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Tech Forum : What's up with my gas mileage?

- BMW E21 Community
   - Tech Forum
      - What's up with my gas mileage?
Daem   Posted Wednesday, Jun 25th 2:49am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 2

OK, USA
82 320i
First off, what kind of gas mileage do you all see? I don't know what's up with my car but it's eating gas like there's no tommorow. I would have thought for only being 1.8 Litres it would get MUCH better mileage than it is. It seems I'm putting $10 gas in it every other day. Is that normal?

My buddy has an '85 2.8 Litre Camaro that gets better gas mileage than my 1.8.

What could be causing this? Is there something I could check? Running too rich? I'm lost. =

Thanks in advance,
Daem

M42powered   Posted Wednesday, Jun 25th 3:27am [Edit] [Quote] [IMS] [View car]
Staff
Post nr. 62
   
Arizona, USA
1981 BMW 320i
Is there a gas smell anywhere? In the trunk perhaps? I'd assume at todays gas prices you're looking at 6 gallons a day? That's a little much for a car to burn unless you've got a Cutlass V8 under the hood. I'd look for gas leaks first. How far do you drive everyday?

Matt   Posted Wednesday, Jun 25th 5:01am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 62
   
SF Bay Area
81 320i
19 mpg

RDAvena   Posted Wednesday, Jun 25th 5:04am [Edit] [Quote] [IMS] [View car]
Staff
Post nr. 55
   
SELECT * FROM users WHERE clue > 0
1981 323i, 1988 545is, 1997 528i
Yep I am with Bic on the leaks. That and when was the last time you had the engine tuned?

With the 323i I get 345 miles per tank of gas (13 gal?) and that is with a buzzing 4 spd. I always check the tires and make sure the mixture is ok (a little rich please for the sake of not overheating).

The obvious would be leaks, tune up, tires(air pressure) , brakes(make sure they are not dragging), and bearings set at the proper torque.

Daem   Posted Wednesday, Jun 25th 5:52am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 3

OK, USA
82 320i
First, there's no gas smell...I don't think it's a leak.

Second, I was looking things over for anything obvious and noticed a green wire by the ignition coil hanging there. I started wondering where it was supposed to be pluged up to and found the other end of the wire down on the exhaust manifold. o2 sensor? Haven't had time to go anywhere tonight, maybe that could have been part of the problem if it was the o2 sensor?

I just had the brakes totaly redone...so they're perfect. No dragging.

As far as having it tuned. I've only had the car for a month or so now. So, no, it hasn't been tuned. I replaced the air filter, changed the oil, new cap/rotor/plugs. All your usual tuneup stuff but nothing in the way of the fuel delivery.

I did bring it in to a reputable shop and had them check over a few things and they said there was no need for them to waste my money and tune it.

Anyways, do I have to bring it to a shop to tune the mixture or is it fairly straight forward for a DIY'er? Also, where does one adjust the mixture at? A picture would be really helpfull.

If that was the oxygen sensor that was unpluged, couldn't that cause it to run overly rich?

Thanks,
Daem

m10_power   Posted Wednesday, Jun 25th 6:00am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 7


CIS (K-Jet) fuel injection system is great but when out of tune it produces horrible gas mileage, it is a rather simple system and straight forward to check out. If a mechanic tells you otherwise, find a new mechanic...

M42powered   Posted Wednesday, Jun 25th 7:57am [Edit] [Quote] [IMS] [View car]
Staff
Post nr. 64
   
Arizona, USA
1981 BMW 320i
That green wire is probably the 02 sensor. Is there a little rubber cap on the end of the wire? if so you've got the 02 sensor. If it was unplugged that means that the 02 sensor is probably bad. The cars run in a suedo closed loop when there is no 02 reading. Shops and stuff seem to unplug them rather than replace them. I'd replace the oxygen sensor and try that first. Might help things out.

You should be getting above 20 MPG... from the factory these cars did 25-28mpg city.

melloh   Posted Wednesday, Jun 25th 8:55am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 41
   
California, USA (San Jose)
1980 320is, 1991 318is
my car (1.8 L) is stock and i get abt 22 @ 75% city, 24 @ ~50% city, abt 27 almost all hwy. All stock, tuned yearly (by mechanic). Also, check that charcoal (black) canister under the battery tray. When my tray finally came off on its own (rusted through, 'forcing' me to move it to the rear) I found a 2" hole in it. Replaced it and got 1-2mpg increase (part of the reuptake system or something of the sort, catching and filtering excess gas fumes?). It had probably been like that for years, after at least 2 tune-ups and one smog check... still went unnoticed and forgotten.

If you have a haynes or similar, wouldn't hurt to go through it, using it as a guide. It would tell you pretty much what RDA and others told you. Also, some have found the breather hose to be a problem? What about fuel filter, fellas? I'm just thiniking out loud, someone else would have to confirm this.

