Fuel/air mixture adjustments

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PatinaBeforePolish
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Fuel/air mixture adjustments

Post by PatinaBeforePolish » Fri Mar 08, 2019 5:45 pm

Hello All,

After having my AFM refreshed by Bavarian Restorations and installing a free flow exhaust I find the car runs very lean going by plug color - light grey. I have not yet used an air fuel mixture meter to accurately measure the mixture. Timing is set correctly and there are no vacuum leaks. It has new plugs, wires, dist cap, rotor and ultrasonically cleaned injectors. The car idles at about 1000 rpm and the AC system is inoperable. It idles and accelerates very smoothly with no stumbles or hesitations.

For those with experience, what is your recommendation to adjust the mixture? I see no guidance in the shop manuals other than measuring it with long thrown out OEM instruments but I see it can be adjusted by removing the top plate of the AFM and adjusting the spring tension which I assume adjusts the flapper tension. Suggestions?
78 F150
79 528i
81 528i parts car
01 540i
86 944

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Mike W.
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Re: Fuel/air mixture adjustments

Post by Mike W. » Fri Mar 08, 2019 9:56 pm

There is a remarkably good tool that came with the car for measuring this exact thing. The Oxygen sensor. If all is well, once warmed up just a little bit, it will show an output, measured while in line and connected, of about a half a volt, bouncing up and down constantly. Technically an average of .45 volts, but constantly cycling up and down, up and down from less than .1 up to .8 or .9. That will tell you if it's rich, lean or just right. Because of the self correcting nature of it, it's actually very difficult to really change the fuel/air ratio.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

PatinaBeforePolish
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Re: Fuel/air mixture adjustments

Post by PatinaBeforePolish » Mon Mar 11, 2019 8:05 am

Thanks Mike, I've looked at that and see it's exactly in the range when idling. But my understanding is the ECU only understands start up, idle, wide open throttle and adds fuel in relation to throttle opening. Isn't there a need to compensate for less potent fuel (compared to the late 70's) and freer flowing intake and exhaust?
78 F150
79 528i
81 528i parts car
01 540i
86 944

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528i-1981
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Re: Fuel/air mixture adjustments

Post by 528i-1981 » Mon Mar 11, 2019 8:36 am

If I understand the lambda control accurately, the O2 sensor doesn't care about these things individually. The sensor is just measuring O2 content as a proxy for the completeness of the combustion. The intake flow, exhaust resistance and fuel quality all impact what happens inside the cylinder and the lambda system adapts mixture to keep the sensor signal at 0.45V without reference to the contributing variables. If the lambda system wasn't doing it's job, that would show up in the combustion process and your reading at the O2 sensor would be out of range. Because you're in range it appears that the system is robust enough to contend with changes to the intake, exhaust and fuel quality.
(oo=00=oo) Eric
1981 528i Manual

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maybeillbuyit
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Re: Fuel/air mixture adjustments

Post by maybeillbuyit » Mon Mar 11, 2019 4:05 pm

I was curious and googled spark plug color. I found at least 6 websites that state the correct or good color for a spark plug is light grey to tan. So maybe your plugs are the right color. What color are you expecting or hoping for? My plugs are a lightish tan. Which means nothing as I'm sure my engine isn't running optimally by a long shot but I'm concerned by that color. I'm planning on putting a larger diameter freeer flowing exhaust on my car so I'm interested in the outcome here.
1977 530i another project
1979 635csi Euro "project"
1987 635csi

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Mike W.
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Re: Fuel/air mixture adjustments

Post by Mike W. » Tue Mar 12, 2019 12:17 am

PatinaBeforePolish wrote:
Mon Mar 11, 2019 8:05 am
Thanks Mike, I've looked at that and see it's exactly in the range when idling. But my understanding is the ECU only understands start up, idle, wide open throttle and adds fuel in relation to throttle opening. Isn't there a need to compensate for less potent fuel (compared to the late 70's) and freer flowing intake and exhaust?
No, not at all. The ECU does get an input for idle off the switch, but that's mostly a fuel cut off above around 1300 RPM, it doesn't seem to affect mixture. WOT does provide some enrichment and at that point is open loop, not reading the O2 sensor. But other than idle or WOT there is no throttle input to the ECU. The AFM kind of provides that, very kind of, but again short of WOT once it's even a little warmed up, the ECU fine tunes things off the O2 signal. There are limits of course to how much correction it will provide, but it's significant and I think in the neighborhood of 25%.

