Internal leaking intake manifold gasket, confirm my thought process

Discussion in 'Engine Performance + Maintenance' started by andy396, Aug 11, 2019.

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  1. andy396

    andy396 I'm Awesome

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    So can someone please confirm my thought process.

    Background:

    7.4L L29 Vortec, completely rebuild engine.

    Full back story here:

    https://www.gmt400.com/threads/andy’s-1995-cclb-7-4l-tbi-to-vortec-swap.45295/

    After I put about 200 miles on it, I ran it WOT and then coasted down. After that it wouldn’t idle down anymore to the desired idle speed. Confirmed that the IAC is completely closed which is most likely a vacuum leak. It’s now gotten so bad it threw a P0507 diagnostic code. I did a smoke test on it. Blowing smoke into the intake yielded no external leaks. Capping off the left side valve cover port and blowing smoke into the right side valve cover, where the PCV valve goes quickly resulted in an intake manifold full of smoke, billowing out the throttle body.

    With how I had things capped off, I’m thinking there shouldn’t normally be a path for the smoke to get from the crankcase to the intake manifold. If that’s the case, I think I’ve found my vacuum leak, it’s internally leaking, a bad gasket allowing extra air to be pulled through the PCV system.

    If all that is true, my next question is how/why would a brand new molded gasket fail? Obviously, the only way to get a certain answer will be once I get it pulled apart.
     
  2. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    Hard to say - bad valve stem seals? If an intake valve is open could the smoke find its way up? Not really probable, huh?
     
  3. evilunclegrimace

    evilunclegrimace Does not always play well with others

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    If he put enough smoke in it would fill the crankcase and could fill the intake if the lower edge of the gasket was not sealed between the head and intake.
     
  4. andy396

    andy396 I'm Awesome

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    Thanks for the idea. I guess that is a possibility, but it would have to be a super leaky valve stem guide to be this big of a vacuum leak. Or possibly every single guide would have to be super sloppy. The valve stem to guide clearance was within spec when I put it back together.
     
  5. andy396

    andy396 I'm Awesome

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    That’s basically what I’m thinking. I just can’t imagine how a brand new gasket would fail like that?
     
    Last edited: Aug 12, 2019
  6. Schurkey

    Schurkey I'm Awesome

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    Is the intake manifold warped? Hard for a gasket to seal a bent manifold. Less-common for the gasket surface of the head to be bowed.

    Debris under the gasket?

    Gasket not installed properly? Manifold bolts not appropriately torqued? (This is the FIRST thing I'd check.)

    Incorrect gasket?
     
  7. andy396

    andy396 I'm Awesome

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    Given my history on this project, that does seem like the most likely cause.
     
  8. andy396

    andy396 I'm Awesome

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    And the winner is @Schurkey. Apparently I forgot to tighten the intake manifold bolts. I remember putting a little bit of RTV around the coolant ports for good measure and I usually like to let that set a little before giving it the final torque. I guess something pulled my attention away and I forgot to final torque them.

    So this leads me to my next dilemma. Do I just torque them and call it a day or should I finish taking it all apart, clean all the surfaces and replace them?

    I could have this thing buttoned up and running in an hour’s worth of work if all I do is tighten them. I could smoke test it again after tightening it, but that won’t tell me if the coolant ports sealed.

    What’s your opinion?
     
  9. PlayingWithTBI

    PlayingWithTBI Desert Old Guy

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    I wouldn't take any chances, the RTV is now cured in this position, if you tighten it down now will it seal properly? If not, you run the risk of getting water in your oil - not good. I'd start over IMHO.
     
    andy396 likes this.
  10. andy396

    andy396 I'm Awesome

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    The opposite of problem solver!

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