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  1. #1
    Fuel Injected!
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    EGR tuning $8D S_AUJP

    Over my last weekend I was able to finally fix the EGR tube on the fiero. Last night I spent some time patching DC values and qualifiers from $24(fiero bin) and other $8D bins. The EGR DC table was propagated using values from $24, along with TPS threshold values. I cant open TP at work but iirc, $24 uses engine load in kpa for its qualifiers to enable EGR DC, also uses a qualifier for MAX threshold(higher loads). $8D uses TPS. On my way into work this morning the EGR looked like it was functioning normally, which is good. I did spot a possible issue, though. Let me start with the 3D EGR DC table, $24 uses a higher resolution table, but doesnt command and EGR DC above 50kpa, and max DC is ~44%. $8D commands 100% DC(which I didnt use those values) upto 80-90kpa, which I didnt use either just zeroed the table above 50kpa. Now in theory there wouldnt be any EGR DC commanded above 50kpa(in my mind) but I noticed EGR DC was 44%(at times) in the 60-90kpa areas. Thats weird to me because the table is zeroed out in those cells? If I dont want any EGR DC above 50kpa, theres nothing in the XDF to disable that. Or am I not understanding that right? There is only the TPS qualifiers, oh and the coolant temp V. EGR DC multiplier table. Im also going to assume that since the EGR DC table only reaches to 2000rpm that the values are interpolated above that? I didnt see a max rpm for EGR DC in the XDF either, possibly not found in hac? Or does it continue to operate no matter what RPM?


    I will try to remind myself to get a datalog on the way home today.

    Thanks!
    '86 Grand National

  2. #2
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    Quote Originally Posted by 34blazer View Post
    Over my last weekend I was able to finally fix the EGR tube on the fiero. Last night I spent some time patching DC values and qualifiers from $24(fiero bin) and other $8D bins. The EGR DC table was propagated using values from $24, along with TPS threshold values. I cant open TP at work but iirc, $24 uses engine load in kpa for its qualifiers to enable EGR DC, also uses a qualifier for MAX threshold(higher loads). $8D uses TPS. On my way into work this morning the EGR looked like it was functioning normally, which is good. I did spot a possible issue, though. Let me start with the 3D EGR DC table, $24 uses a higher resolution table, but doesnt command and EGR DC above 50kpa, and max DC is ~44%. $8D commands 100% DC(which I didnt use those values) upto 80-90kpa, which I didnt use either just zeroed the table above 50kpa. Now in theory there wouldnt be any EGR DC commanded above 50kpa(in my mind) but I noticed EGR DC was 44%(at times) in the 60-90kpa areas. Thats weird to me because the table is zeroed out in those cells? If I dont want any EGR DC above 50kpa, theres nothing in the XDF to disable that. Or am I not understanding that right? There is only the TPS qualifiers, oh and the coolant temp V. EGR DC multiplier table. Im also going to assume that since the EGR DC table only reaches to 2000rpm that the values are interpolated above that? I didnt see a max rpm for EGR DC in the XDF either, possibly not found in hac? Or does it continue to operate no matter what RPM?


    I will try to remind myself to get a datalog on the way home today.

    Thanks!
    Almost all values in GM ECM tables are interpolated. If you go from say 100% @ 2,000 down to 0% @ 2,4oo and are sitting at say 2,200 exactly, you will be sitting at 50%.

  3. #3
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    The table only goes to 2000 rpm. My question for that part was, what is the rpm limit for the EGR DC? Its not listed in the XDF.
    '86 Grand National

  4. #4
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    Scratch that. EGR is functioning properly and my questions have been answered. This morning I observed effects of the EGR and found that it provides no increase in fuel milage. Depending on the duty cycle, the impact negatively affected fuel milage. As soon as DC rose above 30% I noticed an increase in BLM counts. Not satisfied with that observation I toggled EGR on and off while highway mode was active, watching NBO2 readings similtaneously. With EGR DC at or above 30%, the A/F mixture would lean out, ~30-50mV. While it helps with emissions, it doesnt help fuel milage at all, at least in my case. Toggled off, the BLM counts fell back down to where im used to observing them, some cells fell by 6 counts. So I have no use for EGR anymore.
    '86 Grand National

  5. #5
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    not sure how 8D accounts for it, but you did have spark being added due to EGR active, yes?

    i've had good results with EGR enabled on 60V6 applications, after some tweaking of the fueling and spark modifiers for when EGR is active anyways.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  6. #6
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    BLM is only giving you the difference between the airflow the ECM thinks is there (after subtracting out the EGR that it thinks is there) compared with what is actually happening on the engine, not the actual fuel economy. I think that generation of ECM had a BPC vs. EGR calibration in there somewhere and any error in that calibration would show up as a BLM shift. There will be an emissions benefit but you should see a slight mileage improvement when looking at the pulse width at the same vehicle speed - you'd need to log injector pulse width, RPM, vehicle speed and integrate pulse width * RPM and compare against miles (or kilometres) driven to get at least a semi-accurate reading. Also EGR is usually set up to allow more advanced ignition timing - maybe 5 to 8 crank degrees - which should help your efficiency numbers. I don't know if I've ever seen a calibrated EGR system make efficiency drop, but certainly if the timing isn't adjusted up that can happen. That being said, with EGR there is a peak efficiency point where you've lowered NOx as much as you can and advanced the timing as much as you can and if you increase the flow rate any more then you get an efficiency drop - if some is good, more is not necessarily better. Just like tequila. It also should help lower the exhaust temperatures at part-throttle.
    1994 6.5L 4L80E K
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  7. #7
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    Quote Originally Posted by RobertISaar View Post
    not sure how 8D accounts for it, but you did have spark being added due to EGR active, yes?

    i've had good results with EGR enabled on 60V6 applications, after some tweaking of the fueling and spark modifiers for when EGR is active anyways.
    can you share your EGR DC table, unless it is untouched from the factory? the 2.8 in the fiero is bone stock, well, with the exception of the ported headers.
    '86 Grand National

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