But freer flowing has nothing to do with mixture. Here in Calif and some other places there is oxygenated fuel, but the O2 sensor and ECU fully compensates for that. Conceivably spark might require some adjustment, but not fuel. While crude by todays standards, it was absolutely state of the art for it's time.
Mike W.



1980 528i, 3.5 euro, 5 speed conversion
1981 528i, 3.6, Recaros and more. Project
1998 328is, quick and efficient, but not satisfying
2000 528iit, Vacation mobile/wife's grocery getter

PatinaBeforePolish
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Joined: Sun Jan 18, 2015 1:22 pm
Location: Baltimore, Md.

Re: Fuel/air mixture adjustments

Post by PatinaBeforePolish » Tue Mar 12, 2019 3:06 pm

Thank you gentlemen! I've ordered an air/fuel ratio gauge to see the mixture during driving conditions for my own piece of mind.
78 F150
79 528i
81 528i parts car
01 540i
86 944

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maybeillbuyit
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Re: Fuel/air mixture adjustments

Post by maybeillbuyit » Thu Mar 14, 2019 1:44 am

Which air/fuel gauge did you buy?
1977 530i another project
1979 635csi Euro "project"
1987 635csi

PatinaBeforePolish
Posts: 212
Joined: Sun Jan 18, 2015 1:22 pm
Location: Baltimore, Md.

Re: Fuel/air mixture adjustments

Post by PatinaBeforePolish » Thu Mar 14, 2019 1:58 pm

I bought this one on Amazon with the intentions of using it as a test tool and not a permanent installation. I hope to try it this weekend:

78 F150
79 528i
81 528i parts car
01 540i
86 944

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528i-1981
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Re: Fuel/air mixture adjustments

Post by 528i-1981 » Thu Mar 14, 2019 3:41 pm

So this is basically a calibrated voltmeter, correct? What are the failure modes of an O2 sensor? Does it die and return 0mv or just sit still at 0.45 mV (or some other value) and not react?

You should be able to verify that the O2 sensor itself is working by inducing a lean and a rich condition. I think you can do these by pulling out the dipstick (lean) and then stuffing a rag in the air cleaner box intake (rich).

When I got my E12 I just replaced the O2 sensor and it was clearly bad. But now I wonder if there are different levels of performance of O2 sensors? Are some faster than others, more accurate, etc? How often does the ECU check the O2 sensor output in the E12?

I wonder if it’s important to use the OEM sensor for performance reasons and not merely for longevity?
(oo=00=oo) Eric
1981 528i Manual

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Re: Fuel/air mixture adjustments

Post by 528i-1981 » Fri Mar 15, 2019 4:38 am

What are the failure modes of an O2 sensor? Does it die and return 0mv or just sit still at 0.45 mV (or some other value) and not react?
This article was really helpful to me, understanding of course that the E12 has only one unheated O2 sensor and no OBDII. So, junk from the exhaust contaminates the sensor surface over time and causes it to become sluggish. Eventually no voltage is produced at all, resulting in a permanent rich situation, like a cold start. The sensor only returns a voltage at operating temps around 600 degrees.

http://www.obdii.com/articles/Understan ... nsors.html

Evidently you can induce a rich-sense condition by feeding propane into the intake.

Aftermarket sensors appear to work to the same specifications as OEM but likely won't last as long, based on online anecdotes. Periodic checks are therefore a good idea on this engine.
(oo=00=oo) Eric
1981 528i Manual

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