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pressure building up in valve covers, cracked piston?

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Old 09-14-2017, 03:24 AM   #1
kiwivetten
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Default pressure building up in valve covers, cracked piston?

I have a 1990 corvette which I have fitted with a Vortech supercharger. I've noticed that with the pcv system removed, when I put both thumbs over the valve cover holes for a few seconds and then remove one thumb I get a puff of air come out so presuure is definitely building up. Car runs fine but I notice a small amount of smoke coming out the covers.

I know it is normal for some pressure to build up in the engine but the car only has 20000 miles on it. During my tuning process after fitting the supercharger Im sure it was leaned out on a few occasions.

I realise a compression test will tell me what is going on but this car has been converted to right hand drive and with the modified headers, getting a compression tester onto all cylinders probably wont be possible.

Could someone do the thumb test like I have done and let me know if air puffs out??

Any other feedback appreciated!
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Old 09-14-2017, 08:16 AM   #2
ex-x-fire
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Most likely blow by, worn rings. I'd do a leak down test instead of a compression test.
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Old 09-14-2017, 09:54 AM   #3
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You detonated and cracked a piston - period !!! Unless you took at least 10 degrees out at WOT. Lean won't crack - timing cracks. Crankcase pressure - pull engine and start over.
2 caveats: (1) do not lower CR - you'll be sorry !!! (2) get or buy a proper tune etc.
My 2 cents - greg
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Old 09-14-2017, 10:10 AM   #4
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The PCV system is to remove crankcase pressure. If pcv valve is installed in valve cover there should be a slight vacuum on the breather / filter hole. Smoking while pcv valve is installed indicates: rings not sealing or restriction of vacuum through valve to intake manifold. With no PCV crankcase pressure will build up even on a sound engine, and some smoking from uncovered holes in valve covers will occur.
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Old 09-14-2017, 06:22 PM   #5
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Thanks Kenova, after spending more time on the web last night I discovered as you say that even a strong engine will have some blowby and build a little pressure. If I block both valve covers with my thumbs you dont really feel pressure building so it would seem to me that it may be normal pressure occuring and Im being a bit paranoid.

Greg, I have a MSD 6 AL digital system installed and the timing curve setup to retard under boost.

ex-x-fire, I doubt I have worn rings with only 20,000 miles on the clock.
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Old 09-14-2017, 08:05 PM   #6
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A cracked piston is normally noisy + oil smoke from the exhaust.

A remote breather will help.
https://www.summitracing.com/parts/m...SABEgK23fD_BwE

Or install valve cover breathers & wipe off the oil mist on occasion.

If a track car an evacuation system tied into the exhaust is an option.
https://www.summitracing.com/parts/a...yABEgJ-kfD_BwE

A functioning PCV will lean the A/F ratio a bit.

IC engines need the crankcase to breath its normal for blower motors to have more crankcase pressure.
If the crankcase is not vented gaskets & seals will develop leaks.
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Old 09-14-2017, 10:20 PM   #7
kiwivetten
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Quote:
Originally Posted by Churchkey View Post
A cracked piston is normally noisy + oil smoke from the exhaust.

A remote breather will help.
https://www.summitracing.com/parts/m...SABEgK23fD_BwE

Or install valve cover breathers & wipe off the oil mist on occasion.

If a track car an evacuation system tied into the exhaust is an option.
https://www.summitracing.com/parts/a...yABEgJ-kfD_BwE

A functioning PCV will lean the A/F ratio a bit.

IC engines need the crankcase to breath its normal for blower motors to have more crankcase pressure.
If the crankcase is not vented gaskets & seals will develop leaks.


Thanks for the feedback team. Here in New Zealand there aren't any experts on Corvettes so I rely on this community to help me solve problems.
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Old 09-14-2017, 11:04 PM   #8
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Quote:
Originally Posted by Churchkey View Post
A functioning PCV will lean the A/F ratio a bit.
A properly calibrated EFI will compensate for that.

