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Thread: Low Vacuum at Idle - in the tune?

  1. #1
    Lifetime Member Kevin Doe's Avatar
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    Default Low Vacuum at Idle - in the tune?

    I have low idle vacuum, and I'm trying to get to the bottom of it. My latest thought is that its in the tune. I'm seeing ~80 kPa at idle, however during part throttle cruising I often see vacuum in the 30-40 kPa range.

    Here is what I've done.

    I took off the intake manfold and replaced all 8 port gaskets, throttle body gasket, injector o-rings, MAP sensor seal. I started the engine and allowed it to warm up. My idle vacuum was still very low (or high kPa). That lead me to remove and plug the two PCV type lines. First I removed and capped the one from the valve cover. Then I removed and plugged the one from the lifter valley. No change in vacuum, so I reconnected them. I inspected my manifold for cracks also. I feel very confident that there is no leaks in the intake system now. I have drilled my throttle body plate to a larger hole. I went 1/32 larger diameter than stock. I did this because I couldn't get the idle steps low enough at warm idle by only using the set screw/tps reset method.

    I have attached my latest log, where I was logging some AutoVE stuff to get the low rpm area tuned. Basically I'm driving around in 1st through 4th gear just crusing. You can see the vacuum characteristics when I'm crusing in that log. There isn't a good idle in that log, since I'm havin trouble having it come into idle when I stop, so I have just been feathering the gas for the time being. I'll also attach an idle log so you can see idle vacuum.

    My latest thoughts are that the low vacuum at idle is due to idle tuning issues. Does the tune have a large effect on idle vacuum? Possibly increasing timing and decresasing the desired airflow table will bring vacuum back to normal? I'm not sure, just throwing ideas out there. During cruising the vacuum seems very good and normal. Here are my cam specs:

    Its a TSP MS4: 239/242, .649"/.609" on a 111 LSA.

    My engine is a 2004 GTO, so it has the LS6 manifold. My exhuast consists of 1 7/8" long tubes to a true dual 3" exhaust through an x-pipe. The intake consists of a dual 3.5" setup, each going to a filter behind holes in the bumpercover. I am running in open loop speed density right now. I have no MAF sensor at all.

    If you require any more details, please let me know.


    Attachment description:
    SDAutoVEFixed_0004.tun - Tune used on those logs.
    Log_0033.efi - Idle warmup log. From cold engine to operating temp.
    Log_0035.efi - Cruising log. 1st through 4th gear, about 10 minues worth. I was logging to do some VE mapping.
    Attached Files Attached Files
    Last edited by Kevin Doe; October 25th, 2007 at 04:09 PM.

  2. #2
    Lifetime Member Kevin Doe's Avatar
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    And one more attachment. Here is my proposted next tune I'll be uploading on the car.

    I applied the BEN factors, and did quite a lot of manual smoothing, then used the smoothin button afterwards. Then I added 3% to those cells to make sure I'm not over a value of 1.0. I'd rather take baby steps.

    Then I modified the table B3605 to command richer values in the high kPa (high load) columns.

    I also returned the B3618 curve to stock GTO settings. This will allow for safer ratios during some of the high rpm, high load VE mapping.

    I also made a few small tweaks here and there as well.
    Attached Files Attached Files

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    What sort of vaucuun did you expect with a big cam like that , 80-90kpa is about all I would expect.

    Use the custom OS and turn TPS mapping on , thats what its designed for

  4. #4
    Lifetime Member Kevin Doe's Avatar
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    I have very little expereince with large cams and V8s in general. I come from the DSM tuning world, where cams are relatively small and tame.

    From what I read, and people have said, my 80 kPa isn't enough vacuum. Maybe they just weren't aware how agressive my cam is.

    I cannot use the COS5, I have a 1MB flash, its a PCM from a 2004 GTO. The only COS that I can use is COS3, and I was instructed to get a good tune with the stock OS, then upgrade later.

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    You can probably eek out a few more kPa of vacuum at idle by increasing the idle spark advance. Use the bidirectional controls while logging to find the timing needed for best idle quality.

    A bump in idle rpm would go a long ways as well.
    2013 Yukon Denali XL - TVS2300, ID1000, E85
    2013 Silverado CCSB 6.2L - TVS1900, DTH 349Y-1-C
    1972 K5 Blazer - LQ4, TVS2300, 4L80E

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    I'm confused. Why can't you use COS5?

  7. #7
    Joe (Moderator) joecar's Avatar
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    Hmmmm.... Might be a good reason to switch to COS5 (TPS VE table).

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    COS5 is only available for 2001 and 2002 operating systems (512kb PCMs). He has a 1Mb 2004 PCM.

    Justin
    2013 Yukon Denali XL - TVS2300, ID1000, E85
    2013 Silverado CCSB 6.2L - TVS1900, DTH 349Y-1-C
    1972 K5 Blazer - LQ4, TVS2300, 4L80E

  9. #9
    Joe (Moderator) joecar's Avatar
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    That's only 6"Hg of vacuum... that cam is big, but I would have expected a little more vacuum.

    Edit: Hmmmm.... maybe that's all the vacuum you'll get.
    Last edited by joecar; October 26th, 2007 at 03:41 AM.

  10. #10
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    Quote Originally Posted by dc_justin
    COS5 is only available for 2001 and 2002 operating systems (512kb PCMs). He has a 1Mb 2004 PCM.

    Justin
    My confusion deepens:
    http://forum.efilive.com/showthread.php?t=4683&page=2 See posts 17 & 18

    http://forum.efilive.com/showthread.php?t=2599&page=5 See post 48


    I haven't tried moving to COS5 but that has been my future plan, in conjunction with nitrous...

    EDIT: The '04 GTO uses an Aussie build, remember. (w/ Lean Cruise & all...)

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