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Old 03-04-2013, 08:52 AM   #32
OEM ///Member
Join Date: Nov 2009
Location: Fairfax, VA USA
Posts: 22,858
My Ride: 330CiC, M5, X5, E93

Not sure I want to comment on this one!

Here is what I can tell you on this issue.

1. Leave the car in gear, these cars are programmed for fuel cut off under coast conditions. It actually does make an impact on fuel economy. In order for the engine to idle, it needs fuel, under coast conditions, the drivetrain will keep the engine spinning and it does not need fuel.

2. OBC mileage valves can and at times are way off. Not sure how to adjust or tweak them, but it may be possible. I would not waste a lot of effort on this as you can and should manually calculate fuel economy.

3. Not sure, but these cars were likely tested with straight gasoline and not 10% Ethanol blend fuel for fuel economy ratings??? If they were tested with non Ethanol blend fuel then the economy figures would be higher than you are expecting.

4. Given the age of the car, things like MAF, O2 sensors, DISA, VANOS, carbon build up, engine wear, restricted/clogged exhaust and fuel delivery can all work against performance and fuel economy. Something even a simple as a fan clutch that is not slipping as expected, an exhaust flap that is not opening, a auto trans torque converter clutch not working properly, or a different oil blend could lead to reduced fuel economy. Some of the items are cost effective to address, some are not.

5. Given you have a SULEV car, although a lot of things are covered via the SULEV warranty, the warranty does not really cover component degradation. So you will need to make a decision on what you are willing to replace on your own. BMW really only will cover "failed" parts and I would expect you would need a O2 sensor code for like a failed heater or an issue where the car could not pass emissions due to something being WAY out of the norm before BMW picks up the tab.

6. Fuel, I can go on about this for a long time. Readers Digest version.

a. Ethanol percentage and mix can and often is very sloppy. You can buy Ethanol test kits for relatively cheap, however, not sure how accurate they can be, but at least you can compare fuel brands as well as see how consistent the fuel you are buying actually is.

b. Search the term Ethanol Phase Separation. This will open your eyes a bit!

c. When trying to deal with fuel economy issues, there are a lot of variables. An easy variable to change is fuel station/brand to see what happens. One thing to also consider is when was the Bureau Of Weights & Measures Test/Calibration was last performed on the pump(s) you are using. Also what time of the years was the test performed. Most states require annual pump testing/calibrations. Usually there will be a test/calibration sticker will need to be displayed on the pump face. Pay attention to this and the date as well as the time of year the pump was tested/calibrated.

7. Ground temperature of fuel in storage tank. Gasoline expands and contracts with temperature. When you measure by volume, this becomes very important. Not sure if any of the newer pumps are temperature compensated, but I know in Canada the fuel is actually heated before it hits the pumps as the fuel suppliers tend to get short changed with temperature variations. On the flip side, during warmer weather, fuel will expand and you will be short changed by volume and the supplier will make out. Is this enough for you to really notice?? But the fuel suppliers do!

8. Pay careful attention to tire inflation and tire wear ratings. Higher wear ratings likely mean harder compounds and likely lower rolling resistance??

9. Pay careful attention to things like brake caliper maintenance and parking brake adjustment. Sticky calipers, hard and work caliper mounting bushings, parking brake that may not fully release may cause a bit of extra drag that could impact fuel economy.

Many of the things mentioned above may only impact fuel economy in a VERY small way, however, if you end up addressing many small items, you could see a larger improvement in fuel economy.

Some things like O2 and MAF sensors are maintenance items. They do not last forever and you may not get them replaced under warranty so you may have to pony up the cash to replace them on your own dime.

Getting some form of scan tool or software that can read and graph real time data and pay careful attention to engine temp, fuel trims, O2 sensor performance can be a big place to start.

See this thread about software and scan tools that may be informative -

Also suggest you check out some of the smart phone apps that will help you easily log and even graph fuel economy and will allow notes on things like station, pump number, cost, date & time. The may be helpful in tracking things as changes are made to the car.

All I can say is my E39 M5 with a steeper rear gearing that causes the engine RPM to be a bit higher than stock will get 23 MPG during the summer if I say below 70 MPH. Not sure what a 325 SULEV automatic should get, but I think much above where you are currently may be hard to obtain, but it may be possible.

Just do not expect BMW to pick up much of the tab for your improvements.

Good luck.
Solve your misfires, lean codes, rough idle -

Fuel pump failures -

Temp Info -

Hidden OBC Menu - Check Voltage, Temp, Fuel Level -

E46/E39 GM5 Door Lock Info -

Lower hose temp switch O-ring - BMW #13621743299
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