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LZ9 Uplander engine rough/coarse above 4k rpm

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  • #1

    Question : LZ9 Uplander engine rough/coarse above 4k rpm

    Hey guys, I have an '08 Uplander with the 3.9 LZ9 with an Overkill engine/tranny tune.
    Has run great the last 5 years and now has almost 180,000 miles but just lately after getting the AC system repaired the engine has a rough/coarse feeling and resonance sound that's noticeable to hear when the engine hits 4,000 rpm and above.

    No trouble codes and engine feels fairly smooth (no noticeable vibration issue) at idle and up to 4,000 rpm.

    Air filter is pretty new, oil freshly changed and full though the plugs and wires (AC Delco) are nearing 80,000 miles on them since last replaced.

    I didn't know if the 'power valve' in the variable intake manifold could start to malfunction or be an issue without tripping a code possibly?

    It's just the increase in intake/induction noise and the coarseness of the sound at higher rpm that makes me wonder if it's intake manifold related somehow?

    And I've checked the intake tubing, air box, etc for leaks and nothing is loose.

    Just curious if you guys have noticed anything like this before.

    Thanks!

    Jeremy
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  • #2
    Well I finally got time to drop the van off at a local shop while I went to work and finally picked it up. Their verdict: With 179,000 miles on the vehicle, the LZ9s fuel trims are pretty much perfect and no obstruction in exhaust flow based on vacuum tests,etc. So despite a 4mpg drop in fuel economy in the last half dozen fillups at the same Shell station and same driving route each week to work, there's supposedly nothing wrong with the van.
    Oh well lol...

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    • #3
      My car didn't like Shell gas, ran like crap(weak and poor idle). Sounds funny, but gas from other stations didn't do that. Never been back to Shell.
      95 Beretta 3100 with 3400 intakes and TCE TB
      High flow cat and a Magnaflow muffler
      Grand Prix trans with 3.33FDR

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      • #4
        you said it was after an A/C service, right? does it run different with the A/C on or off? when the compressor clutch is disengaged does the problem go away?

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        • #5
          I'm not sure if I've paid attention to that aspect lately, kindof just deal with it now lol. I'll make sure I pay closer attention while driving tomorrow with the AC engaged and disengaged. Thanks for the reminder buddy, I'll report back

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