'97 328iC - Only getting 14-18mpg: Why?

[rasher]

I've been doing a bunch of reading but there's just so much out there...
I have a '97 328iC with 72,000 miles. I bought it last summer and was immediately shocked at the poor mileage. Granted, I do 90% of my driving in stop-and-go city fashion, and granted, this car is super fun to drive, so I probably have a bit of a lead foot. But really, 14-18mpg is pretty bad, isn't it?

Now, I have never had a Check Engine Light come on. I also have never had any pressing problems from the engine/drivetrain. Everything "feels" normal to me, meaning no rough idle, no starting problems.

However: Yesterday, I noticed when making a right turn into traffic that the car didn't accelerate as fast as I'd like. And then I realized that this happens somewhat frequently (but not ALL of the time) and I've always just attributed it to my right foot just not digging into the pedal enough. BUT, I just started reading the "Bogging From Idle" uber-thread and now I'm starting to think I've got some of these symptoms.

Inspection II was done at 68,520 miles (4,000 miles ago). So I have new: Spark Plugs, Fuel Filter, Air Filter, Oil and oil filter (changed 300 miles ago), and they ran through some "Fuel Injection Cleaner".

What should I start with first? O2 Sensors? Mine is an OBDII car, so that means I have 4 sensors, right? Should I change all four, or if not, which ones?

What else causes really bad gas mileage?

[HarryB]

Maybe something as simple as O2 sensors. Just get get your bimmer scanned for codes.

[kyle242gt]

O2 sensors, another vote - supposed to be swapped at 60K I think?

Is your mpg actual factual (gallons to fill after X miles?) or from the OBC?

If you're spending a lot of time idling and winding it out in first and second, your mpg may indeed suffer.

[UltimateDrivr]

maybe you need to go on a diet...lol

but seriously....city/stop and go driving + spirted driving = bad fuel milage

[Fly N Drive]

I get about 23 while in town in my 97' 328i. I get about 28 to 30 on the highway, depending on how spirited I treat the onramp ;)

Have you thought about running something through the injectors to clean those out? I recommend the fuel additive by Techron (comes in a black bottle). I don't swear by these, and they certainly aren't a cure all, but will help with the injectors and maybe give you some better spray into the cylinder.

My other guess if for the O2 sensors. Make sure all the codes are cleared out of the computer. Usually when you have something wrong, the engine runs rich to avoid further problems.

Let us know and anyone can correct me if I'm wrong. If you want the exact name of that Techron cleaner, PM me and I'll shoot it over. I'm at work right now and can't remember the name.

[motoracr22]

Have you thought about running something through the injectors to clean those out? I recommend the fuel additive by Techron (comes in a black bottle). I remember the name.
IIRC. They have 4 bottles for 7-8 bucks at Costco, at least in the PNW.

[rasher]

Well, they ran some injector cleaner through at Inspection II, so I don't know about that.
Right now, my fuel light is on and my odometer reads 185 miles. I bet if I fill it up right now, I'd put between 13 and 14.5 gallons in. That means that I'd have gotten between 12.75 and 14.20 mpg. That's insanely low, even with a lead foot. I'm using 91 octane as well.
Do I have to go to the mechanic and/or stealer to read engine codes, or would a parts store (like AutoZone) have a reader?
Next, if I start replacing O2 sensors, which ones do I replace? All of 'em? Jesus, they're $100 apeice and I have 4! Wifey's going to be P.O.'d let me tell you...
Let me also reiterate: Aside from the crappy mileage I'm getting, I wouldn't have any idea there's anything wrong, engine idles smooth and revs fine.

[camiloteram]

is it an automatic?

[rasher]

Yes, automatic.

[tomt30273]

Yes, automatic.

Then that's about right. stop-n-go, redlights, traffic jams. Rarely makes it into overdrive. 14-18. Put it on the road, 24-28, sometimes more. 3.91 rear diff and no overdrive= low mileage.

[motoracr22]

Try just replacing the 2 pre cat 02's. The rear's have nothing to do with mixture, they only check to see that the cat is operating.

[rasher]

Well, I just got back from the gas station. Boo hoo!
15.3 gallons
201.4 miles
13.16 MPG
That's worse than my pickup truck...

[UltimateDrivr]

Well, I just got back from the gas station. Boo hoo!
15.3 gallons
201.4 miles
13.16 MPG
That's worse than my pickup truck...

poor guy, thats horrible, i get twice that (not trying to rub it in)

[rasher]

OK, so I've got a '97. Where are my Pre-Cat O2 sensors? In the exhaust pipe or in the manifold? Next, anybody want to take a guess at the P/N, RealOEM.com is a bit confusing.
Last: Where's the best price for these little beauties? ($109.95 each at bavauto.com. Ouch!)

[G. P. Burdell]

Each pre-cat O2 sensor screws into its respective exhaust manifold just downstream of where the three pipes merge into one. Scan for engine fault codes before you start throwing $100+ sensors at the problem.

Mileages in the teens are not at all unreasonable for a 328i if you do lots of stop-and-go driving. Go for a long highway drive and see if your mileage improves.

Also, low tire pressures can reduce your fuel economy.

[E36Guy]

Well, I just got back from the gas station. Boo hoo!
15.3 gallons
201.4 miles
13.16 MPG
That's worse than my pickup truck...
:eek: Yeah, something is definately up with your car.. it shouldnt be that bad. Thats what my Chevy 1500 4X4 Z-71 (5.7L) used to get. :(

[amancuso]

If it is an O2 sensor, you would definately get a CEL with an ODBII car. So forget that.

[rasher]

Definitely no CEL, ever. And like I said, the way the car runs, there's no reason for me to think anything is wrong. It runs like a dream. EXCEPT that I'm only getting 13mpg.
Maybe it's my foot...

[rasher]

So now what?

[vr4boy]

i noticed my gas mileage also dropped a month ago and i had my emissions due.i had it tested(emissions) and the car failed-they hooked it up to they're scanner and read p0420(catalyst system efficiency below threshold-bank 1) and searching here, it was due to a faulty 02 sensors(2 up front) so i replaced those and all is well again-gas mileage was back to normal...funny thing is,my car never threw a cel even before the inspection...i bit the bullet and bought the sensors at bavauto...i still wonder if someone has them cheaper....car is a 96 328ic

[berr4880]

More easy things to check

Just out of curiosity have you recently had a tune up? Have you looked at your air filter? I drive pretty hard and still average over 23 mpg in the city.

[rasher]

A new Air Filter was installed at Inspection II about 4000 miles ago. By the way my tire pressure is fine, too.
Besides the Stealership, who is going to have a scanner to check my O2 sensors?

[rasher]

Also, has anyone tried the voltmeter diagnostic test described in Bentley?

[amancuso]

i was getting O2 sensor related codes but also had an intermittant CEL every few cycles it would shut off then come back on again. It turned out to be a broken air intake boot with a faulty MAF sensor.

[Daved]

I have been getting 12~13mpg in the last months...

I don't have O2 sensors so it can't be them, also the car runs well.

[Ronnaferd]

I'm going to college away from home so all my driving is in the city streets and I get HORRIBLE gas mileage. I like to spiritly drive every now and then but I get around 200miles on a full tank/15gal. Should I have this looked at? maybe I need to get new o2 sensors? :help

[labcruiser]

also vote on the o2 sensor. I heard people saying that sometimes they are bad but do not throw CEL codes.