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Thread: Low Vacuum at Idle - in the tune?

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    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.

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