Only 20-23mpg in a 318i M40?

[evilnickwong]

So my DD M40 only gets me about 20-24mpg.

Some info:
- My daily commute to work is about 4-5 miles and 15-20 minutes (slow traffic).
- My typical journeys between home and destination isn't usually further than 5-10 miles.
- I don't have any idle issues, and engine has about 180k miles on it, but still drives fine and redlines without fuss.
- Engine runs on Magnatec 10w-40, changed every 4k miles or so.
- Fuel filter and air filter changed recently, timing belt as well.


I see most M40s and M42s on forums getting somewhat like 30-35mpg? What should I check that might be causing this?

Only thing I can think of is that instead of a 88deg thermostat, I think I'm using a 81 or lower, because my temp only goes to 1/4 max instead of 1/2. But does that make such a huge diff to gas mileage?
What other things should I check besides that?

[M-technik-3]

Is your car fully loaded, Timing Belt? M40s have a chain no belts there. Plugs wires and so on. excessively wide tires? Many factors for determining poor mileage but most are common sense.

[evilnickwong]

M40 uses a timing belt, not a chain.
My tires are 205/45/16, do not think those are wide, but my replica 16x7.5 throwing stars might be a tad heavy. Forgot to mention that.

Boot and rear seat are mostly empty all the time, save for the spare wheel and jack.

[M-technik-3]

M40 uses a timing belt, not a chain.
My tires are 205/45/16, do not think those are wide, but my replica 16x7.5 throwing stars might be a tad heavy. Forgot to mention that.

Boot and rear seat are mostly empty all the time, save for the spare wheel and jack.

Stand corrected, Thought they were chain as well.

They are not known for being all that quick or powerful so the additional rotational mass doesn't help. Is you Cat clogged or does it not have one? O2 sensor?

Do you drive smoothly or jam on the gas when traffic starts to move. I drive like a putz in my Vert so it gets decent mileage. No jerky starts and ease off the gas when I see a light changing to red ahead. People here are impatient and go around you to get one car ahead. To me that wastes to much gas.

I also just switched out the bastardized exhaust that was in my car. It had a standard E30 325e exhaust on an 325i car and it was having trouble breathing. The cat was totally clogged.

[evilnickwong]

As far as I can see, my cat is the original unit, but not sure how to check if it's clogged. I don't hear any funny noises when I thump on it with my hand. Exhaust backbox is a fairly new-ish Ansa unit.

Is there a way to check the O2 sensor? I don't know when it was last changed, I've owned the car for 3 years+ and I haven't changed it in that time.

Other than that, yeah in normal traffic, I do try to drive smoothly, anticipate slowdowns, etc etc. :)

[evilnickwong]

Anyone? Do O2 sensors need to be changed at certain intervals like other wear items? Or any other suggestions?

[Massive Lee]

Bad mileage is often caused by a non-working O2 sensor.

Lee

[Quadrajet]

My M42 only gets 24-26 on my daily commute/weekly use. But on the Freeway on a trip to Portland earlier this month I got 32.

I only say this because your freeway miles may be quite a bit more than your commute miles, from your driving description. I travel about the same amount of miles you do, but I only reach 55 for like 1/2 a mile, and my car isn't really warm till about 1/2 way to work, if I don't warm it up first. But like posted above it could be your O2 sensor.

[evilnickwong]

Hmmm OK thanks guys, probably will have to look at that then since I'm sure it hasn't been changed in years already. Will probably swap the thermostat to the right one on my next oil service too.

[SLUCK]

buy a prius if you want good gas mileage. e30s are for fun