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Rough, lopey idle

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  • Rough, lopey idle

    My small-port 3100 developed a rough idle and sounds like it's aggressively cammed (oh how I wish it were true). Only happens when the engine is idling, in gear or not. My vac/boost gauge is reading a few inches of vacuum less than usual but the RPM is a little lower than normal and sometimes borders on stalling. During acceleration or cruising the engine runs fine but there is a noticeable drop in low-end power.

    Any ideas? I thought vacuum leak at first but that usually results in a high idle. Thanks in advance.
    1995 Grand Am SE

  • #2
    have ya looked at your spark?
    maybe coil/wire/plug is bad

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    • #3
      The front, three plugs look fine and all the wires make a good connection. There's no hesitation while accellerating so there isn't any misfire that I can see. I'm going to check the IAC valve tomorrow and change the fuel filter if I have time. I did fill my gastank a few days ago too. So it could be just a bad bit of gas.
      1995 Grand Am SE

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      • #4
        could be a leaky injector.. my old 3.1 did that all the time..
        Past Builds;
        1991 Z24, 3500/5 Spd. 275WHP/259WTQ 13.07@108 MPH
        1989 Camaro RS, ITB-3500/700R4. 263WHP/263WTQ 13.52@99.2 MPH
        Current Project;
        1972 Nova 12.73@105.7 MPH

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        • #5
          Would the 24x sensor cause it to run that bad? I know the 24x is used for idle and low RPM smoothness...
          sigpic New 2010 project (click image)
          1994 3100 BERETTA. 200,000+ miles
          16.0 1/4 mile when stock. Now ???
          Original L82 Longblock
          with LA1, LX9, LX5 parts
          Manifold-back 2.5" SS Mandrel Exhaust. Hardware is SS too.

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          • #6
            Thanks guys. It turned out to be an easy fix. The minumum throttle set screw on the throttlebody (an old billet FDP unit) came lose and bottomed out, letting the butterfly plate close too much. The IAC must have went crazy trying to compensate. Added some loctite and set it back to where it was. A little bit of driving later and the idle and vacuum returned to normal.
            1995 Grand Am SE

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            • #7
              cool!
              Past Builds;
              1991 Z24, 3500/5 Spd. 275WHP/259WTQ 13.07@108 MPH
              1989 Camaro RS, ITB-3500/700R4. 263WHP/263WTQ 13.52@99.2 MPH
              Current Project;
              1972 Nova 12.73@105.7 MPH

              Comment

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