I have a fresh built ls1, 12:1 comp, mahle pistons, gapped for NOS. LS6 PCV system.

Started up ran fine no leaks, got the fuel dialed in and took it for a drive. When i gave it a good squirt it pushed oil out of the rear main and made a mess.

I changed the rear main (appeared to be nothing wrong with the one that was in it - brand new and installed correctly). Both times a felpro rear main was used.

The engine did the same thing next time i got into it.

I took the oil cap off and had another spirited drive, this time there was no problem so i suspect the crankcase is getting pressurized and causing this.

The LS6 valley PCV i used was from a low km motor and i also cleaned it as best as i could with degreaser before installing it.

Im not sure where to go from here, the friend who built the motor with me thought it might have been building pressure because the rings arent fully bedded in yet (motor has done maybe 30 miles) but i have never come across this before.

I am after some ideas and also some further insight into when the stock PCV system becomes inadequate. I searched and read a lot of threads but was unable to find any clear information on this, i also saw a number of engine builders recommend upgrading the stock pcv system. Is it comp?, bigger ring gaps? or what that causes the stock system to be inadequate?

I will comp test the motor to eliminate doing something silly like putting a ring in wrong and i had wondered about pulling off the valley and checking the pcv isn't blocked, but i could suck air through the PCV and i imagine with the strong vacuum pulling it its certainly not blocked.

There is a decent bit of oil in the intake too but i have thought this pretty normal with LS engines.

 

most people who are having rear main blowouts from crankcase pressure are turbo builds. just to clarify this is NA

 

Is the dipstick staying in place or popping out of it's tube? I would think that enough pressure to push oil past the rear main would easily be able to pop the dipstick out. Have you considered that you might be misaligning the rear main seal?

 

Thanks for the reply, no its not popping out but its a new sump and dipstick and the dipstick locks in pretty firm. I have considered the rear main, i hear the felpro ones arent amazing either with the reverse lip seal arrangement. But i bought maybe 5 of these seals at the same time and have used them on a few motors and never had an issue until this one.

 

Does anyone else have any wisdoms?

I comp tested it today, tested up good.

 

Get in contact with mightymouse, he makes catch cans that might be able to help you out.
Another option you can try is getting a 1-way-out (only lets air out, not in) breather to replace the oil cap (rx makes these for example). Since you didn't have the issue with the oil cap on then at least this should work for you if nothing else it can be a bandaid.
As for the oil you need a good catch can on the dirty line and possibly one for the clean line. For the clean line another possible option would be a to put a one way check valve there only letting air out of the throttle body into the crankcase and not back up, BUT make sure this is paired with the oil cap breather as well so pressure can be vented somewhere.

 

Cheers JD, i am curious why its building a lot of crankcase pressure, i have had similar, even bigger cams in motors and had no problem, this has a lot of compression but ic ant see why this would really cause it. Beyond a blocked pcv on the valley i cant understand what i may have done building it that would cause this.

Plan from here is to run it in a bit make sure the rings are doing their job 100%, test it, put the cap back on, test it, if it leaks clean/replace the pcv, test it and if that all fails catch can

still keen to know when and why the stock pcv becomes inadequate

 

Is this engine stock stroke? When you say the rings are gapped for nitrous, how big did you actually go? Was the engine's cylinders honed with a torque plate?

 

Stock stroke and bore, the block was very low km so i just gave them a quick hone with a brush hone.

.019 and .024 ring a bell.

 

I had oil push out the rear main only once on a fresh engine I built. Of course, I caught **** because everyone thought installed the rings upside down or something. Turned out to be the check valve in the PCV was backwards and the boost from the supercharger (~10-12psi) was pressurizing the crankcase.

If you're sure the system is routed correctly and you've checked the hoses and the LS6 valley cover and there are no blockages, then it sounds more like a problem with the shortblock than an inadequate PCV system. Beefing up the PCV may just be a band aid for the symptoms of excessive blow by.

 

I might check the PCV in the vane hope its that, but i suspect i might be stripping it to see if i have a ring in wrong or one is broken

 

in stock layout crankcase pressure control has nothing to do with the pcv return port and everything to do with the fresh air port

that connection has yet to be mentioned here. for this engine it would be passenger forward valve cover port to top of the throttle body. this is where blowby exits at full load, ahead of the throttle blade

is your stuff correct and not plugged or even partially blocked?

as mods pile up and power increases, the %leaked to the crankcase continues to add up, and unfortunately this is not intuitive for most, and they have a problem before they know it. the crankcase breathing ability needs to grow with the flow you are trying to manage through it

my systems add larger, or even multiple exit points to ensure pressure is never allowed to build, and can shut back down to operate a normal pcv path otherwise.