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Intake Manifold Vacuum Leak(s)

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Red20 View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Red20 Quote  Post ReplyReply Direct Link To This Post Topic: Intake Manifold Vacuum Leak(s)
    Posted: Jan/12/2019 at 9:08pm
A subject as old as time (and this forum) itself.  And yes, I've read every thread ever on this and struggled with leaks off and on for years.

I recently installed a Holley Terminator EFI system on the 360 in the '73 Sportabout.  Right away the ECU couldn't get the idle speed down below 900 RPM and the IAC valve stayed at 0%.  I can close both primary and secondary butterflies and put my thumb over the IAC hole and the car still runs at 900-1100 RPM.  I've plugged off vacuum lines and ensured the throttle body gaskets are sealing - no change.  SO... air must be getting in at the intake to head mating surfaces.

I'm going to have to pull this manifold and seal everything back up.  Any advice?  I usually use indian head or aviation form-a-gasket around the intake ports and a thin bead of Right Stuff or UltraBlack around the water ports.  I also use Edelbrock fiber gaskets.  My valley pan has the mating surfaces cut off and it's tek screwed to the bottom of the manifold.  Crazy, yes, but it works for me.
1969 Javelin SST "Screamin' Banana" - Totaled Feb 2018
1973 Hornet Sportabout X "yellajack" - 360/727/TracLoc DAILY DRIVER
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Post Options Post Options   Thanks (0) Thanks(0)   Quote PHAT69AMX Quote  Post ReplyReply Direct Link To This Post Posted: Jan/12/2019 at 10:44pm
Confirm that the intake manifold mounting bolts actually clamp before bottoming out in the blind tapped holes in the heads.  With the intake off, I screw the bolts into the heads finger tight and measure the distance under the bolt heads and compare that to the intake manifold flange thickness and washer thickness of course.  Fail to remember for sure, but stock intake bolts may have been non-standard length, just a guess or possibility...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Red20 Quote  Post ReplyReply Direct Link To This Post Posted: Jan/12/2019 at 10:52pm
Hey, never even thought about that before.  I hope that’s it!  Then it won’t just be that I suck, lol. 
1969 Javelin SST "Screamin' Banana" - Totaled Feb 2018
1973 Hornet Sportabout X "yellajack" - 360/727/TracLoc DAILY DRIVER
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Post Options Post Options   Thanks (0) Thanks(0)   Quote a401classic Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2019 at 2:41am
https://www.youtube.com/watch?v=HdQSugZFt4s
Start watching at 4:20. This may be a problem for you, it was for me.  I put a FAST EZ-EFI on an El Camino and struggled with the exact issue you describe.  It started out fine, but what I wasn't seeing was that there was so little contact between the intake mounting flange and the TB in one spot and enough vacuum to slowly pull the gasket into the intake and create a leak.  I didn't have room for an adapter, so I went with a new intake, but I should have done a little more homework before getting the Air Gap I did get. Apparently TBI systems prefer single plane intakes to smooth out the manifold pressure signal that the MAP sensor sees.  A fully divided dual plane intake will pulse the MAP sensor.  The Air Gap I have has a 1" cut out of the divider which helps, but not as good as a single plane intake.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote WesternRed Quote  Post ReplyReply Direct Link To This Post Posted: Jan/13/2019 at 3:16am
Originally posted by a401classic a401classic wrote:

https://www.youtube.com/watch?v=HdQSugZFt4s
Start watching at 4:20. This may be a problem for you, it was for me.  I put a FAST EZ-EFI on an El Camino and struggled with the exact issue you describe.  It started out fine, but what I wasn't seeing was that there was so little contact between the intake mounting flange and the TB in one spot and enough vacuum to slowly pull the gasket into the intake and create a leak.  I didn't have room for an adapter, so I went with a new intake, but I should have done a little more homework before getting the Air Gap I did get. Apparently TBI systems prefer single plane intakes to smooth out the manifold pressure signal that the MAP sensor sees.  A fully divided dual plane intake will pulse the MAP sensor.  The Air Gap I have has a 1" cut out of the divider which helps, but not as good as a single plane intake.

Could just try running an open spacer under the throttle body to test this theory.

I remember on Engine Masters, they had some issue when they tried to run the Terminator on a dual plane manifold as well.


