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Vacuum tube to "Accessory"?

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  • Vacuum tube to "Accessory"?

    Good afternoon everyone. I have a problem with my 1998 Pontiac Firebird (V6 Automatic). From the Vacuum diagrams under the hood and the matching diagram from my Chilton there is a U-shaped vacuum tube that connects the Fuel Pressure Regulator to the Manifold Vacuum Source. I purchased my Firebird when she was at 119,000 miles and don't know anything about Wynona's history except that she was owned by two women at two different times. Anyway Google searches give me the impression that this tube is normally not even on the car unless there is some sort of performance-enhancing accessory, and that the FPR is just capped off at its end, but the information was vague and unverified. When I first saw this tube it was connected to the FPR but the rubber connector on the MVS had apparently split and it was disconnected on this end. At this point there were no engine problems. It was oozing miniscule amounts of gasoline from the FPR onto the top of the engine after driving. I went to Autozone and replaced the tubing and rubber connectors, at which point a curious problem developed. Originally upon starting the engine cold it would immediately rev to about 2K RPMs, dropping after a couple seconds to 1.5K until dropping to 1K once warm. This has not changed since I replaced the tube; what has changed from immediately afterward is that, when warm, the engine hard starts. It takes a couple seconds to begin firing, which can be shortened by pressing down a bit on the gas. Once it does begin firing (whether pressing the pedal or not) it bounces in revs from 1.5K to .5K RPMs repeatedly, either dying or eventually stabilizing at 1K after a few more seconds. I also now detect a very slight hesitation in acceleration while driving, whether from a stop or a sudden speed increase. To be honest I haven't even rudimentary knowledge of engine performance add-ons, but I'm almost certain the engine is still stock, so that any such "accessory" isn't even present.

    Also, before I replaced this tube I was getting an engine code telling me that there was a problem with my airflow, the symptom of which was that after the engine was warm and it was placed in park it would sometimes idle at 1.5K RPMs instead of 1K which I presume is normal. The engine icon has vanished along with this problem, replaced by the one I just described. Can someone give me possible explanations to this problem? I can of course give more information if needed, though I feel like I already overdid it, lol.

  • #2
    Not an expert on what is happening. On my 97 TA it has that hose stock from the FPR to the manifold.

    This file here shows what you should have.

    http://s95135784.onlinehome.us/Pages...2_FIREBIRD.pdf
    97 Trans Am A4 more or less stock (Mods: WS6 Ram Air with Fernco & K&N, 12 disc CD changer, power antenna, SLP Fan Switch, LS1 Aluminum DS, Borla Cat back, McCord power plate, Spohn tower brace, Sirius, HID fog lights)


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    • #3
      Right and what is indicated by 11 as a tube for regulating fuel pressure with a vacuum from the vacuum source is what I replaced exactly as shown, which was found disconnected. The problem has persisted and perhaps gotten slightly worse. I am considering just removing it again as reconnecting it seems to have done nothing but cause problems. Perhaps the Manifold Vacuum Source is not working? I don't understand how it creates a vacuum to begin with, and if it wasn't working wouldn't I get a code that would turn on the Service Engine Soon light? I've been reading up on manifold vacuum and it seems to indicate that a lack of vacuum should make it harder to start, not the other way around.

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      • #4
        In case you have not been to this website here, there's lots of good info that may help.

        http://shbox.com/1/4th_gen_tech1.html
        97 Trans Am A4 more or less stock (Mods: WS6 Ram Air with Fernco & K&N, 12 disc CD changer, power antenna, SLP Fan Switch, LS1 Aluminum DS, Borla Cat back, McCord power plate, Spohn tower brace, Sirius, HID fog lights)


        1and1 Web Hosting

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