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Thread: Big off idle stumble, whatt first? IFR? calc.vet? Something else I'm missing?

  1. #21
    Joe (Moderator) joecar's Avatar
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    You can test that MAP using a vacuum hand pump (that has a vac gauge) and by rigging up some shop air fittings (probably have to drill a hole) if you have an air pressure gauge.

  2. #22
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    Well with key on engine off it registers high 70s for MAP so I think I got that portion right. My concern is what was said about IFR vs VE/MAF. If I have injector data wrong, VE won't care so much but MAF will. Now I'll admit it needed a calc.vet before the intake and blower because the 411 engine portion from the van has different MAF data due to its different air intake tube/box setup. It was enough that I needed to fudge PE fueling but the trims elsewhere were minimal as I recall with no stumblewhich is why I was lazy and left it as is for so long. It seems to me the truck ought to run about the same as before at idle and low RPM/low load. Should my next step be to put that 4.8988 IFR back in and do a calc.vet? Or is that stumble an indication that I've missed something else? I just want to make some forward progress here and report back if I'm still having trouble.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  3. #23
    Joe (Moderator) joecar's Avatar
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    I would try doing a Calc.VET with the injector data that is more correct.

  4. #24
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    What's the history of this engine? Did it have the stumble before installing the supercharger? Did it have the stumble before switching to a COS. Can you post the original OS tune file you were using before switching to COS.
    Last edited by statesman; June 20th, 2017 at 04:53 PM.

  5. #25
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    It's never had the stumble. The 411 had COS5 before it was ever ran on my truck and in fact it also had the 4L80E segment swapped in before I swapped it in. Truck ran like a dream that way. The most recent changes (intake manifold , blower, 2 bar) are when the stumble started and I did all 3 things at the same time. Unless I screwed something up with the 2 bar, I don't think that that's to blame here as I indicated earlier where the MAP scaling looks to be correct. The intake manifold is damn near identical to the factory piece but it lacks the spider in the path of the airflow through the plenum which NA guys report makes for more pull starting at 3k. Throttle body has a little more real estate between it and the MAF now with the blower. Bypass valve is working properly. Sorry I'm not home to post anything else but I am thinking of unplugging the MAF and going for a spin to see if the stumble goes away. That would indicate that the MAF is extremely unhappy with the most recent changes, am I right?
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  6. #26
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    Quote Originally Posted by Supercharged111 View Post
    Sorry I'm not home to post anything else but I am thinking of unplugging the MAF and going for a spin to see if the stumble goes away. That would indicate that the MAF is extremely unhappy with the most recent changes, am I right?
    If the IAT sensor is incorporated into the MAF, then it's not a good idea to unplug the MAF sensor.

    Just set the MAF to fail in the tune and test it that way.

  7. #27
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    IAT is separate and has been relocated downstream of the blower to read hot, boosted air. It's going to make the trans shift like crap without disabling those transmission parameters, but I probably won't even get off base before plugging it back in and deciding if it helped.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  8. #28
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    Embarassingly enough I have no gas money to burn on a calc.vet right now, but one observation I made is that unplugging the battery to weld on the truck seemed to erase the IAC trims or something because when I restarted the truck it would barely idle, stumbling around under 500 RPM for a bit. Go for a spin, come back, and everything's fine. Not sure if that's an indicator I should concern myself with or not.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

  9. #29
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    Sounds like you need to do some idle tuning.

  10. #30
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    Yeah I wonder why though? I don't think the intake or blower affected idle VE.
    1998 GMC Sierra K1500 5.7/4L80E, longtubes, 411 w/COS 5, marine cam/intake, Whipple. 91 octane at 6000'.
    1997 GMC Sierra K3500 7.4/4L80E, 411 w/COS 3, Whipple, small cam.
    2004 Corvette Z06 with longtubes.

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