Leak down test
Thread Starter
Melting Slicks
Joined: Jun 2005
Posts: 3,088
Likes: 61
From: NY- 2013 Corvette Challenge Champion-2013 Index Combo Champion 2015 CC/MM Champion
St. Jude Donor '07
Leak down test
I've been chasing a boost leak,I have a SC LS 427,I did a leak down test, & 7 of the cylinders ranged from 88 to 90psi @ a 100 psi coming in,One cylinder was @ 75psi @ 100psi coming in,But air was coming out of the rocker arm bolt hole on the intake side and it was the only cylinder that did it,Could there be a problem with the head,Could that be my boost leak?
Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Did you pressure up the blower system with compressed air and check it all for leaks with soapy water?
Make sure to check the BOV for leaks. A buddy found his Tial 50 BOV with the o ring half off and folded between the piston and the seat.
Make sure to check the BOV for leaks. A buddy found his Tial 50 BOV with the o ring half off and folded between the piston and the seat.
Tech Contributor
Joined: Apr 2007
Posts: 10,924
Likes: 78
From: Forney Texas
I've been chasing a boost leak,I have a SC LS 427,I did a leak down test, & 7 of the cylinders ranged from 88 to 90psi @ a 100 psi coming in,One cylinder was @ 75psi @ 100psi coming in,But air was coming out of the rocker arm bolt hole on the intake side and it was the only cylinder that did it,Could there be a problem with the head,Could that be my boost leak?
Thread Starter
Melting Slicks
Joined: Jun 2005
Posts: 3,088
Likes: 61
From: NY- 2013 Corvette Challenge Champion-2013 Index Combo Champion 2015 CC/MM Champion
St. Jude Donor '07
Did you lock the piston at the top of the cylinder or just put the gauge on it and see what you got? If its the latter, your test is faulty. The piston needs to be up in the bore to prevent from getting skewed #'s with the leakdown test. No way the rocker arm bolt hole is leaking badly enough to cause a significant boost leak. Like AJ said, pressurize the entire system from the inlet of the supercharger to the intake and look for your leaks.
Last edited by DRIVER456; May 31, 2016 at 05:35 PM.
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Le Mans Master
Joined: May 2006
Posts: 9,993
Likes: 1,136
From: League City Tx
Yes they will seap a little air out of the supercharger volute seal.. It's not a big boost leak.
My procharger seaps some. Also if you have any unused holes in the volute, make sure to put set screws (screw plugs) in them to prevent air leaks.
My procharger seaps some. Also if you have any unused holes in the volute, make sure to put set screws (screw plugs) in them to prevent air leaks.
Melting Slicks
Joined: Oct 2008
Posts: 2,946
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From: b.b. n.j.
I had a jeep recently I chased a cylinder misfire issue with and did a leak down, was coming out the exhaust and assumed it was an exhaust valve. Turned out to be the seat, maybe under boost it's bleeding down out seat???
Melting Slicks
Joined: Jul 2004
Posts: 2,844
Likes: 144
From: Northern Ireland
And again your problem given size of boost reduction ( if of course this measurement is accurate ) will still be belt slip, or a coupling not secured correctly.
Tiny boost leaks will not have the effect you describe...because they are tiny.
Where and how are you reading boost pressure ?
Tiny boost leaks will not have the effect you describe...because they are tiny.
Where and how are you reading boost pressure ?
Melting Slicks
Joined: Jul 2004
Posts: 2,844
Likes: 144
From: Northern Ireland
it is not the problem here though
Thread Starter
Melting Slicks
Joined: Jun 2005
Posts: 3,088
Likes: 61
From: NY- 2013 Corvette Challenge Champion-2013 Index Combo Champion 2015 CC/MM Champion
St. Jude Donor '07
If I have both rocker arms off and I try filling that cylinder with air and its leaking out of the bolt hole where the intake rocker goes,Thats a bent/burnt intake valve or broken or leaking valve seat.
Last edited by DRIVER456; Jun 2, 2016 at 06:02 PM.
Melting Slicks
Joined: Jul 2004
Posts: 2,844
Likes: 144
From: Northern Ireland
In that test scenario, and you're filling via the plug hole, then yes something is well ****ed.
Tech Contributor
Joined: Apr 2007
Posts: 10,924
Likes: 78
From: Forney Texas
While you're checking things, make sure the key on the supercharger pulley isn't sheared off. The only way to truly tell is to remove the pulley entirely because when they shear, the top half of the key stays in the pulley and the bottom stays in the shaft and it's always possible they are aligned even though the key is sheared. Some volutes have the bolt holes drilled all the way through into the volute and some don't. If your bolt holes are drilled all the way through, I recommend getting some allen set screws and running them into the empty holes.
Melting Slicks
Joined: Oct 2008
Posts: 2,946
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From: b.b. n.j.
Not really, compression was not that far off and under load miss went away. Via leak down...but as said may not be issue here!
Thread Starter
Melting Slicks
Joined: Jun 2005
Posts: 3,088
Likes: 61
From: NY- 2013 Corvette Challenge Champion-2013 Index Combo Champion 2015 CC/MM Champion
St. Jude Donor '07
While you're checking things, make sure the key on the supercharger pulley isn't sheared off. The only way to truly tell is to remove the pulley entirely because when they shear, the top half of the key stays in the pulley and the bottom stays in the shaft and it's always possible they are aligned even though the key is sheared. Some volutes have the bolt holes drilled all the way through into the volute and some don't. If your bolt holes are drilled all the way through, I recommend getting some allen set screws and running them into the empty holes.
