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Idle suddenly drops when rolling to a stop in neutral???

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  • #1

    Idle suddenly drops when rolling to a stop in neutral???

    I finally got the TA on the road today for the first time in months, and noticed something. When I start it and have it running in neutral, all is fine. Idles at around 800 RPM's. Drives fine from what I can see, pulls hard, no stumble. BUT, as I'm coming to a stop, and either push and hold the clutch in or have it in neutral as car is still rolling, the RPM's drop down to like 500, even 400 RPM's???? And it will stay down there until I've come to a complete stop for a couple of seconds. Then all of a sudden (as though the throttle was blipped) it jumps back to 800 RPM's and stays there per usual??? Wierd for sure. It doesn;t stall out or anything, but is annoying as hell. Because it doesn't happen all of the time (but very often) it's odd trying to launch with a varying RPM. Plus it's odd when it sounds like the car is off and has stalled, but really hasn't. I find I'm always staring at the dash now as I come to a stop...

    Thanks for your help guys.
    BTW, the TA is a 93, 6 speed, 161,000 km's, and runs fine otherwise.

    Cheers,
    Steve
    SOLD: 1993 M6 Trans Am, black with ZR1 rims, Borla cat-back, Random Tech cat, SLP headers and Y, Morosso CAI
    Tags: None
  • #2
    hmmm...

    Taking a guess, but I would check the TPS sensor, and scan it to see if there are any codes. 800 rpm idle sounds kinda high to me, is there any stumbling, rough idle?
    1995 Firebird Formula
    (A4 3.23)Ram Air w/ K&N, TransGo Shift Kit w/3 washers ,SLP~Shorty headers, LoudMouth, Air Foil, LineLocks. Catco Cat. Hypertech 160. TB Bypass. MSD~8.5mm. Walbro 255lph. FT MAF ends. LT4KM. BMR~Adj LCA's, Rear sway, Panhard rod. HAL QA1 rear shocks. Random Tech adj TQ arm. Nitto NT555R's. Covered in Zaino. 13.466@103.85 N/A. [1.948 60' 13.410 @ 102.66 11/20/05]

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    • #3
      I think you can check the TPS with a voltmeter. Probably cheap enough to replace it anyway.
      1995 Firebird Formula
      (A4 3.23)Ram Air w/ K&N, TransGo Shift Kit w/3 washers ,SLP~Shorty headers, LoudMouth, Air Foil, LineLocks. Catco Cat. Hypertech 160. TB Bypass. MSD~8.5mm. Walbro 255lph. FT MAF ends. LT4KM. BMR~Adj LCA's, Rear sway, Panhard rod. HAL QA1 rear shocks. Random Tech adj TQ arm. Nitto NT555R's. Covered in Zaino. 13.466@103.85 N/A. [1.948 60' 13.410 @ 102.66 11/20/05]

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      • #4
        800 rpm idle is perfect on the M6 cars, I would guess that the EGR valve might be sticking open or you have a vacumn leak somewhere...PCV valve seated properly?

        Could also be a dirty MAF sensor, might want to check that.

        -- Flash

        Flash - '97 Black WS/6, Intercooled Powerdyne 11# , LTCC Coil Per Cylinder Ignition, OPTI High Voltage Deleted, 160, Short Stick, BMR-STB, adjustable Shocks, PP+ w/scan, !CAGS, Flowmaster 3" cat-back... 43k miles as of Aug. 2006

        **-** BOOST, it's no replacement for displacement, and there's definitely no replacement for having both!

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        • #5
          He has a 1993. It doesn't have an MAF sensor.

          800rpm is the factory programming for a warmed up engine, with the M6.

          550rpm is the factory programmin for a warmed up engine, in gear, with an A4. 650rpm in neutral.

          The PCM is relatively insensitive to closed throttle TPS voltage. That is why it isn't adjustable on the LT1. The PCM will check closed throttle voltage at startup, set that as 0% throttle position, then pro-rate up for 100% TPP at approx. 4.0V above the closed throttle setting. The PCM will accept any value between 0.20 and 0.90V as closed throttle voltage. Some people feel that 0.60-0.67V provides better performance, but something different thatn that, as long as it is within specs should not cause an idle problem.

