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  1. #1
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    1999 Firebird TransAM WS6

    Slight Rough Idle

    Hello everyone,

    I have a 1999 Firebird trans am m6 with 69k miles that has a slight rough idle i cant seem to find the solution for. Right now it idles at anywhere between 790 and 815, it fluctuates constantly. (mods if this is in the wrong place please move this post to its proper place)

    Mods:

    K&N cold air intake
    borla cat back exhaust

    The troubleshooting i have done is this:
    cleaned maf twice
    cleaned iac twice
    new spark plugs
    msd wires
    iv checked for vacuum leaks multiple times and have not found one yet (i used carb cleaner and sprayed everything pcv/egr related i could reach, no rpm change and i even did that while watching live data and saw no changes at all.)
    i checked the egr valve (a little dirty but not bad)
    Edit* reset ecu (disconnected battery for 25 minutes)
    Edit* replaced pcv valve

    My only other idea i can think of right now are o2 sensors (i have no check engine lights) however i hate to spend money with out knowing they are the problem. I do believe at some point in this cars life it ran nitrous (i found a braided brake line running from the under the pedals all the way to the hatch right where the stock cd changer would have been) however i may be wrong. I have attached data logs in hopes someone can read them and let me know if anything isnt right.

    The logs from the 21st are idling and driving, the logs from today, the 26th are idling only. (i couldnt figure out how to upload the data logs, so here is a link to my google drive where there are, they should be able to be viewed by anyone. https://drive.google.com/drive/folde...DM?usp=sharing)

    Any assistance is appreciated
    Last edited by mbarb55; 05-27-2018 at 08:42 AM. Reason: updated thread

  2. #2
    Veteran pajeff02's Avatar
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    '02 WS.6 / '07 Suburban

    Does it instantly wander at idle when you start the car, or only after it warms up?

  3. #3
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    1999 Firebird TransAM WS6

    It's not instant, it does need to warm up some before it starts wandering.

  4. #4
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    1999 Firebird TransAM WS6

    Just a quick update a friend had a spare pcv valve so I replaced mine, no change. However I also noticed the miss isn't just at idle. I reved to 2k Roma and held it there for about 10 seconds and could feel the roughness there too. I'm not sure what that means. Thanks for the help so far!

  5. #5
    Veteran pajeff02's Avatar
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    '02 WS.6 / '07 Suburban

    Locate your Mass Air Flow sensor (MAF) and unplug the harness connector at the sensor. This will force the PCM into "speed density" mode where it will estimate airflow based upon the MAP sensor readings. If the engine smooths out, you most likely have a defective MAF. Note that this test will set the SES light on the dash, but that the engine will run fine and you can even drive the car to determine if in fact the issue has resolved.

  6. #6
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    1999 Firebird TransAM WS6

    I did that once and didn't notice a change nor a ses light. I will try it again and let you know.

  7. #7
    Veteran pajeff02's Avatar
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    '02 WS.6 / '07 Suburban

    If it is not the MAF, and the idle is not high, then it is likely either fuel or spark.

  8. #8
    Spaz is My Mentor SMWS6TA's Avatar
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    No CEL codes?

    Run a scanner and see if any codes are present, if a tuner got in there, they may have turned off reporting.

    You may need to pull the plugs and see if you see any difference in the plugs.

