[thatg]

Hey all just purchased my x3 a month ago from shift(online only dealer) and noticed some issues with the car when i picked it up. One being the idle was a bit rough on start the other that the gas mileage reader was giving me awful results. I was seeing 13mpg and noticed it continued to drop. I reset it and took it on the highway for approximately 40 miles. When i arrived to my destination i was seeing 22 from strictly highway driving but noticed after maybe 2 miles of city driving it had dropped to 13 almost instantaneously. If i leave the car idling for several minites i can literally watch the mpg count drop several decimals. I reset the mpg and drove around town a bit and was looking at 11mpg. I left the engine idling while i ran a fault code check for 5 minutes and the mopg counter dropped all the way to 6mpg before it gave up and stopped decreasing. I’m hoping(but not hoping) this is a bad sensor that reads fuel efficieny rather than an actual issue with the mileage itself. I’ll reset the mileage counter today and try to bring it down a quarter tank. I understand our tanks at 17.7 gallons so i’ll call a quarter 4 gallons conservitively.


Has anyone else experienced this poor reading for their mpg? Any sensors this could be related to? (Maf sensor?) thanks in advance for your input

[Opie55]

Your post is a bit unclear as to what you are reading. Not sure if a 2012 is exactly the same, but on a 2014 you can read mpg as instantaneous (both in the cluster as a number and as that little thing at the bottom of the tach), as a trip average (both as a number in the cluster and in the trip computer on the screen), and also as a bar graph over time in the iDrive. If you were reading averages as opposed to instantaneous, then the 40 miles of highway at 22 and 2 miles of city at 13 makes little sense. To drop an average of 22 over 40 miles to an average of 13 over only 2 more miles, you'd have to be getting pretty much zero MPG in the city. But as instantaneous readings, they are not too surprising. So clarify what you are looking at.

[Opie55]

And just to add regarding you comment that idling allows you to visually watch the mpg drop. That would be true if you recently reset it. If you reset the mpg to start over, and only drive a mile or two, then sit and wait for a train to pass, you can watch it drop because you have still only traveled 2 miles, but are still using gas. But if you drive 5,000 miles and never reset the mpg, then it would take a long time for idling to cause a drop because the gas you are using while idling gets averaged over all 5,000 miles. But your instantaneous reading while idling will be 0 mpg.

[oVeRdOsE.]

I have also a 35i, but Ive never really noticed the gas mileage, but I might remember seeing a 21.4 mpg. 100% city, no highway. Which is not that bad.

I own german cars, and maintenance stuff to do for gas mileage. :

- spark plug
- coil packs
- carbon buildup clean
- injector ultrasound cleaning
- air filter ( on the x3 its getting dirty very fast)
- BMW approved syntech oil to avoid sludge and wear. Also see if its not overfilled.
- pneumatic wategate , check for leaks and if its free moving. underboost or overboost will change the mpg.
- gas filter, but on the x3 its in the gas tank , so I guess ''a lifetime filter''.
- good gas 91 or more octane so the timing will remain constant.
- tires and air pressure.
- full charged battery and good quality battery to remove load on the alternator.

Also on a new used car, changing all fluids might not increase mpg, but can be great for the car. (transmission/coolant/brake/diff/transfer case/steering/engine oil)

[pokerface]

I would say go tank to tank then do a manual calculation of the MPG which you can then compare it to the computer. I would say that with more distance the readout will be less skewed, but the onboard computers tend to be too optimistic.

[oVeRdOsE.]

Originally Posted by pokerface

I would say go tank to tank then do a manual calculation of the MPG which you can then compare it to the computer. I would say that with more distance the readout will be less skewed, but the onboard computers tend to be too optimistic.

+1 , fuelly app will help OP a lot

[najnad]

As others have said, run a couple of full tanks of gas through it and calculate the mpg yourself and compare to the computer. If they mismatch, it could be a sensor problem. If they match and it is lower than what you expect, start doing maintenance the previous owner may not have accomplished...the list that overdose gave is good, but I would also check the brakes as dragging pads reduces efficiency as well.

Keep us updated

[thatg]

Originally Posted by Opie55

Your post is a bit unclear as to what you are reading. Not sure if a 2012 is exactly the same, but on a 2014 you can read mpg as instantaneous (both in the cluster as a number and as that little thing at the bottom of the tach), as a trip average (both as a number in the cluster and in the trip computer on the screen), and also as a bar graph over time in the iDrive. If you were reading averages as opposed to instantaneous, then the 40 miles of highway at 22 and 2 miles of city at 13 makes little sense. To drop an average of 22 over 40 miles to an average of 13 over only 2 more miles, you'd have to be getting pretty much zero MPG in the city. But as instantaneous readings, they are not too surprising. So clarify what you are looking at.

