TBI vacuum leak

[Daveo91Burb]

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I’ve discovered a vacuum leak between my TB adapter and intake manifold. I’m using the adapter that came stock on the early TBI big blocks (GM didn’t come up with a TBI specific intake until 91 - before that they just used a q-jet one with this adapter). The OEM gasket at that interface is very thin and I replaced with brand new one this weeked; did not help at all. I can spray carb cleaner right below the pipe plug in the pic and rpm dips noticeably.

I’m thinking about just trying a typical q-jet base gasket which is much thicker, like 1/4”. Thoughts on that? Other suggestions?

Note that the pipe plug is not the source of the leak. That just closed up a coolant passage that I’m not using. They put it there to aid cold weather operation but I don’t seem to need it.

 

[4WDKC]

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I’ve discovered a vacuum leak between my TB adapter and intake manifold. I’m using the adapter that came stock on the early TBI big blocks (GM didn’t come up with a TBI specific intake until 91 - before that they just used a q-jet one with this adapter). The OEM gasket at that interface is very thin and I replaced with brand new one this weeked; did not help at all. I can spray carb cleaner right below the pipe plug in the pic and rpm dips noticeably.

I’m thinking about just trying a typical q-jet base gasket which is much thicker, like 1/4”. Thoughts on that? Other suggestions?

Note that the pipe plug is not the source of the leak. That just closed up a coolant passage that I’m not using. They put it there to aid cold weather operation but I don’t seem to need it.

Id check that plug just to make sure, second I would take fine sand paper and a true block or sandstone and move over the surface of the adapter to identify where the issue is. When I did my truck I just used the cheap flat metal plate from transadapt. I check the length of bolts and spaced them accordingly to prevent them from hitting the intake and lifting the plate.

 

[bucket]

You could probably use one of the flat plates that Edelbrock sells, with two gaskets obviously. It gives better support to the areas with less coverage. Is that a square bore intake?

 

[Daveo91Burb]

Id check that plug just to make sure, second I would take fine sand paper and a true block or sandstone and move over the surface of the adapter to identify where the issue is. When I did my truck I just used the cheap flat metal plate from transadapt. I check the length of bolts and spaced them accordingly to prevent them from hitting the intake and lifting the plate.

Thanks. Yeah, got the adapter out tonight and went over the sealing surface with scotchbrite. Found one nick in that area that caused a tiny raised bump. I wouldn't think that would be the culprit, but I couldn't find anything else.

You could probably use one of the flat plates that Edelbrock sells, with two gaskets obviously. It gives better support to the areas with less coverage. Is that a square bore intake?

So I actually have a flat plate adapter that I bought off ebay several years ago, but I've never used it. I decided to use the OEM big block adapter since it has a vacuum port that I'm using for my brake booster. The manifold only has one vacuum port so anything else vacuum uses that, brakes from the adapter. I probably could put everything on one port, but it just made me a little squeamish. There's also a small port on the front of the TB that I'm not using. I could rearrange my vacuum lines and try that, but I'm still hoping to get the OEM one to seal.

Bottom surface of the adapter, after scotchbrite:

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Gasket I'm using. Leakage occurring at below the pipe plug, by the adapter part number.

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Here's my flat plate from ebay. I realized tonight I'm not sure what base gasket you're supposed to use with these... the OEM adapter one is not right, nor is a typical q-jet base gasket.

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[bucket]

I was talking about just the flat steel plate with a square opening, it has both bolt patterns in it. They are generally used for putting a square bore carb on a standard Performer manifold. But I've found they work well in most situations where there's any carb surface that doesn't match up well. It could be run between the manifold and the factory adapter.

But I figured the issue was on the sides, not that area with the part number. To leak there, it must be warped. Maybe a quick shave at the machinist's will take care of it.

 

[4WDKC]

you have to use the gasket to match the intake, since you have a spread bore intake based on using the 454 adapter, you should use that gasket on bottom and a tbi gasket on top.

 

[Daveo91Burb]

I was talking about just the flat steel plate with a square opening, it has both bolt patterns in it. They are generally used for putting a square bore carb on a standard Performer manifold. But I've found they work well in most situations where there's any carb surface that doesn't match up well. It could be run between the manifold and the factory adapter.

But I figured the issue was on the sides, not that area with the part number. To leak there, it must be warped. Maybe a quick shave at the machinist's will take care of it.

Thanks. I’ll check for warping with a straightedge tonight. I’m also thinking about a light bead of sensor- and gas-safe silicone in the problem area. Would that work?

 

[Daveo91Burb]

you have to use the gasket to match the intake, since you have a spread bore intake based on using the 454 adapter, you should use that gasket on bottom and a tbi gasket on top.

I agree. Manifold is spreadbore and has two bolt patterns. A q-jet and the oem adapter use the inside bolt pattern but the eBay adapter uses the outside bolt pattern and I’m not sure how to specify that when trying to buy the bottom gasket. Maybe the gasket for the adapter that @bucket mentions above?

 

[bucket]

When I made my first suggestion, I assumed it was leaking on the sides.

If it checks out with a straight edge, you could maybe try stacking two gaskets. I've never done it, so I don't know how reliable that would be. But I would try that before rtv.

 

[Rusty Nail]

How did it start keaking?

I mean wtf? It happened magically or what? Out of the clear blue sky it all of a sudden deceloped a vacuum leak?
Under what condition AND what had you done just prior?

 

[Daveo91Burb]

This is a relatively new build - it’s probably been there since the beginning. A small vacuum leak on TBI is not very noticeable. My idle was occasionally high but other time normal and it didnt hunt - I was diagnosing the odd idle and the bigger more noticeable surging on deceleration when I decided to check for vacuum leaks with carb cleaner - bingo. My bad for not checking earlier but I’ve never had this problem with brand new gaskets.

 

[Daveo91Burb]

When I made my first suggestion, I assumed it was leaking on the sides.

If it checks out with a straight edge, you could maybe try stacking two gaskets. I've never done it, so I don't know how reliable that would be. But I would try that before rtv.

It passed straight edge test with flying colors, no warping at all that I could see. Cleaned up mating surface real nice and bolted it back to manifold with single gasket, dry. Used a torque wrench to get all four bolts torqued same. Had the TB completely off and out of the way (last time I just slid new gasket in and bolted it up). I’ll try this one more time and if it still leaks I’ll try the two gaskets or just go to my other adapter. Thanks

 

[Daveo91Burb]

Well it still leaked after all that so last night put on the other adapter and did some vacuum line reconfig. Both brake booster and small lines for HVAC controls run from big port on back of manifold now. Put it all together and no leaks. The mystery of the leaking OEM BBC adapter may never be solved.

 

[bucket]

Well that's an odd one. Didn't you have that flat adapter for sale? Good thing you still had it!

 

[Daveo91Burb]

Well that's an odd one. Didn't you have that flat adapter for sale? Good thing you still had it!

Yep, I did post it up here a while ago, before I knew there was a vacuum leak. Good thing it didn’t sell!