So when i first bought my car in january, i was originally filling up and getting close to 400 miles per tank. Recently i have been getting about 300 miles or so (i drive like a normal citizen). I thought that it might be a one time occurance but it turns out that its been happening everytime now!! I have already tried changing the fuel filter, spark plugs (performed myself), both O2 sensors, and air filter. What else would possibly affect my gas mileage? low oil? no air flow?


i just put $40 of gas in my car and i got a little less than 3/4 a tank. But 130 miles later, i only have 1/4 a tank left. wtf?

someone please help. thanks.

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what's your mileage? could be pre-Cat O2 sensors. I'd get the ECU read for any codes

Well, lucky you, NO VANOS...LOL!

+1 on the Pre-Cat O2's (how many miles on 'em?) but it could be a number of other issues i.e. clogged cats, bad injectors, faulty DME, bad MAF, vacuum leak, etc.. More details please...

i have the same issue... sometime i park the car overnight with 1/4 of a tank and in the morning its close to empty.. i swear it goes from like 80 miles to 40 in minutes of driving...
i might have a vacuum leak cuase i do hear a whinning noise when i accelerate but other than that i have no clue what to do .. its a 525i and im gettin close to 13mpg

i have the same issue... sometime i park the car overnight with 1/4 of a tank and in the morning its close to empty.. i swear it goes from like 80 miles to 40 in minutes of driving...
i might have a vacuum leak cuase i do hear a whinning noise when i accelerate but other than that i have no clue what to do .. its a 525i and im gettin close to 13mpg
WHAT?!? A 525 gettin 13mpg! Dude, you have a performance issue with your bimmer. My 540 is gettin 16mpg in the city MIN with spirited driving. My recent average is: 21mpg with an average speed of around 40mph so you have a project to tackle. :devillook

Try a performance tuneup. New filters, plugs, clean MAF, etc. and tell us if you are seeing improved mpg. Also remember to check on the VANOS issue. I read that BMW updated the software to prevent codes from being thrown regarding the VANOS problem (set the ECU to ignore and compensate :shifty). One of the major symptoms is reduced fuel economy. Otherwise you might as well get a V8 or V12 'cause that's the same mpg I was gettin in the city with my 750iL! :eyecrazy

EDIT: Have you already posted this problem here? This is THE place to have your problems diagnosed you know...:redspot

How does one fix a vacuum leak?

How does one take out the MAF for cleaning?

How does one fix a vacuum leak?

How does one take out the MAF for cleaning?

you need to look at the vaccum lines and see if they have any trips/tears/breaks in them, over time the rubber becomes brittle and brakes

It it was a vacum leak, the car would be sputtering most likely, I would say the o2s are going, going, gone

Read on the MAF cleaning here: http://forums.bimmerforums.com/forum/showthread.php?t=833346

Courtesy of 12CoolDude...

forgot to mention, i already changed the O2 sensors! both of them!

how much is it to replace the o2 sensors? i feel like the tuneup of my car will run me a alot of money that i rahter put towards a 540:(
then id feel any money i put into it would be worth it... the 540 that is..

The biggest thing is that it is running rich, so if it isn't your o2s then it probably has to do with your pcv, probably clogged, my hoses went out on me, the car ran like shite and I immediately got it to my mechanic. You can also clean out your maf

Have you tried a carbon cleanout yet? Could be a buildup of soft carbon on the injectors and intake valves (and everywhere else). There is a procedure that will flush all that "cholesterol" from your motor. Some people endorse a product called "Seafoam" but I've read that MotorVac CarbonClean is a much more effective procedure. The "Seafoam" can be purchased at your local auto parts store for around $4.50 (varying prices and success) and the CarbonClean is done at your local indy for about $99 (with 100% success). Your call...GL!

Have you tried a carbon cleanout yet? Could be a buildup of soft carbon on the injectors and intake valves (and everywhere else). There is a procedure that will flush all that "cholesterol" from your motor. Some people endorse a product called "Seafoam" but I've read that MotorVac CarbonClean is a much more effective procedure. The "Seafoam" can be purchased at your local auto parts store for around $4.50 (varying prices and success) and the CarbonClean is done at your local indy for about $99 (with 100% success). Your call...GL!


ive bought the lubro moly tune up kit from bav auto which includes 6 bottles jectron, 6 bottles of ventil sauber, 1 bottle of motor clean, and 1 bottle of motor protect. Im hoping this helps out a bit.

from everything that is mentioned so far, im starting to lean towards the problem either being the MAF or a vacuum leak. How exactly do i go about finding a vacuum leak? how much does a fix for a vacuum leak cost?? thanks for all the help.

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whats a PCV? i feel like my cars lost so much power (not that it had much to begin with)
also which are the vaccum hoses?

whats a PCV? i feel like my cars lost so much power (not that it had much to begin with)
also which are the vaccum hoses?
PCV=Positive Crankcase Ventilation and it is part of the EGR (Exhaust Gas Recirculation) system, which is better known as your Emissions Control. It is a one-way valve that ensures continual evacuation of gases from inside a gasoline (http://en.wikipedia.org/wiki/Gasoline) internal combustion (http://en.wikipedia.org/wiki/Internal_combustion) engine's crankcase (http://en.wikipedia.org/wiki/Crankcase).

As an engine runs, gases from the cylinders leak past the piston (http://en.wikipedia.org/wiki/Piston)'s sealing rings into the crankcase (containing the crankshaft (http://en.wikipedia.org/wiki/Crankshaft) and other parts). This leaked gas is sometimes referred to as "blow by" because the pressure within the cylinders "blows" them "by" the piston rings. These gases include compounds harmful to an engine, particularly hydrocarbons (http://en.wikipedia.org/wiki/Hydrocarbons) (unburned fuel), as well as carbon dioxide and water vapor (http://en.wikipedia.org/wiki/Water_vapor). If allowed to remain in the crankcase, or become too concentrated, the harmful compounds will condense out of the air within the crankcase and form corrosive acids and sludge (http://en.wikipedia.org/wiki/Oil_sludge) on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present. :buttrock

nice, very detailed... so basically what should i do to the PVC as preventive maintenance? replace it completely??

Clean it out there is a write up about cleaning it with intake/brake cleaner.

Search for it.