It’s definitely an average mpg its displaying amd not the current mpg. I did about 32 miles today and its showing 13.6 mpg. I climbed up to 18mpg when the miles were lower and i was on the highway for a bit and the miles after reset were low. But now that im at 30 something miles its in the 13.5 range but drops as I idle at a light then might increase a bit if im steady on the gas.

At 32 miles i’ve used almost a quarter tank. Which at 4 gallons a quarter tank puts me just over 8mpg, since i havent used the whole quarter tank. I’ll continue to drive a bit, but this is more of the same. It looks like the car is just getting horrendouse mileage for some reason right now. My v8 m3 gets 13.5 average and seems like im making less trips to the gas station with that.

[pungo]

I would be concerned about damaging components if the engine is running way too rich. Fuel washing oil off the cylinder walls, cat damage from raw fuel. An exhaust gas analyzer (sniffer) will tell for sure.

[MichiganMike]

I own two 2016 X3 35i. Over the past >5000 mi, each has averaged over 22mpg in mixed driving.

Fuelly.com reports:
"Based on data from 7 vehicles, 374 fuel-ups and 116,400 miles of driving, the 2012 BMW X3 35i gets a combined Avg MPG of 20.55 with a 0.19 MPG margin of error."
http://www.fuelly.com/car/bmw/x3/201...bmodel_id=1955

Your reported fuel economy appears much below average. As others have suggested, you might start with a full gas tank and manually calculate the fuel economy at the next fill-up to eliminate potential errors due to the on-board mpg calculation.

There are numerous service issues that might contribute to poor fuel economy, including:
a dirty air filter, bad oxygen sensors, malfunctioning fuel injectors, defective spark plugs or coils, low tire pressure or wrong motor oil.
https://www.carsdirect.com/car-buyin...ad-gas-mileage

[MichiganMike]

Excerpt from another article on likely causes of poor fuel economy:

Dirty Oxygen Sensor: An oxygen sensor measures exactly how rich or lean the exhaust gases are when they leave the combustion chamber. The data is used by the vehicle computer to adjust the amount of fuel entering the engine. If the sensor is dirty or failing it can lead to a drop in miles per gallon. It can also lead to failed emission tests and a rough idle.

Dirty Fuel Injectors: Fuel injectors spray fuel into the cylinders where it is mixed with air and ignited. Over time the fuel injector system can become clogged. This can lead to a drop in fuel efficiency as well as slow acceleration and the car not having enough power. If caught early, simply cleaning the injectors can solve the problem. As it progresses, the injectors may have to be replaced.

Bad or Dirty Spark Plugs: Spark plugs ignite the fuel in the combustion chamber. If they are dirty or working incorrectly, it can cause the engine to misfire, leading to poor engine performance. This can lead to a lack of power as well as a big decline in fuel efficiency.

Malfunctioning Mass Airflow Sensor: A mass airflow sensor detects the amount of air coming into the fuel injection system. It delivers that information to the vehicle's computer, which crunches the numbers and then delivers the proper amount of fuel to the air in the vehicle. A dirty airflow sensor will degrade fuel efficiently and lead to a rough idle and even stalling as the problem progresses.

Misaligned Tires: Tires that are low or out of alignment can lead to a drop in fuel efficiency. Checking the tire pressure and having the alignment checked on regular basis can help prevent this problem.

Defective Fuel Pump: Fuel delivery issues can drastically affect fuel efficiency. The fuel pump pulls fuel from the gas tank to the fuel injectors. The fuel pump can become clogged or malfunction. A bad fuel pump can lead to a rough running engine because it is not getting enough fuel. This will lead to a decline in gas mileage. If this issue is not addressed it will lead to a rough, idle, sputtering and stalling.

Clogged Fuel Filter: A clogged fuel filter will also cause problems with gas mileage. The fuel filter screens out contaminants in the fuel, and over time it will become clogged. Fuel filters need to be replaced periodically.

Stuck Brake Caliper: A stuck caliper on a disc brake or a stuck shoe on a drum brake can lead to a serious decline in gas mileage. In addition, any brake issue can make the vehicle dangerous to drive. If the decline in efficiency is accompanied by the vehicle pulling to one side, the brake system should be inspected immediately.

[malicem3]

Our N55 averaged about 19.8mpg in only city. 21.9 in mixed. 26.8+mpg highway.

[383vett]

As mentioned, fill up the tank, note the mileage. Drive a few hundred miles, refill and divide miles by the number of gallons. Forget the gas gauge and the computer readout.