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Thread: Recording Wrong RPM on ECM

  1. #1
    Electronic Ignition!
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    Recording Wrong RPM on ECM

    Hey guys,

    Did some more datalogging and had overlooked a major issue.

    This application is my boat.

    350 CID
    TBI with 16168625 ECM

    I had the boat running down the river at WOT. Really moves out fast. I know that the Tach on the dash is correct. Engine is racing. It was running at approx 4900 RPM on my dash, yet my ECM logged only ~3600 RPM. How can this be? bad distributor?

    Also at 3100 RPM on the dash, ECM is recording 2400RPM.

    This would make sense as to why all my data is lean. If the ECM thinks it is only running 3600 when it is actually running 4900, would cause a problem.

    Thoughts?

    -Fallflight

  2. #2
    RIP EagleMark's Avatar
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    What distributor is in the engine?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  3. #3
    Super Moderator Six_Shooter's Avatar
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    This seems to be an issue with the P6 (IIRC that's the designation) PCMs, where they control both trans and engine. The engine indicated RPM seems to be lower than the trans data stream engine RPM.

    It's something I need to look into a little more on the '7060, as well.
    The man who says something is impossible, is usually interrupted by the man doing it.

  4. #4
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    The only p/n that I see on it is 1103479 which is 1 number short of the GM nomenclature. Does this help?

    -fallflight

  5. #5
    RIP EagleMark's Avatar
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    Quote Originally Posted by EagleMark View Post
    What distributor is in the engine?
    Quote Originally Posted by Six_Shooter View Post
    This seems to be an issue with the P6 (IIRC that's the designation) PCMs, where they control both trans and engine. The engine indicated RPM seems to be lower than the trans data stream engine RPM.

    It's something I need to look into a little more on the '7060, as well.
    When I did the kids IH truck with the 16197427 we just used the extra white wire off coil like always and it's spot on to TP data. His 8625 is same type PCM, so not sure what the differnce is there or what's happening?

    Quote Originally Posted by fallflight View Post
    The only p/n that I see on it is 1103479 which is 1 number short of the GM nomenclature. Does this help?

    -fallflight
    No, is it a small cap EFI distributor? Just wondering if you had a tach filter with the howell conversion you got going on? Also could look at tach and see if it's set to 4, 6 or 8 cylinder, like said above I just used the extra white wire normally off coil and it worked fine... have we looked at your bin file on that chip yet?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  6. #6
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    It is the small cap distributor. I dont lknow that it has a tach filter from Howell. I was just over to see them as I live close to them. they didnt mention anything about it, and we discussed this RPM issue. How would I know if there is a filter?

    Also, if it is displaying the 3600 RPM, is the ECM/PCM thinking the engine is running at this RPM or ? This would explain the lean conditions

  7. #7
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    hmm.... almost seems like the V6 and V8 RPM conversions are getting mixed up? my P66 unit displays the RPM perfectly.

    perhaps swap out the reference period RPM for the normal RPM X 25 one to confirm?
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  8. #8
    RIP EagleMark's Avatar
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    The small cap EFI distributor would not have a tach filter. If it's not the tach then I would have to think the chip/Bin is not correct?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  9. #9
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    Thanks for all of your input.

    Is the incorrect RPM that I see while datalogging also what the computer is using to calculate everytthing or is it for my viewing purposes only?


    Thanks,

    Fallflight

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    depends. if it's the reference period based RPM, that's only really for datastream viewing, since the ECM creates the RPM value based off of that and a cylinder count value to create the other RPM values.

    the other one is used for a LOT of stuff.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  11. #11
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    Where do I look for # of cylinders? I just can get a feel for how this software works other than datalogging.

    I might just need to get another BIN. I have the data, can anyone look at it in detail and tell me what I need?

    I see things like TABLES, SCALERS, and FLAGS, but I cant make sense of it. Is there a course that I can take?

    sorry for the frustration.

    -Falllight

  12. #12
    Super Moderator Six_Shooter's Avatar
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    The cyl number or cyl count will be found in Scalers. It is usually near the top of the list.
    The man who says something is impossible, is usually interrupted by the man doing it.

  13. #13
    RIP EagleMark's Avatar
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    We could look at data if you post it, but does not sound like anything is getting there?

    Are you using the AutoProm to Emulate and the cable for data logging?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

  14. #14
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    Guys,

    I am not emulating. Only datalogging. I am going to attach files involved. I attached 2 log files, while short can show what it is doing.

    note BLM

    Also how low the O2 sensor is reading both in closed loop and open (Reminder that this boat does have an 02 sensor)

    Thanks! Thanks! Thanks! Thanks! Thanks!!


    I PLAN TO EMULATE.....Still trying to understand how this whole system works
    Attached Files Attached Files

  15. #15
    RIP EagleMark's Avatar
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    The highest tempreture I saw was 140f so it's never going to run right without major chip changes. Then it only went Closed loop for a few seconds, rest of log is Open Loop so O2 sensor is not working correctly, BLM mean nothing in open loop. There is also a code 44 set...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
    -= =-

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