I borrowed a datalogger and tried to do some tuning of my S-AFC. I found some "quirks" that might be the cause of tuning problems. A) all three fuel trims are maxed out at 140. I tried to lower them by bumping up the S-AFC settings to about +25 over the board. After a 30 minute drive the trims had barely dropped, maybe to 137 or so. If I wanted to get to the ideal 105-110 range the settings would likely have to be in the +40's. With the mods I have this should not be the case. The modified MASS can't be flowing that much more air, right?
B) With a knock count of zero any throttle increase results in the timing getting pulled back. Why? With no knock the ECU should advance the timing or at least maintain it.
C) O2 voltages cycle between 0.4 and 0.86. I would have thought they should go up to 0.96.
EGT's are ok and I get about 0.85 under WOT. Could my O2 sensor be going bad resulting in the bad fuel trims and slightly low O2 readings? It's about 4 years old but only has about 50K on it.
Here is a pic of the datalog.

Mods are on my web page...

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'91 Laser RS Turbo-"Sound Advice" guy.
www.geocities.com/motorcity/speedway/5051
Turbo + Intercooler = The replacement for Displacement

[This message has been edited by Chris Holmes (edited October 31, 2000).]

Timing will generally decrease as RPM increases. This is normal for the DSM ECU.

If you have zero knock, the ECU will set timing according to the preset maximum values. These values decrease as RPM increases.

Oxygen sensors vary. Your oxygen sensor may not have ever read more than 0.86. I doubt it is responsible for your fuel trims, because it cycles correctly. The ECU doesn't care about the high and low points of the O2 reading, only that it switches around periodically.

I haven't reviewed your mods. However, it is possible to mod the car so the ECU will max out all the fuel trims.



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S.
1000 AAQ: 1000q.dsm.org
ECU Primer: members.home.net/costall/ECUprimer/index.html

Chris

I think maybe you are forgetting something...Fuel trims are for closed loop ONLY. If you are trying to adjust the fuel trims using WOT, it will never happen. Use knock and 02's/egts to tune WOT and fuel trims to adjust lean/rich closed loop



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PurplePlymouthGuy
1992 Laser AWD
The 11 second car with the 13 second driver...

True, but I would have to add TONS of fuel for the low S-AFC settings to get the trims down. I shouldn't have to add that much when compared to cars with similar mods. I think my O2 sensor is getting slow and is not responding as fast as it should. I'm noticing more that it does not cycle as fast as before and the range which is cycles over is decreasing. I'm going to look at replacing it as a start.

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'91 Laser RS Turbo-"Sound Advice" guy.
http://www.geocities.com/rsturbo.geo
Turbo + Intercooler = The replacement for Displacement

Chris after you got wot are you right off the gas letting teh car slow down? I can't quit tell if you are of ir you're going back into a constant speed drive. I'm just curious to why your o2 sensor voltage does not start cycling after that point.

I have yet to get into understanding fuel trims properly so I can't help ya there.

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Quadcylla
92 Laser RS-T FWD

Mike, I'm usually just coasting after giving it a bit of gas so that's why the O2 voltages drop to zero. The picture aboe was mainly to illustrate that the timing is being retarded for minimal throttle increases with NO sign of knock. I then went WOT to see if knock was detected and you can see that it wasn't.

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'91 Laser RS Turbo-"Sound Advice" guy.
http://www.geocities.com/rsturbo.geo
Turbo + Intercooler = The replacement for Displacement

Chris, there is talk on the tmo list about part throttle knock caused by out-a-whack fuel trims...

are you still running with a resistor on the maf?

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PurplePlymouthGuy
1992 Laser AWD
The 11 second car with the 13 second driver...

Yes, I have the resistor still and I do sometimes see knock for no apparent reason.
Kind of like this....



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'91 Laser RS Turbo-"Sound Advice" guy.
http://www.geocities.com/rsturbo.geo
Turbo + Intercooler = The replacement for Displacement

And what would happen if you took it out? Do you have a rheostat in there too?

might be fooling the ECU too much. I thought the resisitor was for non afc users not for fuel management systems...

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PurplePlymouthGuy
1992 Laser AWD
The 11 second car with the 13 second driver...

Nope, the flapper door is for non - afc users...

We found that with hogging out Craig's MAF the AFC couldn't compensate enough for the extra airflow. We put in a resistor, and Craig was able to lean out the mixture with the AFC a bit to get his fuel trims from +100 to -100 (can't remember the exact numbers..)

HOWEVER, we didn't use a resisitor as "large" as Al Blaha does. Chris, maybe try replacing the 02 sensor (due to the extra high fule trim values.. I'm guessing on this though..) and try replacing the reisistor with one of about 1/2 the value. I can't remember what Craig used (Craig.. feel free to jump in.. ) but it was a fair bit smaller. Yet it was enough to knock his fuel trims below 100%.

Hope some of this yabbering helps!

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1992 Eagle Talon Tsi FWD
Superiority after 60'..
ICQ # 9622983 My Home Page...