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Thread: Warm spark bias

  1. #1
    Fuel Injected! TheApocalyptican's Avatar
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    Warm spark bias

    I've had a problem with bogging in my 90 Sub with 350 tbi and 770r4 w/ 7747 ecm. Not sure if I posted about it here before. Basically have a intermittent low rpm bog. Have been chasing my tail trying to find the problem. Today I decided to play with the tune some and tried pulling out the warm spark bias which was at 9.XX. All the other bins I have downloaded suggest that this is the correct number to have. However, when I pulled it out, my bog disappeared almost completely. Had it do it a tiny bit, but nothing like before. What is the reason for the warm bias? I understand the cold bias, but I would think if there was a subtraction in spark wanted when warm it would be easier to just set the main table correctly. What's the reasoning behind having the warm bias this way?

  2. #2
    It's explained in the XDF file, IIRC. The memory can't hold negative numbers, so there is a warm spark bias applied to the ENTIRE table.

    Thus, if the table says 10 degrees and the warm spark bias is set at 15 degrees, the actual computed timing is now NEGATIVE 5 degrees.

    Some XDF tables AUTOMATICALLY compensate for the warm spark bias. Some do not.

    At least that is how I understand it...
    Familiar with 1227747 and 16197427 PCMs

  3. #3
    Fuel Injected! TheApocalyptican's Avatar
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    I guess that makes sense as a whole....but in my main spark table, the only cells that are low enough to have the warm bias make the spark negative, are cells that basically can't ever actually be achieved. Like the 400rpm 100map cell. That's the big reason I don't know the point of it(in my case).

  4. #4
    That's GM for you. LOL!

    I don't know why it's there, either. Maybe some engines need the negative spark and they just left it in the code to cover all situations. <shrug>
    Familiar with 1227747 and 16197427 PCMs

  5. #5
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    Is this the coolant temp bias were talking about. If it is, it's supposed to pull timing once it hits those temps. But to me it seems like it pulled timing no matter what. I had results by zero'ing out the coolant temp bias as well as the egr bias. I just about zero'd out most of the bias's. Now I'm not saying to do this as I have been told to be careful as the timing may not be retarded if it needs to. Just don't do too much at once and watch knock counts. HTH

  6. #6
    RIP EagleMark's Avatar
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    FSJ Guy is exactly correct.

    In the 1227747 $42 small block you just added 9.84 degrees to your entire spark table!!! Now if you look at a Big Block bin file warm spark bias is 0. There's so many other spark corrections and bias that doing this takes time and testing with timing light to ensure you know..... wait for it......

    So where is your timing really at?

    I'd put it back for now and look into other things first. Like where is your base timing set? This is timing light marks at balencer with wire disconnected stock should be 0. Lock distributor and recheck, reconnect wire, clear codes. Then you can use the hack in $42 XDF and see actual timing in ADX. if you need more you can add to timing table, with bias and vehicle fully warmed up you'll notice what your timing table says is 9.84 degrees off. Look up data tracing as well when data logging.

    This could still be a mechanical issue, these small cap distributors cap and rotor can be completely destroyed and still run, there's also a possibility of a faulty EST or even a fuel pressure issue? Injectors clogged, injector O rings leaking, do you see a good spray pattern or lots of drips, etc... is it a high mile vehicle? Min air setting? What's your IAC count totally warmed up closed loop? What I'm getting at here is with age the wear closes throttle blades and when engine is running there's hardly any air mixing with fuel through throttle blades and all air is coming in from IAC, when you hit gas throttle blades are being sucked closed and it jerks open dumping fuel that has been collecting on top, when engine not running throttle blades open smoothly...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  7. #7
    Fuel Injected! TheApocalyptican's Avatar
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    That's what I figured with the spark table. Didn't drive it long that way. Just was using it as a test. The problem also started back up again before changing the setting back.

    Timing is at 3 btdc. And balancer has been verified to be correct.

    I've got a new distributor. I've got the spark test set to pass instantly and therefore never run. Also, still have a non working knock sensor. With things this way, is the EST still doing anything?
    Fuel pressure is at 13psi. I've tried from 11-15 and also cranked it up to 18 to see if it would help the problem. The injectors have a nice cone shape spray. I went thru the min air setting procedure. I'll have to look at the iac counts next time. I thought they were at like 40, but could be mistaken.
    After putting the warm bias back, I tried the main spark settings, and egr spark settings from the Stage 1 5.7 bin on this site. Just like with the bias it helped the problem at first, and then my problem came back. I swear something is messing with my timing, but the ecm is commanding the right advance, the dist is new( had the problem with the old one too), the spark test is set to pass, and the knock sensor is for all intents and purposes disabled.