Also, is $10 abt 6 gallons? Thats ~1/2 a tank. After how many miles? One digester found that the hose connecting the 2 tanks was pinched, allowing only 6-8 gal's per fill. Perhaps yours may be pinched or leaking? But, this would probably be a PITA to check, so do the easy checks first (tires, brakes, canister, o2...).

Much luck with it.

CAVEAT: As a warning, I'm clearly not an expert and never claim to be. I'm just tossing out ideas that I haven't seen posted yet to perhaps be confirmed or to be set straight by our resident experts (something like trial and error learning).

Matt   Posted Wednesday, Jun 25th 10:54am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 65
   
SF Bay Area
81 320i
Ahh, I almost forgot. Check your valve lash. That can do it majorly, that's y mine is so bad. I haven't though about the charcoal canister and should check that, thanks...

Tricord   Posted Wednesday, Jun 25th 2:16pm [Edit] [Quote] [IMS] [View car]
Administrator
Post nr. 87
   
Belgium, Europe
323i Baur
That's a good suggestion Matt. If your valve clearance is not correct you will have lack of power and other problems. When your engine is idle you should listen close to the cylinder head. If you hear distinct clapping and clicking, I suggest you open up to adjust the valve clearance. You should listen with the engine hot, of course.

With a correct setting there is absolutely nothing clicking to be heard except for regular noise of the turning of the fan and the pulleys.

By the way Ray, that's a pretty damn good fuel economy for that car! That's over 500km with a 55L tank. The 320/6 does about 440km with a full tank, or less depending on my driving style
I think the 320/6 has an avg. of 12-13l/100km, which is quite a lot.
I have yet to see how much the 323i burns, but I suspect it's more or less the same.

Daem   Posted Thursday, Jun 26th 1:17am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 4

OK, USA
82 320i
I just had the valves adjusted by a BMW service center. They did a good job and they're a lot quieter but the gas mileage problem still exists.

Exactly where can I adjust the fuel mixture? I'm thinking it's to rich. Might also explain some of the smoke from the exhaust.

What's the procedure to check an o2 sensor? If there's some way I can check the one on it before I buy another, that would be great.

When I get a chance I'm going to replace the fuel filter and check the charcoal canister. Can't hurt to replace them anyways.

Anything else you guys can think of to check?

I will win this mpg battle.

Thanks again,
Eric

M42powered   Posted Thursday, Jun 26th 1:23am [Edit] [Quote] [IMS] [View car]
Staff
Post nr. 69
   
Arizona, USA
1981 BMW 320i
You can check the oxygen sensor with a volt meter. If it's creating voltage when the car is on and warm it's a toss up. But if no voltage exists than you KNOW it's bad. I think there are specs on what the voltage should be in the Haynes manual. If the 02 sensor is reading lean then you know it's malfunctioning, because the concensious is that your car is rich right now.

I'd take a picture of where the mixture adjustment screw is, but my M10 is no more. Perhaps someone else would be kind enough to take one? I know there is a picture on the net somewhere... just can't seem to find it.

Good luck!

melloh   Posted Thursday, Jun 26th 8:40am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 46
   
California, USA (San Jose)
1980 320is, 1991 318is
Just curious... but when they did the tune-up, didnt the check mixture?? Sorry, i've never tampered w/ my mixture so I can't formulate a mental picture (and no digi-cam). Meanwhile, we should assume you've checked the other common faults like no brake rubbing?

Yety   Posted Thursday, Jun 26th 10:44am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 11
   
Europe, Romania
320i/6
Methinks, all possibilities I can think of were mentioned, so I can't add anything significant to all that was said on this topic...

All I can say "more" is that my father's 318 carb. "eats" around 13 liters/100 km in city and a little over 8 liters/100 km on open road. My 320/6 w/ Solex A4 "eats" around 15 liters/100 km in city and a little over 9 liters/100 km on open road. This confirms once more (if needed) most of the figures mentioned here about fuel consumption.

Maybe this is stoopid, and I apologize in advance, but how does the exhaust gas smell ? Normal or gas-like ? I'm thinking about unburnt fuel, due to sparks, ignition or God-knows what else. I'm too used to have the "problem" right before my eyes... ...so I really don't know what to say more, 'cause it seems to me that all "normal (usual) causes" were already mentioned.

Matt   Posted Thursday, Jun 26th 11:37am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 70
   
SF Bay Area
81 320i
Adjust w/ a 6" 3mm allen

melloh   Posted Thursday, Jun 26th 12:05pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 47
   
California, USA (San Jose)
1980 320is, 1991 318is
Matt.. that was just loooovely.

Yety, I dont think thats a stupid comment at all. Mine was running very rich and neeeded a valve-job (full tune-up) when I could smell gas from exhaust after it was warm. It fixed many problems, and mpg went from ~19/23 to ~22/27. However, i'm still hunting down some residual problems and curiosities (like how some get 30-PLUS mpg).

Nic: did you already mention your mileage on the m42? I've always wanted to know the comparison btwn m42 and m20 2.5/2.7 swaps.

