[Kurt-06-325i]

I have had my '06 325i for two years now, I have put about 35,000 miles on it and always average about 28 mpg. In the last 6 weeks mileage has dropped to about 25 mpg and staying steadily there.
No changes in my driving. No codes or warnings. Car runs as good as ever. Currently at about 117,000 miles. I am going to replace the air filter but struggling to think of anything else.

Any thoughts about the longevity of O2 sensors? Just thinking they can change performance before getting far enough out of range?

Thanks,

Kurt

[MarkkyyMan]

Originally Posted by Kurt-06-325i

I have had my '06 325i for two years now, I have put about 35,000 miles on it and always average about 28 mpg. In the last 6 weeks mileage has dropped to about 25 mpg and staying steadily there.
No changes in my driving. No codes or warnings. Car runs as good as ever. Currently at about 117,000 miles. I am going to replace the air filter but struggling to think of anything else.

Any thoughts about the longevity of O2 sensors? Just thinking they can change performance before getting far enough out of range?

Thanks,

Kurt

I'd consider picking up a fresh set of 02 sensors.

Ever replaced the plugs?

[Kurt-06-325i]

Yes, plugs were replaced about 10,000 miles ago.

Thanks, I think I will get the sensors.

[Taskmaster]

Originally Posted by Kurt-06 -325i

I have had my '06 325i for two years now, I have put about 35,000 miles on it and always average about 28 mpg. In the last 6 weeks mileage has dropped to about 25 mpg and staying steadily there.
No changes in my driving. No codes or warnings. Car runs as good as ever. Currently at about 117,000 miles. I am going to replace the air filter but struggling to think of anything else.

Any thoughts about the longevity of O2 sensors? Just thinking they can change performance before getting far enough out of range?

Thanks,

Kurt

Oil changed? Air filter clean? Tires inflated? Cleaned the MAF? Clean the Vanos solenoid? You know we are in winter, the gas formula changes causing lower fuel economy, right?

[Kurt-06-325i]

I will check the MAF and clean it. You are correct about winter conditions. I did not see this change last year or the year before.

Thanks,

Kurt

[631twentyeighteye]

That's a small enough drop that I think bad/winter fuel would account for it. Do you always fill at the same station?

If not that then o2 sensors would be my first guess. Use a scanner to see what your o2 trims are, it's easy to figure out which one if any is wonky this way.

[mpaone23]

Originally Posted by Kurt-06-325i

I have had my '06 325i for two years now, I have put about 35,000 miles on it and always average about 28 mpg. In the last 6 weeks mileage has dropped to about 25 mpg and staying steadily there.
No changes in my driving. No codes or warnings. Car runs as good as ever. Currently at about 117,000 miles. I am going to replace the air filter but struggling to think of anything else.

Any thoughts about the longevity of O2 sensors? Just thinking they can change performance before getting far enough out of range?

Thanks,

Kurt

Well I must be missing something, cause my 06 325xi is averaging about 19mpg...

[rick100]

Originally Posted by mpaone23

Well I must be missing something, cause my 06 325xi is averaging about 19mpg...

too many variables. Do you drive mostly city ? highway only? automatic? manual? rwd? awd? etc etc

I bet even if the OP and you live in the same house and work at the same place , you ll still get different mpg.

[fravel]

Just a thought, but I've noticed a considerable bump in efficiency since changing my thermostat and putting in fresh coolant. I know the coolant temperature is variable to maximize efficiency and I was also running a diluted coolant mixture due to having to fill up with distilled water on a roadtrip last December.

At the same time, I've noticed that my coolant temperature seems to average 5-7* higher since the change. I'm wondering if the dying thermostat + the reduced effectiveness of my diluted coolant didn't combine to shave efficiency. I didn't get any codes until an SES popped up just a few weeks ago indicating the dead thermostat.

[Kurt-06-325i]

Thanks all who have been writing...

My mpg indicator was "28.3 or 28.6" for almost two years, then dropped to "25.3". I drive about 75% highway and 25% city, my driving patterns are pretty constant. I confirmed actual mileage has dropped, not just the indicator.

I replaced the water pump and thermostat a year ago, no notable changes from that.

I always burn premium fuel but nothe always at the same place. I stay away from the bargain places.

I will make a few checks and let you know what I am finding.

I appreciate your comments.

Thanks!

