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Thread: TBI vacuum leak at manifold

  1. #1
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    TBI vacuum leak at manifold

    I have a vacuum leak at the TBI adapter to intake manifold interface. Last night I replaced gasket and if anything made problem worse. I can spray carb cleaner in one specific spot at that interface and RPM dips noticeably (with IAC unplugged it will kill it).

    my setup in a nutshell: 383 with vortec heads, GMPP vortec intake for q-jet spread bore, stock TBI to 4-barrel intake adapter from a 87-90 BBC TBI, 46 mm rebuilt and re-bushed TB, 454 74 lbs/hour injectors.

    Iím using stock style gaskets for the BBC application both above and below the adapter. Theyíre both very thin, but Iím wondering if I should be using a traditional q-jet bas gasket which is much thicker? Any other suggestions?

    Hereís a pic. The leak is right below the pipe plug, center front. Leak is not being caused by the plug - the adapter has coolant passages to improve cold weather operation; Iím not using that feature.

    7DC004F9-92DA-4057-AD3F-42F5C02F62ED.jpg

  2. #2
    Fuel Injected! S.O.B.B.'s Avatar
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    If u put grease like bearing lube etc on the gasket surfaces both sides it makes seeing the leak easy if it won't seal.it sometimes leaves the gasket in non reusable state if paper surfaces peel.but still makes where the leak is easier to see when the TB and adapter are lifted off.I'm using self made adapter because I'm using a dodge TB where the gm one was but I'm using the thin style gaskets with out issue.the grease trick I learned years ago from an older mechanic and I've used it ever since with out any issue.hth
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    87 sjimmy powered by a 92 cadillac 4.9L pfi on 7427pcm /700r4/ 8.8 3.55 rear/disc brakes.

  3. #3
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    Quote Originally Posted by S.O.B.B. View Post
    If u put grease like bearing lube etc on the gasket surfaces both sides it makes seeing the leak easy if it won't seal.it sometimes leaves the gasket in non reusable state if paper surfaces peel.but still makes where the leak is easier to see when the TB and adapter are lifted off.I'm using self made adapter because I'm using a dodge TB where the gm one was but I'm using the thin style gaskets with out issue.the grease trick I learned years ago from an older mechanic and I've used it ever since with out any issue.hth
    Thanks, I may try that. I took the adapter out tonight and it's not obvious where/why it's leaking.

  4. #4
    Fuel Injected! CDeeZ's Avatar
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    Ditch that spacer ring that raises the aircleaner housing up. GM used a drop base aircleaner, but it hit the coil or anything else so they then threw that little spacer choke ring on there. TBI breathes from the side since the injectors and pod hang over the throttle bores. But that spacer chokes off the air from the side. Hypertech sells something called a "powercharger". You can make the same thing out of a salad bowl, that's what I did.

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    As CDeez mentioned, I just went out and bought a salad bowl roughly the size I wanted and trimmed the base out of it and added a rubber strip top and bottom.
    I'd have maybe bought a powercharger but the freight cost to NZ meant making my own was way cheaper.
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  6. #6
    Fuel Injected! CDeeZ's Avatar
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    Looks as good as the powercharger. But you made it so it's better!

  7. #7
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    Quote Originally Posted by CDeeZ View Post
    Looks as good as the powercharger. But you made it so it's better!
    Agreed, that's pretty cool. Do you have an outlet for the PCV system, or do you just use a breather mounted on the valve cover?

    So I'll definitely consider that mod, but I first need to figure out this sucking air problem. I've had the adapter out four times now with different gaskets and/or light coating of grease, but nothing helps. I can actually hear the leak now and can change the sound of it by touching the part of the gasket that overhangs in front. I've come to the conclusion that the 454 adapter I'm trying to use does not bear correctly on the GMPP intake. I don't really see why, but I have no other explanation, the adapter is not warped or gouged. Fortunately I have another option. I also have this aftermarket adapter and I'm going to switch to it. I didn't use it originally because the OEM one has a vacuum port that I was using for my brake booster and there's also a coolant passage to aid in cold weather operation. But it's running fine in cold weather without heat and I'll use the port on the manifold for brakes.

