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Compression and leak down test results-Thoughts?

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Old 03-04-2018, 09:40 AM
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azvrod
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Default Compression and leak down test results-Thoughts?

Hi guys, I wanted to post my compression and leak down results (I was trying to eliminate a cracked piston as the source of my blow by and oil blowing out of the oil breather issue). I also now know may catch can setup is completely inadequate and have ordered a new setup with a breather on it. Now, my disclaimer is that I dont have a lot of faith in the $30.00 harbor freight leak down tester I bought.


#1= 165 psi, 30% leak
#3= 155 psi, 30% leak
#5= 150 psi, 35% leak
#7= 155 psi, 25% leak

#2= 155 psi, 25% leak
#4= 155 psi, 30% leak
#6= 160 psi, 30% leak
#8= 155 psi, 35% leak

So I think they are well within each other, but arent the compresson and leak down tests indicating that the engine is uniformly having too much blow by? Is this a result of running too much boost on a stock bottom end? So in other words, no one cylinder is the issue, its all the cylinders combined?

Other notes: The car does not blow smoke and seems to be making plenty of power.

Thanks for your feedback and input!!

KJ
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Old 03-04-2018, 09:59 AM
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Where is the leak going? Leak down can be by the rings or through the valves.
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Old 03-04-2018, 11:46 AM
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Originally Posted by juanvaldez View Post
Where is the leak going? Leak down can be by the rings or through the valves.
Definitely into the crank case. No symptoms of a bad head gasket, or valve issues. I could hear the air hissing through the dipstick with a stethescope.
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Old 03-04-2018, 11:57 AM
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Matt Gruber
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I'd run it as long as it makes good power.
In the old days i'd run a 3/4" hose from a breather to the air cleaner base. that would suck all the blow by back into the engine.
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Old 03-04-2018, 06:17 PM
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A compression and leakdown test are two different tests.

The fact the compression test shows even across the board I wouldnt have too many concerns.

Was the test done with all plugs out and throttle held open ? Depending on CR/cam the actual numbers seem a little low, but again...they're consistent which is a good thing.

And what do the plugs look like ?

Cheap USB bore scope camera ?

Probably worrying over nothing
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Old 03-04-2018, 09:13 PM
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Originally Posted by stevieturbo View Post
A compression and leakdown test are two different tests.

The fact the compression test shows even across the board I wouldnt have too many concerns.

Was the test done with all plugs out and throttle held open ? Depending on CR/cam the actual numbers seem a little low, but again...they're consistent which is a good thing.

And what do the plugs look like ?

Cheap USB bore scope camera ?

Probably worrying over nothing

Sorry, yes ran 2 seperate tests, a compression test, and then a leak down test.

All plugs were out, throttle open, and looked OK. No not a cheap camera, I could easily see when when TDC. Stock C5 Z06 cam

Ther car runs very strong, so you guys are making me feel better!

KJ
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Old 03-05-2018, 05:31 AM
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I dont care where TDC is, I mean get a visual down the bore of the piston crown. The cameras are dirt cheap and handy to have.
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Old 03-05-2018, 12:51 PM
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Originally Posted by stevieturbo View Post
I dont care where TDC is, I mean get a visual down the bore of the piston crown. The cameras are dirt cheap and handy to have.
Yes we could see the crowns of the pistons, no obvious cracks and or gunk or damage on top. And yes you are right I love that camera..I bought a Bosch for about $100, and other than crunching the tip with the piston when we left it in too far..it worked great.
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Old 03-05-2018, 02:51 PM
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I bought a couple of USB bore scopes for $15, always handy to have in the laptop bag.
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Old 03-09-2018, 04:18 PM
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Whats the CR and cam specs? Those numbers look like a lower compression motor or one with a cam with lots of overlap. The fact that they're consistent across all cylinders is great.
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Old 03-09-2018, 09:15 PM
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Originally Posted by 5 Liter Eater View Post
Whats the CR and cam specs? Those numbers look like a lower compression motor or one with a cam with lots of overlap. The fact that they're consistent across all cylinders is great.
Stock c5 z06 heads, cam and internals. I have some serious doubt about the Harbor Freight leak down tester, so bought a better one and will retest.
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Old 03-10-2018, 06:44 AM
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Is this a 2001 ls6?
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Old 03-11-2018, 12:46 PM
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Originally Posted by jimbos'ss View Post
Is this a 2001 ls6?
No 2003 LS 6, its does not have an issue with burning oil

BUT a new update: I reran the leakdown test with a better gauge, a Lang CLT 2PB, and the results were much better, all cylinders were between 10-15%..still not great, but much better than 25-35%. Lesson learned..buy a decent gauge..not the $40 job from HF.
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Old 03-12-2018, 09:41 AM
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I've seen a fair amount of motors die with the ring lands breaking between the 1 and 2 rings, where nothing can be seen from the top. Once you lose the motor, your blowby will skyrocket.

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Old 03-12-2018, 11:23 PM
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That is why I suggest monitoring and logging crankcase pressure using a sensor which has good resolution between about -30" H2O and +40" H2O.

By monitoring you will be able to record what a healthy ring-seal is supposed to look like, and recognize immediately when you lose a ring or other oil seal in one of the cylinders it will become apparent right away. I would recommend a warning light to go off when pressure goes above norm if you have the space.
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Old 03-13-2018, 11:19 AM
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Originally Posted by azvrod View Post
Stock c5 z06 heads, cam and internals. I have some serious doubt about the Harbor Freight leak down tester, so bought a better one and will retest.
I'd expect ~190 PSI from that motor. So it does seem low, but consistently low so
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Old 03-14-2018, 08:59 PM
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If you had cracked a ringland blowby would be like a steam engine out the oils fill port. Since they are close itís probably fine itís going to have some blowby when boosted. Make sure itís vented well. Cranking compression can also be lower based on where you live like higher elevation.
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Old 03-16-2018, 07:38 PM
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waaay too much leakdown
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Old 03-19-2018, 12:46 PM
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Originally Posted by slow ride View Post
If you had cracked a ringland blowby would be like a steam engine out the oils fill port. Since they are close it’s probably fine it’s going to have some blowby when boosted. Make sure it’s vented well. Cranking compression can also be lower based on where you live like higher elevation.
Ran hard this weekend and the MM catch can solved the oil blowing out problem!

Last edited by azvrod; 03-19-2018 at 12:47 PM.
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