Siamese Intake Sealing Woes

[Rumblin70SS396]

Hello all,

It's been awhile since I have posted here. Unfortunately I have very little time to work on the car so it has been sitting. Any help is appreciated to get this Z up and running again.

A few years ago we replaced the coils, fuel pump, spider and all 16 injectors in this car. The original gaskets had a silicone sealer around each port and the water jackets. I used the paper gaskets available from one of the sponsors here without sealer. Unfortunately almost immediately it was leaking coolant and had a very high idle. After that I shut it down, tucked it in the corner and just got back to it here recently.

Fast forward to present day, I took another set of gaskets and used a non hardening Permatex gasket sealer I have used successfully on other cars. I put the intake back on and torqued the bolts in 20ft lb in the proper sequence. Once the car started, the water issue is gone but the high idle (2k) is still there. I can only assume it's a vacuum leak on the intake as the car never had these issues previously.

What's the best way to seal a siamese intake? Is there something else I am missing? Thanks in advance for any help.



Tony

[XfireZ51]

Is this happeningnwhen u first start the motor or its always idling at 2k?

[Rumblin70SS396]

Hello there, it is always idling at 2k and I get a light surge after it warms up.


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[XfireZ51]

Originally Posted by Rumblin70SS396

Hello there, it is always idling at 2k and I get a light surge after it warms up.


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So it idles at 2k then surges at 2K when warmed up?

[DRM500RUBYZR-1]

Have you blocked off the air horn completely in order to confirm the presence of an intake leak?

Can you isolate it using a propane source and observing engine rpm increase to see exactly where it is coming from?

Where in Delaware?


Marty

[Jitse]

Please check your plenum also for straightness, there can be some problems because of this.
The plenum is cut open , and after it they welded , if they not supported it on a stiff metal plates with the bolt holes tighten to it , it can be that the plenum is bend if this is not done proper.
I had the same issue with my Siamesed plenum, using Jerry’s thicker gaskets, and a bit more torque about 30Nm solved my vacuum leaks.
How did your plenum looks from the inside, did they smoothed it after welding...

[Rumblin70SS396]

Originally Posted by DRM500RUBYZR-1

Have you blocked off the air horn completely in order to confirm the presence of an intake leak?

Can you isolate it using a propane source and observing engine rpm increase to see exactly where it is coming from?

Where in Delaware?


Marty

Hi Marty,

I will try blocking the air horn, haven't tried that yet.

Never tried the propane trick but I can do that as well.

I am located in Newark Delaware.

Tony


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[Rumblin70SS396]

Originally Posted by XfireZ51

So it idles at 2k then surges at 2K when warmed up?

I only ran the car for about 15 minutes and once warmed up it idled a little over 2k with about a 100rpm surge.


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[Rumblin70SS396]

Originally Posted by Jitse

Please check your plenum also for straightness, there can be some problems because of this.

The plenum is cut open , and after it they welded , if they not supported it on a stiff metal plates with the bolt holes tighten to it , it can be that the plenum is bend if this is not done proper.

I had the same issue with my Siamesed plenum, using Jerry’s thicker gaskets, and a bit more torque about 30Nm solved my vacuum leaks.

How did your plenum looks from the inside, did they smoothed it after welding...

The plenum was sealed previously so I really didn't think it would be an issue.

I used Jerrys thicker gaskets as well on mine and will have to try the additional torque. The 20ft lbs felt awful tight though and I did re-torque them. Did you use any sealer on them?

I have some pictures of the porting but I don't have them with me. I'll be honest I can't remember if they were smoothed or not but it looked like a pretty nice job.

Tony


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[A26B]

It's hard to seal a full boogie mono runner. Engine de-cel creates very high vacuum and there's simply not much area to provide sufficient clamping friction to keep the gasket in place.

Adhesive:
It makes cleaning difficult, but use an adhesive on one side only, such as the injector housing, so you can apply it easily. Laquer glue gives you more time, super glue is anaerobic and sets quickly.

Standard Adhesive:
Lacquer glues require one side to be absorbent. Gorilla glue looks to be feasible.
https://www.zoro.com/gorilla-glue-al...13/i/G0912441/

Anaerobic Adhesive (Super Glue) (#30)
http://www.stewmac.com/Materials_and...uper_Glue.html

Recommended procedure:
1. Spray silicone onto only the top surface of the "thicker-3/64" (10168684-X) gasket & allow to dry.
2. Apply the adhesive to the injector housing
3. Lay the gasket in place on the injector housing
4. Mock install the plenum & bolt in place, tightening with the proper sequence
5. Allow 24 hours before removing the mock installed plenum

Now do all the under plenum attachments for final plenum install

The silicone will help prevent the gasket from being damaged during subsequent removal(s) & the adhesive will hold the gasket in place.

added note

Re: Warping from welding. Everyone I've talked to that has built a mono-runner plenum encounters warping. It can be severe and require extensive metal removal during re-machining of the gasket mating surfaces. It needs to be checked with a precision straight edge from side to side and length ways. Gaskets can only do so much to seal irregular surfaces, typically in the 12~20% range. Example, our thicker gasket is 0.046" thick, uncompressed. Maximum irregularity would be in the 0.007"~ 0.012" range.

We had a customer working on a Lingenfelter 415 with a mono-runner plenum, encountering difficulty with plenum sealing. It was severe enough to require gluing/laminating 2 gaskets together & gluing to one surface.

Over torquing can work, just depends on how bad the warp is.