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Thread: Vacuum Leak ?

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    Member G Winter's Avatar
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    Default Vacuum Leak ?

    How do I locate a vacuum leak on the L98 if I have one?


    Glenn

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    sometimes you can hear them, sometimes you can go through the pull it off and see what happens routine. Do you have a manifold Vacuum guage? that might help.

    I will cheat, I have a TIF6500 vacuum leak detector that I got when I was a Mfg Rep for them in the mid 90's. I have never used it, but I am going to use it on the vette this weekend,. I also ended up with about 10 or 12 different instruments that are all for cars in the 90's, never thought I would need them but I am glad I kept them.

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    Quote Originally Posted by G Winter View Post
    How do I locate a vacuum leak on the L98 if I have one?


    Glenn
    use a plumbers propane torch with a hose. turn on the propane. (don't light it of course) and move the hose close to all the fittings, vac hoses, throttle body etc, if the motor races up there is the vac leak

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    Now that I think of it, I used WD40 and would spray it aroung the area I thought was the problem. Used it around the Carb, or the Intake. I forgot about that. The engine will increase in rpm's if is sucking the fluid in.

    We used WD 40 , I guess you could use Carb cleaner.

    Be Carefull!!

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    Member G Winter's Avatar
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    I had the plenum off my 90 and now the idle pulses a little and it jumps to about 2500 rpm on startup then drops down to near normal speed. It also does not drop RPMs when I shift gears. Seems to take a bit for it to slow. I need to find out if the plenum to runner gaskets are leaking or if it is the lower injector O rings.
    Speaking of hearing it , I do have a sound of air but thought it was coming from the air pump.

    Glenn

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    start it up and spray wd40 around the flange.

    Aluminum is tough to seal. Did you tighten it using a torque sequence?

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    Member G Winter's Avatar
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    Yep, used torque sequence. The problem was that the
    runners fit pretty tight putting the plenum in. Afraid
    I may have damaged the gasket.
    May have to study this to make a tool to spread the runners just a little to install the plenum.

    I really don't think it is the injectors leaking , if it was I would think I would have a fuel leak also.

    Glenn

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    Quote Originally Posted by G Winter View Post
    Yep, used torque sequence. The problem was that the
    runners fit pretty tight putting the plenum in. Afraid
    I may have damaged the gasket.
    May have to study this to make a tool to spread the runners just a little to install the plenum.

    I really don't think it is the injectors leaking , if it was I would think I would have a fuel leak also.

    Glenn
    i have damaged the plenum gaskets a few times in my day.. sucks air in the bottom.. did you check the iac? the pintle sticks if you tried to clean the TB without removing the IAC. all that junk washes down in the little hole behind the Blades.. Don't forget the IAC is a controlled vac leak..and it must work properly for the engine to idle correctly.

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    Quote Originally Posted by G Winter View Post
    How do I locate a vacuum leak on the L98 if I have one?


    Glenn
    get out your oxy-acetolene torch or a propane torch, don,t light it but turn it on so gas is flowing and place the tip near any suspected leak points, while a buddy watches the tach, anytime the gas gets sucked thru a crack the rpms will jump higher in direct relation to the amount of extra fuel the engines getting, no, oil, fluid,or cruds or stains are left on the intake and its simple too do process

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    Member G Winter's Avatar
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    I tried the propane torch trick by myself and couldn't hear
    any diff in engine rpm. With my exhaust I would think I would hear it. Also tried carb cleaner, nothing. I do now have this odd buzz just after the engine stops.Enough vacuum in the
    plenum that I would here the leak?

    Glenn

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    I've heard 1 way vacuum check valves make a buzzing sound immediately after shut down,even heard one sound like a Swianett!!
    Quote Originally Posted by G Winter View Post
    I tried the propane torch trick by myself and couldn't hear
    any diff in engine rpm. With my exhaust I would think I would hear it. Also tried carb cleaner, nothing. I do now have this odd buzz just after the engine stops.Enough vacuum in the
    plenum that I would here the leak?

    Glenn
    It's got so it seems to me that the Internet has created a lot more Horses A$$'s than we have Horses!
    junk!~!!
    Founding Member: 10 Corvettes Anonymous


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    Do not force spread the runners they are aluminum and will buckle. Just remove the top runner bolts and loosen one side of the bottom and wack the plentum with a rubber mallet and lift out.

    Quote Originally Posted by G Winter View Post
    Yep, used torque sequence. The problem was that the
    runners fit pretty tight putting the plenum in. Afraid
    I may have damaged the gasket.
    May have to study this to make a tool to spread the runners just a little to install the plenum.

    I really don't think it is the injectors leaking , if it was I would think I would have a fuel leak also.

    Glenn

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    Default Engine numbers

    Hello Junk- Have a question for you. I have an 88 C-4 and the engine has been changed to a 350 HO 355HP Chevy crate motor. Where would I find the engine numbers so I can figure out the specs/yr. for the engine I have ?


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    No one got the proper technique for vacuum leak detection...

    FIRST; block the pedal, or block the throttle linkage. YOU HAVE TO DO THIS TO GET A STEADY IDLE , around 1,250 rpms, or a little higher. If you don't, you can fish a propane hose, or WD40, or Krylon, or hair spray, or pee, anything, until you need a sweater in Hades, but if the motor is lopin' already from a vacuum leak, you ain't gonna' find any vacuum leak, unless it's so big that Little Jack Horner can stick his thumb in it and pull out a pineapple NOPE.

    AFTER you get a steady idle, THEN, you can spray starting fluid everywhere - that's the best detector, because it's vapor pressure is very high - a LITTLE at a time, until you get the SURGE. Follow the vacuum lines to the EVAP cannister too. You might even have to follow the EVAP lines from the EVAP cannister to the gas tank too, but that vac leak will show up as a DTC 32 EGR fault, and won't normally show as loping idle, unless the EVAP cannister has been by-passed (been there, saw that uh-huh).

    If you get a surge in a tight spot, and can't tell exactly where it is, light a book of matches, blow out the match heads while they're still burnin', and feed the smoke to the tight spot.

    Fear not the starting fluid; you cannot put the can down fast enough and light the matches fast enough to catch the starting fluid, because the vapor pressure is so high.

    Bookmark this post YUP.

    Wrench Wizard OUT!

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    I don't know if you can rent this or not, but I have a TIF 6500 Vacuum leak detector with the 6501 transmitter and it works great, no guess no flammable liquids.

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