  8. #8
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    I looked at trying to add a hose from the salad bowl to the rocker cover but in the end I just fitted a breather into the rocker cover.
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    Quote Originally Posted by Kitch View Post
    I looked at trying to add a hose from the salad bowl to the rocker cover but in the end I just fitted a breather into the rocker cover.
    Cool, thanks. Yeah, I'd probably go that way too. Looks like you're using the GMPP TBI-specific intake for Vortec? Happy with it? I wish I would have gone that way even though they seem to be a lot more expensive. I wouldn't be having the vacuum leak problems I'm having now.

  10. #10
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    The manifold seems okay, I'd have liked something a little more high rise but the GMPP manifold is the only one I know of with the factory style water passage that links the front of the heads to the back of the heads. It was expensive and then I had to get it bored out for a 46mm TBI unit and block off the EGR ports.
    I also decided to block the EGR ports in the bottom of the runners because I thought fuel might pool in there.
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    Quote Originally Posted by Daveo91 View Post
    I have a vacuum leak at the TBI adapter to intake manifold interface. Last night I replaced gasket and if anything made problem worse. I can spray carb cleaner in one specific spot at that interface and RPM dips noticeably (with IAC unplugged it will kill it).

    my setup in a nutshell: 383 with vortec heads, GMPP vortec intake for q-jet spread bore, stock TBI to 4-barrel intake adapter from a 87-90 BBC TBI, 46 mm rebuilt and re-bushed TB, 454 74 lbs/hour injectors.

    I’m using stock style gaskets for the BBC application both above and below the adapter. They’re both very thin, but I’m wondering if I should be using a traditional q-jet bas gasket which is much thicker? Any other suggestions?

    Here’s a pic. The leak is right below the pipe plug, center front. Leak is not being caused by the plug - the adapter has coolant passages to improve cold weather operation; I’m not using that feature.

    7DC004F9-92DA-4057-AD3F-42F5C02F62ED.jpg
    You say the leak is not being caused by the pipe plug but did not say how you verified that. The adapter is hollow (coolant passage) and pipe plugs are tapered. I am wondering if the plug when tighten, is distorting the adapter? If you were to remove the pipe plug, do you note any changes?

  12. #12
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    Have you thought about trying a different adapter for your TBI. TransDapt, SPR Performance, Holley, and a few others make them. You might check and make sure the base on your adapter is flat and not warped a little bit or cracked and you can't see it. You can also run a raised base air cleaner should clear the distributor, at least it does on mine. Hope this helps.

  13. #13
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    Quote Originally Posted by Eds View Post
    You say the leak is not being caused by the pipe plug but did not say how you verified that. The adapter is hollow (coolant passage) and pipe plugs are tapered. I am wondering if the plug when tighten, is distorting the adapter? If you were to remove the pipe plug, do you note any changes?
    I was using carb cleaner with a straw to direct the spray and it was pretty apparent exactly where the leak was at the interface between manifold and adapter. Possible that the pipe plug was causing distortion, but it was not in there tight and the other night I used a straight edge to check the bottom surface of adapter every which way and it was perfectly flat. The mystery of the leaking adapter may remain permanently unsolved - I switched to a different adapter tonight and problem solved.

  14. #14
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    Yeah, I switched to a different one tonight and it solved my problems. There's a link to the one I used in post #7. The reason I used the OEM one originally instead of this one is the OEM one has a vacuum port big enough for a brake booster. But I just used the one on the back of the manifold for brakes and tee'd into it for the small HVAC control lines. Also had to cut my own gasket - All the q-jet gaskets I found used the inside bolt pattern on the manifold, but the adapter uses the outside bolt pattern. And the gaskets I found that matched the bolt pattern were for other applications that didn't match the TBI bores or the manifold bearing surface.

  15. #15
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    Quote Originally Posted by Kitch View Post
    The manifold seems okay, I'd have liked something a little more high rise but the GMPP manifold is the only one I know of with the factory style water passage that links the front of the heads to the back of the heads. It was expensive and then I had to get it bored out for a 46mm TBI unit and block off the EGR ports.
    I also decided to block the EGR ports in the bottom of the runners because I thought fuel might pool in there.
    Yeah, the q-jet manifold for Vortec is easier to find used. I've bought two now, both only slightly used for about $150 each. Mine has EGR provisions as well, and I'm not using it. I've got a block off plate where the valve goes, but I didn't do anything with the ports in the runners. You've got me a little nervous now - when I had everything off tonight I could see a small amount of fuel down those holes. What did you use to block them?

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