Edited by WesternRed - Jan/13/2019 at 3:59am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2019 at 8:30am
Red20, does the 360 in the Sportabout have the factory intake manifold or aftermarket?

Is this just the Holley system that acts like this? Or all aftermarket throttle body EFI ?



This doesn't sound good at all.

I have an MSD EFI system that is planned for install on the White70 with an Edelbrock performer intake. Which (correct me if I'm wrong) is a dual plane intake.

I suppose I should remove the wall between the two sides before I install the manifold?  Anyone have the MSD EFI on a Performer intake? Does it function correctly?


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Post Options Post Options   Thanks (0) Thanks(0)   Quote pit crew Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2019 at 10:25am
Originally posted by White70JavelinSST White70JavelinSST wrote:

I have an MSD EFI system that is planned for install on the White70 with an Edelbrock performer intake. Which (correct me if I'm wrong) is a dual plane intake.

I suppose I should remove the wall between the two sides before I install the manifold?  Anyone have the MSD EFI on a Performer intake? Does it function correctly?
I had the MSD Atomic on my Performer then I switched to the Holley Sniper.

I had all kinds of issues with the MSD that turned out to be ignition based and not the EFI itself.
An additional reason for the switch was the amazingly POOR support from MSD that I experienced at that time. Now that Holley and MSD are one company I would hope the MSD support has improved.

I heard of the dual plane vs single argument but if the Sniper is any indication the MSD will run like a champ on the Performer so long as the ignition system is not an issue. My car did fantastic on the chassis dyno without modifying the Performer but keep in mind that was with the ignition system sorted out and with the Sniper. Results may vary and all those other disclaimers.




Edited by pit crew - Jan/14/2019 at 10:37am

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2019 at 10:58am
Originally posted by pit crew pit crew wrote:

Now that Holley and MSD are one company I would hope the MSD support has improved.


Don't bet on it.  I have two friends with Holley systems and they both claim that tech support is non-existent.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2019 at 11:50am
Red20,

I was thinking....maybe you should try placing small balls of clay or something similar against the sealing surfaces on the heads where they mate to the intake manifold and set the manifold in place to verify that the mating surfaces are actually somewhat parallel. You could check how much each ball of clay got squashed. If all of them are the same, or nearly so, at least you'll know the surfaces are parallel.


Edited by White70JavelinSST - Jan/14/2019 at 12:03pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote White70JavelinSST Quote  Post ReplyReply Direct Link To This Post Posted: Jan/14/2019 at 11:55am
Originally posted by pit crew pit crew wrote:

Originally posted by White70JavelinSST White70JavelinSST wrote:

I have an MSD EFI system that is planned for install on the White70 with an Edelbrock performer intake. Which (correct me if I'm wrong) is a dual plane intake.

I suppose I should remove the wall between the two sides before I install the manifold?  Anyone have the MSD EFI on a Performer intake? Does it function correctly?
I had the MSD Atomic on my Performer then I switched to the Holley Sniper.

I had all kinds of issues with the MSD that turned out to be ignition based and not the EFI itself.
An additional reason for the switch was the amazingly POOR support from MSD that I experienced at that time. Now that Holley and MSD are one company I would hope the MSD support has improved.

I heard of the dual plane vs single argument but if the Sniper is any indication the MSD will run like a champ on the Performer so long as the ignition system is not an issue. My car did fantastic on the chassis dyno without modifying the Performer but keep in mind that was with the ignition system sorted out and with the Sniper. Results may vary and all those other disclaimers.




I have an MSD Pro Billet RTR distributor, IIRC the MSD EFI and this distributor are supposed to interface in a good way. I have a 6A box and MSD Blaster coil as well. Hopefully all these parts will  play together nicely. You'd think so anyway for what they cost.
 I didn't buy the 6A box, it came free with the MSD Atomic EFI. Not sure how good of a box the 6A is ?? Or how versatile.  I'm not even sure I want to use it. It must be one of their early versions that they wanted to get rid of cuz the lot of them were taking up too much warehouse space. LOL  I believe the 6AL is a more versatile unit. 


Edited by White70JavelinSST - Jan/14/2019 at 12:01pm
70 Javelin SST, second owner, purchased 1972
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