The MPG Quest again!

I posted this a min. ago in the Tuning & Tech Forum:

Need a bit of help/advice. MY MPG sucks compared to the many other E46's I'm Currently @ 16.3. The highest I've gotten was 16.8 and that was before changing my O2 sensors. I installed 2 new 02 sensors last week, though i haven't taken her on the hwy as yet. I'M WONDERING WHAT CAN I DO TO FURTHER INCREASE MY MPG?

I have no lights on: SES CEL. My plugs are new (NGK's), Belt's new, Fuel Filter- new, Oil is fresh (Luiqi-Moly 5-30LL) Air Filter is new (within past month and a 1/2) I seafoamed her twice and Deep Creeped it once. I use prem/ultra 93 however I was forced to mix 2 weeks ago with the Hurricane situation here in NYC, but I used octane booster within 24 hrs of mixing 1/4 tank with half a tank of reg 87. I've inspected around the VANOS and don't see any leaks or issues. I also took off my TB, DISA and ICV all for cleaning and inspection, all seals are good and all parts are clean. WTF else to do? Do I have to drive to DC and back?

Last week i was at 16.3 and took a drive out to montauk and back to Brooklyn and only went up ONE STINKING point to 16.4 i was there till yesterday. I didn't use this car sat or sunday (yest) and when I started her up to move for alternate side parking I noticed the 16.3 MPG

I'm assuming the O2's you replaced were the pre-cat sensors?

yep!

That's really bad. I usually get 27.8. If I drive less roughly and use cruise control more often I can just about keep the average at 30. I'm in a 328ci. I also have the A/C on all the time and it's an steptronic.

Stupid question, are you using regular gas? I always fill up with premium unless the gas prices are close to $4/gal (it's usually under, next year though who knows?).

Okay, first of all, when's the last time you reset your average mpg? If it's been 5000+ miles at 16.3mpg and then you take a 100 mile highway trip at 30mpg, you're only going to notice a negligible difference in your average since your trip is a tiny bite out of the total.

Also, do you take short trips often? When the car isn't up to temp it is not efficient. If you're starting it up in the winter and mainly driving it 3 miles to school/work/whatever and shutting it off you are going to average complete crap for mileage because it's not ever reaching operating temp. On longer trips it should fare better.

Third, if you haven't replaced the Vanos seals yet you should. The old ones may "seem" fine but if they are original they have degraded. Just do it, it's 60 bucks and a day of your time. This won't improve your mileage from 16 to 25 mpg, but you should notice more low-end and about a 10% increase in mileage.

Mixing the gas shouldn't have been a worry. The M54 DME is relatively advanced, it noticed pretty quickly that you were knocking and retarded timing.

Have you checked for vacuum leaks?

I'm at about 16mpg average in my 330 6 speed commuting 3 miles, but I get about 30 on the highway if I cruise at 80 or so.

It's amazing that my 11 year-old engine can still deliver the efficiency that the BMW engineers said it would. I usually get about 30 on the HWY, 18 in the city because, well, I like to REALLY drive the hell out of it.

As above.... ! Reset it by holding the trip computer button when it shows the mpg .

To clarify, you hold the OBC button (on the turn signal stalk), not the trip odometer reset button on the cluster.

Press the OBC button each time till you get to the MPG screen. Now press and hold the OBC button till the MPG shows ----- .

This will reset your MPG display. Same can be done with the MPH display as well.

However, seafoam and deep creep is not going to do anything to MPG. All it will do is burn a little of the carbon deposit.

Keep your tire pressure good and probably its time for your VANOS. I believe the camshaft sensors also play a role in how the engine runs and consequently what mpgs you get. I have a bad exhaust CPS and I am changing it this week once it arrives, so I will report back once I change it.

Also, consider taking a longer drive to free things up in the engine.
16 is towards really low side. I get around 23 in mixed driving with my small engined auto.

fuel pump/filter, spark plugs, vanos seals, tires, diff fluid, trans fluid, there is no end to things that contribute to the car's efficiency

I've found that the E46 gets wildly different gas milage depending on how its driven. It's upper potential is truly stunning. I drive a 2005 330Ci with the performance package (ZHP). I bought it used in March 2006. I drive it to and from work and very little on weekends. My compute is 40 miles of limited access highway each way. Just a couple of traffic lights. To avoid tickets, which I've gotten with past BMWs, I drive it every day in a way to optimize gas mileage. Over six years and 120K+ miles I've averaged 36.4 MPG with a high for a full tank of 41.2 MPG. This is actual MPG, the OBC usually indicates about 1.2 MPG higher.

The OBC is showing im getting like 15 mpg That's all city, shifting below 3k. Car is a 02 325i 5MT with 74k miles.

Rocket3360, is your car an auto transmission or manual? I've got a 330Cic ZHP auto that barely gets 21 MPG highway / 17 MPG city and that is with a light foot. (~59K miles on the odometer)

Owned the car since new and it seems as if the car has lost a bit of efficiency (and power)....used to get almost 24MPG highway, but this number was never retained for more than 1 tank of gas.......it almost always converged to an average of 22 MPG.

I blame my trans for terrible city MPG since 1st gear doesn't grab completely until over 2000 RPM - this has been a long standing issue (- when under warranty, two dealers told me that is normal auto transmission operation - BS!). My RPM's seem to be a bit high cruising at 70MPH on a level road - transmission problem? Maybe, but I still have reverse.......It'd be awesome to get 30MPG highway .....but the 26MPG goal that has been unobtainable as of yet .

Recently replaced (within the last 500 miles)
- New Tires (all 4)
- New BMW Struts/Shocks
- New BMW Front Control Arms
- New BMW Front Control Arm Bushings
- New BMW Crack Case Vent Valve/Hoses/dip stick o-ring
- New OEM Spark Plugs
- New BMW Fan Clutch
- New BMW Water Pump
- New BMW Thermostat/housing
- New OEM Air Filter
- New Intake boots (upper and lower)
- New VANOS seals (not BMW - Beisan)

..........with no change in MPG at all!!! On the other hand, none of the above was in the pursuit of better MPG, but expected 60K maintenance.