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Thread: Twenty Thousand Degrees Advanced

  1. #1
    Carb and Points!
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    Twenty Thousand Degrees Advanced

    Hi, I'm new to the forum.
    I have a problem I haven't been able to find a solution for.
    94 GM TBI 7.4 Truck. 395 ECU BHDF Moates G1 adapter, Tunerpro RT.
    I can rev up moderately just fine, but if I WOT from idle, it instantly reads 6375 RPM and 23000 degrees spark advance.
    (about 1200 RPM by the external tach) It then backfires through the throttle body.
    It does this with the stock $OE bin, I also tried a stock $OD bin with the same results.
    This is in Open loop, map reading of 80, TPS 2.1 volts.
    I replaced cap/rotor/wires to rule out secondary ignition misfire, but this seems more of a computer issue.
    Could I have a bad ECU?
    Could I have a bad Memcal?

    other unrelated issues:
    ECU reads the O2 constant 446mv, but I measure varying .90-.93v at the pin.
    On hot restart, it struggles against the timing. Even though it's reading 0 degrees advanced.
    If I reset the ECU by disconnecting the battery, it starts up with ease.
    Has anyone seen this before?

  2. #2
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    Quote Originally Posted by ericburgundy View Post
    Hi, I'm new to the forum.
    I have a problem I haven't been able to find a solution for.
    94 GM TBI 7.4 Truck. 395 ECU BHDF Moates G1 adapter, Tunerpro RT.
    I can rev up moderately just fine, but if I WOT from idle, it instantly reads 6375 RPM and 23000 degrees spark advance.
    (about 1200 RPM by the external tach) It then backfires through the throttle body.
    It does this with the stock $OE bin, I also tried a stock $OD bin with the same results.
    This is in Open loop, map reading of 80, TPS 2.1 volts.
    I replaced cap/rotor/wires to rule out secondary ignition misfire, but this seems more of a computer issue.
    Could I have a bad ECU?
    Could I have a bad Memcal?

    other unrelated issues:
    ECU reads the O2 constant 446mv, but I measure varying .90-.93v at the pin.
    On hot restart, it struggles against the timing. Even though it's reading 0 degrees advanced.
    If I reset the ECU by disconnecting the battery, it starts up with ease.
    Has anyone seen this before?
    I wouldn't rule out a bad Memcal or ECM, but I would start by locating the ECM grounds and cleaning them as well as where they attach to the block. I am not as familiar on the 454 with their location, but on the same truck with a V6 or smallblock V8 the grounds are on the thermostat housing and on the rear of passenger side head.

  3. #3
    Carb and Points!
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    Good call on the ground, and thank you for your response!
    o2 is working and now i've discovered i was extremely rich.
    I leaned out the VE tables and also rerouted the #7 plug wire away from the wiring harness in case it was causing interference. This has nearly cured the misfire.
    I still have the hot start issue which feels like i'm overly advanced although I'm reading 0 degrees.
    I'll have to get the wife to crank while I check with a timing light to be sure.
    My gut says the memcal is faulty, even though I don't know exactly how it works.

  4. #4
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    You might have a mismatch between the ADX and the calibration. There's really no way the ecm should be sending a message for 23k degrees timing.

  5. #5
    Fuel Injected! JeepsAndGuns's Avatar
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    I remember something like this happening to me once. If I remember correctly, I changed a spark advance setting, maybe a bias setting, and did not reset the ecm before I installed the chip and run it.
    I just remember the spark advance number on my ADX jumping to some crazy stupid number and it running like doo-doo.
    I have no idea if this has anything to do with your issue, but just figured I would share.
    79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy

    93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver

    99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.

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    i get the feeling that the spark is being reported as a 16-bit value but someone never set the box for signing.

    23,000/.35 = 65,714, which is right around the 16-bit limit, indicating spark is happening negative to reference(or TDC, i don't know the late TBI code too well).
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  7. #7
    Carb and Points!
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    I think Robertlsaar is on to something.
    It makes sense that the ecu would try to retard the timing, but then mis-interperet the negative data.
    It also makes sense that super-retarded timing wound backfire the intake.
    Would a ADX make difference here?
    Could you guys suggest/link one for my $OE bin?

  8. #8
    Fuel Injected! steveo's Avatar
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    lets see the adx you're using it can probably just be fixed

    also i really dont see why the calibration shouldn't be set up to clamp timing to 0 degrees minimum anyway. since when has a car engine actually wanted spark after top dead center?

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    lowish RPM, WOT with a quick throttle transition to get there(triggering burst knock protection or torque management in the process)?

    i could see it going ATDC from that.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  10. #10
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    how do i upload my ADX to this thread?

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    If you click "go advanced" you'll be able to attach it as a file. Or send it to me and I can attach it.

  12. #12
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    My 3 files
    Attached Files Attached Files

  13. #13
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    well, i was right: 16 bit, but no sign.

    try this out. it will likely fix the advance being reported incorrectly, but it won't help with how the engine is running.
    Attached Files Attached Files
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


  14. #14
    EFI GearHead ! EagleMark's Avatar
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    Do all 16 bit data have to be signed? Because the BPW is 16 bit and not signed too...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  15. #15
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    not required of all 16-bit, some 8-bit values are even signed at times.

    if there is a value that can go negative and it doesn't have a negative offset in its equation, that is a good indication that it is a signed value. otherwise, you get the whole wraparound issue that happened here. if spark happened 4.9* BTDC, the raw value is 14 (14X.35=4.9). if it happened 4.9*ATDC, the same 14 comes into play, but it is subtracted from 65,535(or 65,536, i can never remember), so 65,535-14 = 65,521.

    it's kind of like a compliment situation, except the MSB of the value determines the sign. 0-32,767 is positive and 32,768-65,535 is negative. with 8-bit values, 0-127 is positive and 128-255 is negative.

    while the MSB is set(so, things are negative), the scales are reversed(large raw values are actually small real-world values).


    it's still kind of confusing to me at times.
    1995 Chevrolet Monte Carlo LS 3100 + 4T60E


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