reasons for poor gas mileage - United Bimmer Community

**cruzin 323is** · 04-18-2005, 03:29 PM

can some 1 help me out.. i mean i get about 180 miles to the tank!! and its all city!! thats a horrible number.. and i don't even drive hard!!

on a normal in traffic take off, the mpg needle stays hidden untill about 3rd gear.. btw, its a automatic

**jrhaile** · 04-18-2005, 03:53 PM

Not good man. First cause is poor air intake. Check your air intake filter and make sure it's not dirty. I've also seen failing MAF (Mass air flow) sensor's cause this. A bad or dying cat converter(s) is another. Dirty fuel filter is another cause. Also take it to autozone or something and have them see if there are any codes. How's the car's performance?

I drive a lot of city and if that's all I do, I get around 300-330 Miles per tank and if I do a lot of highway it's around 370-410 Miles Per tank.

**bitcore** · 04-18-2005, 06:14 PM

I dont think a dirty fuel filter will cause bad fuel mileage, if anything, cause you to run lean (even though the entire system is regulated and computer controlled) Decrease in proformance, yah.. But, you did an engine swap so you probably know more than me.

Everything he said, plus if you buy crappy gas, you will find yourself pushing the accelerator down further to get the same power effect that you use to get with better fuel. Try other gas stations, run the tank to almost try and then put 15$ (like, 5 galons) in the tank from one station, drive around for a while and see how that proformance is, then try another station.

It really depends on what distributer the particular station buys it's gas from, and you never can tell. Sometime a station will have crap gas, sometimes it has great and powerful gas. And no, I'm not getting a placebo effect.

Pour a dose of fuel injector cleaner with about 1/4 of a tank left (so the concentration is higher) and on the next tank, fill up the tank with fuel (don't buy X dollars, fill the tank) This may help if you have not done this in the last 30,000 miles.

**TMac325is** · 04-18-2005, 06:34 PM

eeeekk and i thought my gas mileage sucked, im running about 17 mpg, up to like 25 mpg if its all highway driving

**Addicted?** · 04-18-2005, 06:35 PM

i would take it to a well known german mechanic and tell them exactly what you told me, i bet they can pretty much pin point the problem in a few minutes and they wont charge ya... You should really check your air filter though, When i drove my 318 mine was filthy and my car could barely breath... i changed it and mpg went up a good amount. I have also heard it could be the cat

David

**97alpM3** · 04-18-2005, 07:05 PM

Aight.... a few words on gas mileage..... lets think here... hmmm

First thing that comes to mind.... O2 sensors.... the entire O2 sensor system... I've seen more than one car... absolutely suck the gas because of a faulty O2 sensor... weather or not it has failed might be the problem... if it hasnt exactly failed yet.. telling the OBD to give you a CEL then it might be giving you a bad signal witch will lead to a bad fuel ratio... check into that... you can test them out... with a voltmeter

Second thing... which IMO goes back to just good car maintenance.... fuel filters... air filters and oil filters.... make sure they are upto date and within a change cycle... BTW... if your using shitty oil... then your probably getting under par results... friction causes slower movement with the same amount of power put forth to the same part. Physics...

Another main contributor of bad gas mileage is spark plugs.. using the right ones.. and spark at the right interval for BMW's crazy distributor-less ignition system... is rather important... a degraded plug will do some work on gas and power alone.

Another fault path would be to check your injectors... if old and rather crusty... they are done.. and need to be replaced...

Just a few to note of.... and to add to the topic...

Peace,

Les

**Rooz** · 04-18-2005, 07:10 PM

what's your average mpg? Mine's 19.4 and I drive pretty hard

**bitcore** · 04-18-2005, 07:57 PM

Les has some excellent points.

**jrhaile** · 04-18-2005, 08:02 PM

Les is the man. Something I'm wondering though. I wonder if your gauge isn't faulty because that's reallly low! At what point and how many gallons do you put in on average?

**jrhaile** · 04-18-2005, 08:06 PM

Hey I saw in another thread you just sold your CAI. When did your poor gas mileage start? So I'm assuming you're using your stock airbox again. Did you EVER change the filter and did you hook it up right so that it has air coming in properly with that little ugly-molded plastic piece?

drz · 04-18-2005, 08:53 PM

The gentlemen above have pretty much covered it. David's suggestion is perhpas the first thing you should do. Don't mess with anything, just take it to a German car specialist to get it checked out (aka, getting a free estimate). Then come tell us what they tell you -- to see if it is a DIY job (very likely will be). Man, you are getting pretty bad gas mileage. I get about 25mpg evenly distributing my travel between highway and city.

**cruzin 323is** · 04-18-2005, 09:21 PM

i change oil filter everytime i get an oil change.. and i just changed the air filter 2 months ago...

so ya.. i guess i'll get it checked out...

thanks for the imput

**jrhaile** · 04-18-2005, 09:24 PM

Read my other post about your gauge? Let me know what the outcome is. I'd like to make a sticky/FAQ about this problem.

**cruzin 323is** · 04-18-2005, 09:27 PM

ya... i took the intake off a while ago.. but i just sold it...

i changed the air filter as soon as i put the air box back in..

but its been like this since i got the car over a year and a half ago..

and, 2 months ago i changed the air filter

**jrhaile** · 04-18-2005, 09:28 PM

oh okay. Then spark plugs/injectors and or Cat is what I would guess. Good luck man.