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Still have a strange cold start

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  • Still have a strange cold start

    I have a strange issue that is still lingering even after a complete fuel system overhaul (new fuel pump, power-cleaned injectors and throttle body).

    On a cold start, it will fire up and rise to 1100RPM, but drops to 800RPM and stay there for 1/2 second, and finally rise back to 1200RPM and stay there until warm, then go back to 800RPM (normal warm idle). The idle speed pretty much stays put and never stalls or surges except that little bit on a cold start, but I do notice a little roughness at a warm idle at 800RPM (tach needle doesn't move but I do feel occasional "jolts" as the engine idles). When I drive normally (or drive at WOT), car runs awesome. No stumble or hesitation as I press the gas pedal. No SES light either.

    Any ideas? Already tried another MAF sensor with no change. Does the throttle body power clean service include the IAC valve too? The car used to just go straight to 1200RPM on a cold start (problem did exist before having all this work done). Thanks!

  • #2
    any ideas on this one? Warm starts are still normal (revs up to 1400rpm before falling back to 900rpm). Cold starts used to be like that, but went to 1200rpm cold idle vs 900rpm hot idle. Now on a cold start it revs to 1200rpm, briefly drops back to 900rpm for 1/2 second, then go back to 1200 and is stable after that. Seems to run fine though.

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    • #3
      Re: Still have a strange cold start

      Check all ur vacuum lines around throttle body. One may have a small crack and be messin with cold temp readings. If all good, then try a new iat sensor. But I feel quite sure u are pulling vacuum somewhere.

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      • #4
        Scan it and tell us what the idle solenoid is doing.... Or just get a known good used idle solenoid from the forum and swap it out to test it.

        You say you cleaned the TB. Did you actually take it apart to clean it? If not, you could of worsened the problem by letting gooped up cleaning fluid setting in the idle circuit. Proper cleaning requires removal and disassembly.
        Tracy
        2002 C5 M6 Convertible
        1994 Z28 M6 Convertible
        Current Mods:
        SLP Ultra-Z functional ramair, SS Spoiler, STB, SFCs, Headers, Clutch, Bilstein Shocks, and TB Airfoil. 17x9 SS rims with Goodyear tires, 160F T-Stat, MSD Blaster Coil, Taylor wires, Hurst billet shifter, Borla catback with QTP e-cutout, Tuned PCM, 1LE Swaybars, 1LE driveshaft, ES bushings, White gauges, C5 front brakes, !CAGS, Bose/Soundstream audio, CST leather interior, synthetic fluids

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