Kurt

[Kurt-06-325i]

Replaced air filter today and cleaned maf sensor. I'll let you know how its going.

[thomas88162]

Winter air and fuel quality drops MPG... 02 sensors are either working or not; them switching voltage is what is important. Don't waste your money there unless they are getting lazy. This would result in a diagnostic code.

[mpaone23]

Originally Posted by rick100

too many variables. Do you drive mostly city ? highway only? automatic? manual? rwd? awd? etc etc

I bet even if the OP and you live in the same house and work at the same place , you ll still get different mpg.

I wouldnt expect it to be the same, but at least close. I am about 60/40 city/highway, automatic, and awd. Changed oil, plugs, coils, air filter, and cleaned my MAF all relatively recent.

[TittoBlunt]

Try running a fuel injector cleaner like Chevron Techron. I do this once a year to make sure the injectors are clean. Last year I tried out BG44K and noticed an increase in fuel economy. Will be using it again this year.

[Three_thirty_I]

I would check the hidden cluster menu and check that the coolant temperatures are normal and that the engine is not running over cooled. Thermostat could be faulty and there won't be an error code, and this can affect fuel consumption of course.

Lots of good advice above, hopefully you can sort this out.

[Kurt-06-325i]

I ran fuel injector cleaner, replaced air filter and cleaned MAF sensor. Ran full tank of gas then checked next tank. Actual mileage was 27 mag which is good. This is close to where I was before, I will accept cold weather as the culprit now.

The mileage calculator seems to indicate lower than before. For example, it used to reat 28.3 mpg for my regular driving. Actual was 27 - 28 mpg. Now it reads 24.5 - 25 mpg with 27 mpg actual mileage... very strange.

I will go to the hidden cluster and look at coolant temp.

I hope this is a good reference for others with similar concerns.

Thanks to all!

Kurt

[dragonsi]

Originally Posted by 631twentyeighteye

That's a small enough drop that I think bad/winter fuel would account for it. Do you always fill at the same station?

If not that then o2 sensors would be my first guess. Use a scanner to see what your o2 trims are, it's easy to figure out which one if any is wonky this way.

This or check your tire pressure

[chris82]

My mileage has been horrendous lately. On long highway trips (1,2,3 hours) I'd consistentlyaverage 29 mpg, not even taking it easy either or driving slow. My trip to work is 20 miles each way. I'll occasionally go out to lunch and make some other trips during the week. But my last fill up I put 13.5 gallons in and my mileage was 263. Idk if it's the winter tires, or what the hell is going on, but I'm getting killed.

[torkles]

Did they move to an ethanol blend in your area?

[cdgatti]

that modest a decrease is definitely to be expected with the very frigid temps you've been getting in IN lately, someone already asked about tire pressure but remember you lose about 1 psi per month through natural permeation and 1 psi for every 10F.

[Zernaff]

I too have noticed a drop in fuel economy for the last 2 months or so. I used to get 28mpg consistently (29 indicated) but now I'm getting 23-24.5 (with indicator showing 19-20). Very puzzling. A full 16 gallons might get me 350 miles.

I got the 24.5 on shell gasoline and 23 with quiktrip (Multiple tanks).

Last winter I got more like 26 mpg and mine dropped to 24 this when it was still warm out (80-90 F).

I am also in a 325i.

So, injector cleaner?

Edit: Driving home today, fan stayed on and coolant temp stayed around 80 Celsius. So, could MAF sensor be causing this? Im thinking the coolant temp being low is causing my poor fuel economy.

[thomas88162]

Originally Posted by Zernaff

I too have noticed a drop in fuel economy for the last 2 months or so. I used to get 28mpg consistently (29 indicated) but now I'm getting 23-24.5 (with indicator showing 19-20). Very puzzling. A full 16 gallons might get me 350 miles.

I got the 24.5 on shell gasoline and 23 with quiktrip (Multiple tanks).

Last winter I got more like 26 mpg and mine dropped to 24 this when it was still warm out (80-90 F).

I am also in a 325i.

So, injector cleaner?

Edit: Driving home today, fan stayed on and coolant temp stayed around 80 Celsius. So, could MAF sensor be causing this? Im thinking the coolant temp being low is causing my poor fuel economy.

Sounds like either a thermostat fault or a limp mode situation... It is hard to say without diagnostic data.