    My problem is odd because the bog comes and goes. Once warmed up it does it most of the time. But then every once and a while will behave as it should. Long idling periods of like say 5 mins at idle make the problem go away for a stop light or two, and then the problem comes back.

  8. #8
    RIP EagleMark's Avatar
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    What spark test?

    If you change fuel pressure you should also change BPW, look at my last post in other thread and grab the injectorsizing calculater.

    3 BTDC should also be set in Base timing in bin. Or those 45 degree numbers on decel limited to 41.84 max are 44.84 and firing wrong cylinder. Hence soaking your O2 sensor with unburned fuel and air and making the ECM have a panic attack!

    40 IAC is fine. Coming and going is not? Hmm? Got a data log with the issue happeneing? Coming and going sounds like a mechanical issue...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  9. #9
    Fuel Injected! TheApocalyptican's Avatar
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    Well, I've been back at 13 for a while. The others were just a test.
    The 3 BTDC is set in the bin....actually it's like 3.16 or something.
    I'll post up a log. I haven't seen a problem anywhere in the logging, but maybe you(or someone else here) might see something I'm not.
    As far as a mech issue.....any ideas on what it could be?

  10. #10
    RIP EagleMark's Avatar
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    Coming and going bog issue.... hmmmmm... fuel pressure! Voltage steady? You in a salt stat and got rust on your grounds especially the one from fuel pump to frame?

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  11. #11
    Fuel Injected! TheApocalyptican's Avatar
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    Fuel pressure is what I thought originally, but it sits at a steady 13psi. I've got a pump that can supply quite a bit more pressure too. Had the stocker in and thought maybe it wasn't keeping steady so I switched it with one capable of higher pressure. My system voltage never goes below 12.8v, but the ecm is somehow seeing as low as 11.8v, but I've had it work correctly when the ecm is reading 11.8v. Not sure why the ecm is seeing less. I even ran a 4 gauge wire from the batt to the junction box on the firewall that goes to the fuse box. I read that the fuse box is where the ecm gets it voltage reading. So I'm not sure why it's a full volt lower. Fuel pump ground is good. I live in Las Vegas, so salt isn't an issue. Thought I had my location set, but I guess not. Is now though.

  12. #12
    RIP EagleMark's Avatar
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    11.8 ay? With the truck running? Mine reads 13.8 running and has no issues.. doing a lot of conversions anything below 12.0 spooks me. Pretty sure the voltage comes from fuel pump relay like $42. But you have 13 PSI at 11.8? Voltage go up with RPM? Lets see a log...

    1990 Chevy Suburban 5.7L Auto ECM 1227747 $42!
    1998 Chevy Silverado 5.7L Vortec 0411 Swap to RoadRunner!
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  13. #13
    Fuel Injected! TheApocalyptican's Avatar
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    Still need to get a log recorded. Found out the ECM B fuse reads at the lowest 12.8v. The problem is the ecm 1 fuse. It's only reading at 11.8-12v. Ideas on why it's so low? According to the diagrams I have....this means that the fuel pump relay is getting at the least the said 12.8v. But the ecm is reading the ecm-1 fuse as the voltage, and not the ecm-b.
    Last edited by TheApocalyptican; 06-03-2012 at 02:26 AM.

  14. #14
    RIP EagleMark's Avatar
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    They are in the same fuse block, how can that happen? Check your ground wires, should be one on front of intake and one on back of intake manifold. What about with vehicle running? Please don't say that was running...

    I have the exact same truck, just looked at my last log and I have 13.5 to 13.8 volts running

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

  15. #15
    Fuel Injected! TheApocalyptican's Avatar
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    12.8 is with the system fully loaded. A/C, the a/c fans, stereo and amps on, also it's about 105 out today, and the alt is at 13.4. When the alt seems to be fully charging at 14.3v the ecm b is at like 13.8. The ecm-1 stays between 11.8 and 12.1 irregardless. Grounds are both good. Something's gotta be funny with the fuse box part of this.


    I've pretty much traced the problem back to the ignition switch. I'm only getting 11.8v-12.1v out of the switch.
    Last edited by TheApocalyptican; 06-03-2012 at 04:44 AM.

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