Yety   Posted Thursday, Jun 26th 3:37pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 12
   
Europe, Romania
320i/6
Thanks for the appreciation, melloh

I found the same gas smell coming from the exhaust of my father's 318 carb. but only when the engine was cold... so I "loosened" a little his cold start system and everything went fine, especially the consumption which made him quite happy as a "daily city driver"

A "30-PLUS" mpg could theoretically be achieved on highway with a poor gas mixture (meaning lack of power and torque) and very "soft" driving around 50-55 mph, but IMHO it's quite "hard" to achieve that with our "gas-hungry little monsters"

Tricord, from what I've heard from other 6-in-line E21 owners around, the consumption depends quite much on the driving style. I'm a slow driver in city-mode, and I never went under 14 liters/100 km, but in winter I managed to burn more than 15 liters/100 km in rally driving style on a snowy steep mountain road with sharp curves.

thug   Posted Tuesday, Jul 1st 5:42am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 9

Pagosa Springs, CO.
bmw 320/6
I was getting about 24 mpg with my 320/6 just driving. if I was going nuts driving, it would drop to about 16, at worst, that was the record time from my house to town (15 minutes, 26 miles, through twisty roads)... so, i have heard that it really depends on how you drive too... and how the motor is tuned...

JDBeyer   Posted Tuesday, Jul 1st 6:46am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 21
   
N. Humboldt county Ca.
82 323i on steroids
I am getting about 18 mpg city and 26 freeway in my 323 but I have a 3.91 diff, with a O.D.5spd. to try and help me remember that cops get mad if you cruise at autobahn speeds. When I had the 3.45 in it I think mileage was about 20 and 28
and I was always finding myself doing 90 mph.

Tricord   Posted Tuesday, Jul 1st 12:05pm [Edit] [Quote] [IMS] [View car]
Administrator
Post nr. 106
   
Belgium, Europe
323i Baur
Too bad the US people use mpg while us europeans use l/100km

You measure how much distance you can drive with a given amount of fuel (a gallon) and we measure how much fuel (litres) it takes to drive 100km.

Yety, the 320/6 manual says the consumption should not be that high. It says that highway at 90km/h should burn 9l/100km max, but I have never been able to do that. I don't know about the 4-bangers, but with our six' you don't even have to bother driving sparingly. You burn too much anyway, so you might as well have a little fun doing it

Yety   Posted Tuesday, Jul 1st 1:46pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 26
   
Europe, Romania
320i/6
Frankly speaking, Tricord, I was never able to drive "softly" on a good road in order to achieve a consumption under 9l/100km - I always seem to find out that my right foot is pushing "the pedal to the metal" quite often, while my ears wait for the magnificent sound made by my empty rear silencer when all 4 barrells of the Solex are wide open at over 4000 rpm. Sorry for being so out of topic, but I just can't help it - I enjoy too much the roar of my beloved little beast, so that's why I "burn" so much fuel, but at least I have fun doing it, just like you said

Tricord   Posted Tuesday, Jul 1st 7:39pm [Edit] [Quote] [IMS] [View car]
Administrator
Post nr. 109
   
Belgium, Europe
323i Baur
Heheh. Honestly I think I like the sound of the 323i more by a long shot. It's a deeper purr and more creamy, also the throttle and induction system makes it sound nicer than the 320/6

Daem   Posted Wednesday, Jul 2nd 7:02am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 5

OK, USA
82 320i
I'm still looking for the source of my problem. Without luck.

I'm having a hard time locating the proper tool for adjusting the air/fuel mixture. Every 3mm allen wrench I find is 4" long. Not nearly long enough. Where can I find one? I was thinking I'd just find the longest 3mm allen and weld it on to the end of a long, skinny screw driver and make my own tool.

Any ideas on that front?

Almost out of ideas here.

JDBeyer   Posted Wednesday, Jul 2nd 7:44am [Edit] [Quote] [IMS] [View car]
Member
Post nr. 24
   
N. Humboldt county Ca.
82 323i on steroids
Hazet tool co. makes a nice one but they are a specilty tool co. so you would need to find a Snap On,Mac or Matco tool dealer(your in the U.S.?) They can get specilty tools for you.
Or make one like you are thinking, You might try asking local BMW Techs. were the they might suggest you find one.

Tricord, now I will have to figure my next tank of fuel the euro way l/100km!

Matt   Posted Wednesday, Jul 2nd 10:40pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 95
   
SF Bay Area
81 320i
Craftsman p/n #46801
Long Hex Keys
$12



M42powered   Posted Wednesday, Jul 2nd 10:44pm [Edit] [Quote] [IMS] [View car]
Staff
Post nr. 104
   
Arizona, USA
1981 BMW 320i
I still have my mixture adjustment tool... just sitting around... Daem, you get first dibs. It's no good to me now.

Daem   Posted Wednesday, Jul 2nd 11:39pm [Edit] [Quote] [IMS] [View car]
Member
Post nr. 6

OK, USA
82 320i
M42powered, how much?


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