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Old 09-15-2017, 09:47 AM   #9
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Quote:
Originally Posted by BlowerWorks View Post
You detonated and cracked a piston - period !!! Unless you took at least 10 degrees out at WOT. Lean won't crack - timing cracks. Crankcase pressure - pull engine and start over.
2 caveats: (1) do not lower CR - you'll be sorry !!! (2) get or buy a proper tune etc.
My 2 cents - greg
I did not read your post correctly - or too many beers !

Anyhow the cracked piston test is easy. With vehicle at a hot idle remove the oil fill cap. Place palm loosely over fill hole - maybe 1/2 inch above. If you feel 'puffing' you have a cracked piston. Smoke - even a lot is normal or not so normal (worn rings) blow by. I have seen new engines with a blower go from virtually no-blow by to a lot (oil out everywhere) depending on who built to what specs. A great build will NOT need catchcans which in my opinion are a fix for a poor build or choice of parts !!! greg
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Old 09-15-2017, 09:49 AM   #10
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Agree 100% - maximum venting = max power !!!!!!!!
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Old 09-15-2017, 09:49 AM   #11
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Agree 100 %
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Old 09-17-2017, 08:18 AM   #12
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Thanks for your feedback Greg - your info on supercharging was so helpful when pulling together my supecharger install. Im only running an old Vortech A-trim with about 4 pounds of boost but man it got the car going! Vortech tell me they can upgrade my A-trim to the latest trim spec so I'm really looking forward to having that done at some stage soon!
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Old 09-17-2017, 02:33 PM   #13
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I would try to get a leakdown test on the cylinders. I know you said it would be difficult. If someone put sparkplugs in the thing, you should be able to snake a leakdown test hose into the holes. That will help answer all your questions. It's actually easier to get the hose thread started because it's flexible thank to put plugs in. May need to go from the bottom. I you don't have access to a compressor, at least try a compression test.
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Old 10-23-2017, 06:47 AM   #14
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just confirming it must be blowby as went to the drags on the weekend and had my best run ever of 12.7 so engine is good!
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Old 10-23-2017, 01:57 PM   #15
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Just an FYI, I ran 2 tenths off my best time fogging the track with two lifted ring lands and multiple cracked piston skirts with the ProCharger. Stock shortblock (hurt) will still run deep in the 11s.


I knew it was hurt when I took it to the track, but figured I would give it go. That weekend I also yanked that engine combo out of the car.
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Old 10-23-2017, 01:59 PM   #16
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If its literally smoking out the valve covers its tired or damaged- no matter what your et is
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Old 10-24-2017, 05:13 AM   #17
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It's not smoking
It's not using oil
It's not noisey
There's no puffing out valve covers - just a little pressure building which seems like normal blowby
And its running better quarter mile times than it did years ago so I'm guessing the engine is fine!
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Old 10-24-2017, 01:06 PM   #18
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You need to Google how a PCV works. With positive pressure in the intake manifold the PCV becomes a check valve and prevent air flow from the intake into the crankcase. What I'm saying is with a supercharged/turbocharged motor the crankcase pressure has no where to go but out the seals on the motor unless you provide another vent path for it.

If you look modern turbo or supercharged motors they have additional vent path back to the air inlet duct (not the intake itself) that has a check valve itself to prevent flow during startup and idle when the normal PCV can function (no or low boost).

Hope this helps more than it hurts and please let us know what you do for this.
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Old 10-24-2017, 10:36 PM   #19
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Yep, aware of that and PCV system already removed. Currently running an electric pump to provide good ventilation
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Old 10-25-2017, 02:59 PM   #20
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Curious as I've never seen an electric evacuation pump. Most are belt driven mechanical and others use exhaust flow to create a low pressure to entrain gas out the exhaust pipe. If you search U-tube you can find street driven cars can get 'bout 4"Hg vacuum from the exh driven system while optimum is near 12"Hg vacuum.

Good luck and let us know how it works.
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