / highway driving combined, or even driving it every day on the highway, its just once in awhile when the conditions are right it is capable of "getting" 30 MPG, but dont count on that happening every day and every week.

If I drive ALL highway no stop lights or traffic jams then yes I can average 27 MPG, for the length of time that I remain on the highway, but once I get off then its going to become considerably lower, so your AVERAGE of 30 MPG is more like 40 MPG on the highway, and then 25ish on the street..where as my max is 30 MPG on the highway and 19ish on the street.. there's a difference.

But for what these cars really are they offer very good fuel economy and I was able to fully rely on my 01 Z28 with 2.73's as a full commuter car without worrying about gas.

DANOZ28

iTrader: (1)

if anybody would like to take the test and prove your milage feel free! go to your fav station , fill er up, remember pump# & milage & post. rip around a week or 2 then at the same station same pump top it off & post your receipt & milage. let us do the math. nobody has been ever willing to step up. cheers.

Midnight02

iTrader: (8)

I've always been amazed by some of the mileage claims I see on here. I can get roughly 20 mpg for a tank if I'm cruising at highway speeds without any city driving, but under normal daily driving conditions for me (I too have a short commute -- 3 miles) I usually get 10-11 MPG consistently.

Granted, not an apples-to-apples comparison based on my mods (heads, cam, intake, 4.10 rear, etc.) however I don't expect I'd ever see 15-17 mpg with my normal driving habits.

One item of note in your post is the K&N FIPK. I have one on my car because the previous owner installed it and I haven't gone through the pain of converting back to a lid setup. In short, I hate it. The filter gets dirty very easily. I'm good about cleaning it and using as little oil as possible for the "recharge", however despite my precautions it will still push filter oil through on the MAF sensor -- this can have an immediate impact on performance and fuel economy. I keep a can of CRC MAF cleaner close by and usually clean it after every 1,000 miles or so. If you haven't done this already, you definitely should.

At the end of the day, I don't think you'll find a silver bullet here that unlocks a substantial amount of additional fuel economy. Nonetheless, it's still worth taking a look at some of the items that are potential sources of lost "economy" (using that term loosely).

OSU Football Fan

iTrader: (23)

I have a 60 mile daily commute, so I check my MPG a few times per year to make sure everything is running like it should. The easiest way to do it is to reset your trip meter right after you fill up. Then the next time you fill up, divide the number of miles on you trip meter by the number gallons of gas you buy.

I have a 2000 Z28 M6 with bolt-ons. I typically get right around 23.5 MPG with around a 75% highway & 25% stop and go driving split. The best I've ever gotten was 27 MPG coming home from vacation on a flat highway.

For what these cars are, I feel the gas mileage is exceptional.