Poor gas mileage

[89Bimmer89]

I am getting poor gas mileage, like 15-18 mpg in town and 20-22 mpg on the highway. My car is stock, so what could be problems causing poor gas mileage. I could not really find anything searching. BTW I have about 120,000 miles and have recently done a tune up with new spark plugs, air filter, fuel filter, etc. When I filled the gas tank in the summer the range on the obc said close to 300. Now after I fill the tank it reads 230. Is it the temperature outside?

[Daved]

I know some guys are used to very high mileages but 15-18mpg in the city is not bad at all, I usually get 16 and my engine is .5L smaller.

[89Bimmer89]

Back in the summer my obc gave me a range of 300+ miles on a full tank and now it is closer to 230 though. I am going to run some injector cleaner through the engine and see if anything changes.

[radcc]

could be the o2 sensor. My milage dropped and I switched the o2 sensor out and it seems to be back to normal. I think when the o2 sensor goes the computer uses an extra rich a/f ratio so your milage suffers.

[lbraasch]

MAF on the way out could also be a culprit. same reason as stated above.

[Balthazarr]

Pull the sensor and clean it.
See if that makes a difference before throwing money at this.

[WytLytnyn]

Fuel filter? Air filter? I also agree about cleaning the O2 sensors and checking the Air flow meter. When was the last time the car had a tune up?

[cgrow]

Is the temperature gauge reading normal? Could be a bad T-stat causing the car to run rich.

[Johno]

Cold weather makes an impact.

However, at 120K, like you, I just broke down and bought 4 O2 sensors this morning. I am seeing 17 mpg after adding a TS supercharger, despite being told it should not decrease fuel economy. I suspect my O2 sensors are a culprit, and I've already driven 20K beyond their replacement interval.

Cleaning a 120K mile part seems like a bad compromise, considering you are doing almost the same work to remove and clean as to remove and replace. Plus when you replace there's less doubt whether the exercise worked.

If you use injector cleaner, get Techron. It is a really legitimate product. If nothing has changed since I worked at the Oronite PR firm, Techron is the retail version of "oronite," which Chevron sells to other fuel makers to add to the gas for its detergent effects. You can read up on it here: http://www.chevron.com/products/oronite/

[M13]

New plugs and Techron fuel injector cleaner brought me up to 350/tank; before I was under 300.

[pdub]

For seasonal variations in fuel economy, think about changes in fuel. I don't know if ALL states do, but many will add ethanol to the gas to help with CO emissions in winter months. It will drop your power some, as well as fuel economy. I don't know realistically how much it would affect economy, but it's an idea...

[Johno]

When I had a mitsubishi eclipse turbo, fuel economy dropped a lot in winter. The dealer tech had a newspaper article clipedd and on display so many customers were bitching.

I have never noticed seasonal variations in the M3. If mileage seems down, trust your feelings Luke.

In my case, my horespower just jumped about 150 percent, and now I'm trying to figure out if 17.5 mpg is because of new twin-screww driving, bad 02 sensors, or what have you. What's complicating is that I never measured before with anything but the OBC. Now it says I get 50 MPG, so it's even less accurate than before.

If I could do it all over I would track milage on paper at every fillup or so. A drop in fuel economy is one of the easiest trouble indicators....

[fiveightandten]

2 simple things to check out:

-Frozen brake caliper: Jack each end up real quick and make sure none of the brakes are dragging. That will kill your mileage in a hurry

-Tire pressure: it will drop quite a bit when the ambient temp plummets.

Like mentioned above, O2 sensor could definitely do it as well.


-Nick

[89Bimmer89]

I have not checked the tire pressure yet, but have been meaning to.

My calipers should be good as they are pretty new. Might check them later anyway.

I think I am due for O2 sensors, but are they not supposed to throw a code when bad?

I need to change the fuel filter too I suppose.

[spitvenom]

didnt read the thread but

tire pressures
good maintanence, make sure fuel filter, air filer etc keep on top of maintanence.

also my car waste more gas if I use the cheap stuff make sure you run good gas..

make sure your o2 sensors are good!