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Old 08-17-2011, 10:35 PM   #1
Discostar
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Why is my fuel economy so bad?

I own a 1992 325is non-vanos motor. I read about people getting well over 500k per tank on these things, yet somehow I just barely break the 500k mark on fumes with super conservative driving.

I replaced my spark plugs, fuel filter, air filter. Nothing made an impact.

I am planning on cleaning the idle sensor soon. I hear that the O2 sensor has a big impact. Please donít tell me about driving style, I know thatís a major factor. I only get 400km/per tank average with normal driving.

How is there such a big discrepancy between cars and fuel mileage? Could compression be playing a factor? Anyone else experience this issue?


Thanks.
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Old 08-18-2011, 12:33 PM   #2
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It's gotta be your O2 sensor since you've done the other stuff. Although when it's bad, doesn't it throw a code?
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Old 08-18-2011, 01:27 PM   #3
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I experienced poor fuel mileage for about a month before my O2 sensor threw a code (1221). After replacing the sensor, mileage was back to normal. 500km per tank isn't terrible if that's all stop and go city driving. 400km is bad.
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Old 08-18-2011, 06:32 PM   #4
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yea if u get 500k in the city that's ok
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Old 08-18-2011, 06:54 PM   #5
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That's really shitty. My 2.8 is getting about 500 highway right now, and its running in closed loop (o2 is shot)
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Old 08-18-2011, 08:14 PM   #6
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O2 sensor does next to nothing on long highway trips. I know this because I ran no o2 sensor for years and still had great gas mileage in my old 325e.

In town having no O2 sensor, or a malfunctioning one wastes more gas.

I'd say have the emissions tested, this should point out a problem if there is one with the engine/fuel-air mix.

On another note, when you stop driving, quickly walk around the car touching all the rims. If one rim seems hotter than the others you could have a siezed caliper. Also a dragging parking brake can cause some drag. I would also check your tire pressures.
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Old 08-18-2011, 09:29 PM   #7
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Oil separator? (PCV)
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Old 08-18-2011, 09:54 PM   #8
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O2 sensor does next to nothing on long highway trips. I know this because I ran no o2 sensor for years and still had great gas mileage in my old 325e.

In town having no O2 sensor, or a malfunctioning one wastes more gas.

I'd say have the emissions tested, this should point out a problem if there is one with the engine/fuel-air mix.

On another note, when you stop driving, quickly walk around the car touching all the rims. If one rim seems hotter than the others you could have a siezed caliper. Also a dragging parking brake can cause some drag. I would also check your tire pressures.
I run my tire pressures pretty high (40-42 psi cold) so I doubt it would be that, and I did the breaks and replaced my e-brake hardware and backing plate about the same time, but I'll recheck the adjustment.

However, I also had an emmissions test about 6 months ago. The car failed the first time around, then just scraped by on the second try. So I'm guessing that would suggest O2 sensor. That was also before I replaced the air filter, plugs, and fuel filter.
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Old 08-18-2011, 09:58 PM   #9
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Oil separator? (PCV)
Are you suggesting pressure in the crank case? how would that have an effect?
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Old 08-19-2011, 02:36 AM   #10
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well from what i've read, and take with a grain of salt... if your PCV is leaking... then oil/waste could be flowing back in to your engine. the PCV is supposed to keep it clean... I would double check with someone more knowledgeable than me...

On a side note, i noticed when i changed my belts (serpentine and a/c) i got way better fuel mileage. I am guessing the serpentine had more to do with it than the a/c.
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Old 08-19-2011, 08:43 AM   #11
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Originally Posted by InfiniteDice View Post
O2 sensor does next to nothing on long highway trips. I know this because I ran no o2 sensor for years and still had great gas mileage in my old 325e.

In town having no O2 sensor, or a malfunctioning one wastes more gas.

I'd say have the emissions tested, this should point out a problem if there is one with the engine/fuel-air mix.

On another note, when you stop driving, quickly walk around the car touching all the rims. If one rim seems hotter than the others you could have a siezed caliper. Also a dragging parking brake can cause some drag. I would also check your tire pressures.
obd 1 and 2 most certainly use the o2 sensor while cruising. at wot and when engine is cold? disregarded.

I pull over 500km on my e36 m3 in the city.

OP try o2 sensor
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Old 08-19-2011, 09:46 AM   #12
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It could very well be the O2 sensor. All I'm saying is even if you pull the O2 sensor and run open loop it won't affect gas mileage that much especially if you mostly cruise on the highway. I've run open loop for years and could go about 600km on a 55l tank at the time.

I would pull the plugs, re-check the condition and gap.

I'm just wondering why the car failed the first time then passed. How do you drive the car? Is it automatic? I'm asking this because the engine needs to run above 3k for at least a 10 minute period straight to blow out the crap once in a while (source:87 325i owners manual). This may or may not be the case for an m50 powered car.
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Old 08-19-2011, 11:48 AM   #13
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I'm just wondering why the car failed the first time then passed. How do you drive the car? Is it automatic? I'm asking this because the engine needs to run above 3k for at least a 10 minute period straight to blow out the crap once in a while (source:87 325i owners manual). This may or may not be the case for an m50 powered car.
Its 5-speed, and I track the car 4-5 times a season so I doubt that would be an issue. For the emmisions test, it could be as a result of the cats being cold. When I first took the test the car had been sitting for about an hour and it was a pretty cold day. Perhaps that also had some effect on the O2 sensor during the test. For the second test I took it for a spin to heat everything up, then it passed.

Thanks for all the help. I'll look into the condition of the O2 sensor and see what that does.
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Old 08-19-2011, 02:57 PM   #14
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All I'm saying is even if you pull the O2 sensor and run open loop it won't affect gas mileage that much especially if you mostly cruise on the highway.
yes it will on any obd1 and up car.
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