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Was that on just one cylinder (#8)? If you're adding air to the cylinder, there is only a few places for it to go. Past the rings, past a valve, past the head gasket, or through a crack in the head. Plenum is beyond that chamber, so the plenum isn't causing that leak. It may be a contributor to your overall issues, but it is not going to change that leak down issue one bit. You will be wasting some time and your intake gaskets, because you will have to pull the intake off again when you realize it didn't cause the issue and have to pull the head anyway. I hear you on the ASE opinion, perhaps he wouldn't describe it as you did. I think I'd get the ASE to tell you how he thinks the plenum fix is going to change a chamber leak, you'll either get to the bottom of it - or decide that he isn't all that the ASE should ensure.

Good luck !

 

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I've been doing this kind of thing for more than 40 years, the number 8 intake valve, or valve seat, has issues. Nothing you do with the intake manifold is going to fix that. You need to pull the valve cover, and check the valve springs, and whatever else you can see there. (like, how the valve moves, etc.) If everything there looks ok, time to pull the head.

 

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Need to see what the problem actually is. We already know it involves the number 8 intake valve, from stuff previously done. You can peek in there with a bore scope, and see if anything obvious presents itself..... but, I would bet real money that the head is going to need to come off.

 

Its not the intakes job to seal the combustion chamber. So correct, there is no way the plenum is going to cause a combustion chamber leak. Its job is to mix fuel/air and deliver it to the intake valves. Watch some videos on youtube to gain perspective, I'm sure there is someone making a cool movie on combustion chamber dynamics.

 

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I usually recommend you get a good used engine, demonstrated running, and toss it in there. You can do the compression testing on the replacement before you buy. Faster and cheaper, that pair doesn't usually go together too often. Then you can tear down your original and learn things.

 

From what I'm reading here, have you reperformed a leakdown test with positive results? As mentioned by others, any problem with a leakdown test or a compression test involves the combustion chamber which makes the plenum totally irrelevant. Unless the leakdown test was performed incorrectly with the #8 cylinder in a postion other than TDC on the compression stroke. If you performed the leakdown test at TDC at the beginning of the intake stroke, you might actually hear air coming out of the throttle body which is not a valid leakdown test. Something is getting lost in this post. A failed properly performed leakdown test, if you hear air coming out in any quantity, means that at a minimum the heads need to come off. There is some wiggle room in a properly performed leakdown test but actually hearing air escaping means that the engine is not going to run properly no matter what you do.