          Lack of smooth TPS voltage change through the range of rotation can cause a problem, and if the throttle blades are hanging up when they come closed, then release, it could cause the problem. Check to smooth throttle shaft rotation. Check that it isn't hanign up on closing.

          I would look at the IAC (idle air control) motor. That is what controls idle speed. You need a scanner to see how the PCM is adjusting the IAC "counts" (its a stepper motor) in response to the problem. The valve may be sticking and moving too slowly to keep the idle under control with a sudden change in throttle postion. Add to that the fact that the PCM keeps the idle speed much higher when the car is still moving (VSS not equal to "0") and you see the need for a very fast response from the IAC motor when the car acutally stops and the VSS output drops to "0". The IAC motor and the passages that supply air to it (the small hole at the bottom of the divider between the throttle blades) should be cleaned.
          Fred

          381ci all-forged stroker - 10.8:1 - CNC LT4 heads/intake - CC solid roller - MoTeC engine management - 8 LS1 coils - 58mm TB - 78# injectors - 300-shot dry nitrous - TH400 - Gear Vendor O/D - Strange 12-bolt - 4.11's - AS&M headers - duals - Corbeau seat - AutoMeter gauges - roll bar - Spohn suspension - QA1 shocks - a few other odds 'n ends. 800HP/800lb-ft at the flywheel, on a 300-shot. 11.5 @ 117MPH straight motor

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          • #6
            You're right on Injuneer, my bad, scratch the MAF idea. Just had one other thought, I've seen just plain bad (watered etc...) gas cause this kind of behavior also... might want to treat or re-fill the tank since it's been sitting a while.

            -- Flash

            Flash - '97 Black WS/6, Intercooled Powerdyne 11# , LTCC Coil Per Cylinder Ignition, OPTI High Voltage Deleted, 160, Short Stick, BMR-STB, adjustable Shocks, PP+ w/scan, !CAGS, Flowmaster 3" cat-back... 43k miles as of Aug. 2006

            **-** BOOST, it's no replacement for displacement, and there's definitely no replacement for having both!

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            • #7
              My money is on bad IAC and or dirty IAC passages
              96 WS6 Formula: Ram Air, 383 Stroker, Ported LT4 Heads and Manifold, 1.6 Crane Rollers, 58MM T.B., AS&M Headers, Borla Exhaust, Meziere Elec. H2O Pump, Canton Deep Sump Oil Pan, 100 HP OF TNT N2O!! , T56 Conversion w/ Pro 5.0 shifter, SPEC Stage 3 Clutch, Hotchkiss Subframe Conn., Lakewood Adj. Panhard Bar, Spohn Adj. LCA's, BMR Adj. T.A., Custom 12 bolt w/ 3:73's, Moser Axles, Eaton Posi, Moser Girdle
              11.6 @ 123mph (1.6 60' - getting there )

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              • #8
                I've had this problem for awhile i was wondering what it is.
                ______________________________
                94 Camaro Z28 T56

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                • #9
                  Also check your Alternator

                  Another thing is that your alternator maybe cutting out a low RPM's. Check the volt meter when it drops like that again. If the meter drops below 12V then the alternator is droping out and your battery is taking over for a moment.
                  '95 Formula A4, 17"x9" wheels, F1 275/40-ZR17, TB bypass, 160 deg. TS, LS1 drive shaft, WS6 muffler, LCA, tie rod Soon to be LT4 heads intake and HOT cam.

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                  • #10
                    COOL! Thanks guys! I'm going to try and diagnosis this problem this weekend. It's really wierd because it isn't happening all of the time. I'm sure everyone knows how annoying the "little things" can get. Thanks again and I'll keep you guys posted on what it was...

                    Cheers,
                    Steve
                    SOLD: 1993 M6 Trans Am, black with ZR1 rims, Borla cat-back, Random Tech cat, SLP headers and Y, Morosso CAI

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