    Also you could have an injector failing or clogged. Hook up a fuel pressure gauge and see if it is a constant pressure reading or fluctuates.
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  9. #9
    Senior Member Redphoenix1998's Avatar
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    One thing to check also which I have had personally happen is the check valve to the brake booster. If the valve goes bad it could induce a small vacuum leak. I would also check your TB and make sure it's clean. A plugged IAC hole can be the culprit. Also hook a scanner up and check what your O2 sensors are reading. If you're rough at 2k it may possibly be your O2's are slightly out of fuel control. Agree with the other statements above. A clogged injector would not throw a code so pulling your plugs and checking the condition on each should be one of the first things you should do.
    98 WS6 M6 ASC #2647
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  10. #10
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    Thank you everyone. I will update the best I can. So the same friend who had the spare pcv valve also had a new fuel filter he was going to toss (no longer owns his LS1). So I took that and installed it no change. As pajeff02 suggested I unplugged the maf and ran the car, I didnt change. However I did notice that the check engine light never came on. Smws6ta I hooked up both scanners I own, no codes at all. I have the data logs linked in my first post, the O2 sensor data is in there. If you have the stock numbers the o2's should be I can compare them or if you would like you can take a look at the data logs yourself. Also redphoenix I ran the car and sprayed the brake booster check valve with carb cleaner, I didn't hear or notice a change. If there is a better way to test it, let me know. My thought at this point is bad o2 sensors (they are not cheap) or a bad tune on the car. I appreciate all of the help so far.

  11. #11
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    Hey everyone, here's my update so far. Per pajeff02's suggestion i unplugged the Maf and ran the car, i didn't notice any change except when i held the revs this time i didn't notice the roughness. Apart from that it was still slightly rough at idle. Also i did notice that i my check engine light never came on, at all at any point. Per smws6ta i hooked up both scanners and i did have the code for the Maf being unplugged however my check engine light/CEL light never came on. I know that light on the dash works i see everytime the car turns on (when all of the lights are on briefly). Does that mean some tuned around the check engine light being on? My data logs i have linked have the o2 sensor data in them, if anyone has data on what the o2 sensors should look like i can compare them. Also Redphoenix i ran the car and sprayed carb cleaner at the brake booster check valve looking for a vacuum leak, i didn't notice or hear any difference. If there is a better way to check it let me know and will give that a go. Also a friend of mine (same friend with the pcv valve) had a new fuel filter so i installed that also, it didn't help unfortunately. The fuel injectors are next on my list of things to check. I should be able to check them today, once i research the best way to do that. I appreciate the help so far everyone!

  12. #12
    Spaz is My Mentor SMWS6TA's Avatar
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    How are you data logging?


    Are you using HPtuners or Holley EFI Live?


    Do you have a buddy that has a 98-02 Camaro or Trans Am/Formula that you can "borrow" their MAF for a test. Swap theirs over and see if any changes.
    Last edited by SMWS6TA; 05-30-2018 at 08:30 AM.

  13. #13
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    I have been data logging with the Torque app (google play store) and my Bluetooth obd2 scanner. I wish i had HPtuners or Holely EFI Live but i don't quite have the money for that. That said the data torque pulls seems to be useful (hopefully). I unfortunately don't have anyone i could swap MAF'S with, iv been asking. Also the car just threw a p0332 (knock sensor i believe, check engine light still didn't come on.) I saw that code quite some time ago however i assumed (because the check engine light never came on) the previous owner hid the code. I cleared then and at that time drove around and never saw it come back, until now. I dont know if that code could be causing my rough idle or not. I apologize for not spotting the code earlier.

  14. #14
    Spaz is My Mentor SMWS6TA's Avatar
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    You may need to find a tune shop or a local tuner that can go in and see if reporting has been disabled. Knock sensor should have tripped the lightas well as if the MAF is dead.

    Another option but it would mean your car is down is to contact Frost and see if they can look at the ecm to see if anything has been turned off.

  15. #15
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    Sorry to revive a old thread but i have an update. The issue still exists, i have since replaced my 02 sensors (no change) and had a cylinder head replace (my own fault). I have done more data logging since and troubleshooting and have found something interesting. I drove the same route, data logging both times and noticed misfires in every cylinder except cylinder 4, seemingly random with no pattern to discern. On the third data logging pass i went back to my house turned the car off, unplugged the maf and drove the same route again. this time the idle was far better and there were no misfires at all. I know that just because it ran better with the maf disconnected doesn't mean I have a bad maf. I have also noticed fuel smell, which is new, normally only on startup. I am open to suggestions.

    edit: for those who would like them i can provide the data logs for all three drives.

    Thank you everyone who has helped so far and who are continuing to help.
    Last edited by mbarb55; 05-18-2019 at 06:08 AM.

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