I recently finished some oil system upgrades on my motor and noticed there is quite a bit if crankcase vacuum at idle. Pulling the dipstick or oil fill cap while idling is how I noticed it. Idle drops slightly and hesitates a bit when either of those are pulled. The vacuum seems pretty strong. To be fair, I never pulled one of these with the motor idling in the past so it may have been there before. I didn't have any help when I fired the motor so I could not check with the throttle open past idle. Motor otherwise seemed to run very well.
Prior to todays startup I had the car apart for a while doing some oil system modifications to hopefully improve oiling during hard road course use short of doing a dry sump. I already had an oil cooler and accusump prior to these modifications. This is what I recently did:
Smoothed out all internal casting on the heads and ported out oil drains.
Rebuilt the heads, valve job, very minor mill, new PAC springs, clean valves, new seals, valve seats cut, etc.
Replaced factory lifters and buckets with link bar hydraulic lifters to improve drainage.
Installed pushrods with .040" hole to restrict oil to the heads slightly.
Comp rocker arm trunnion upgrade.
Cloyes timing chain set.
Ported and shimmed oil pump.
Switched from amsoil 10-40 dominator to 5-20.
I have dual catch cans from before on the car as well as an LS6 valley cover. I am assuming the fact that the valley cover tube is plumbed into the intake manifold behind the throttle body is why I am getting so much vacuum in the crankcase at idle. I just want to make sure nothing is wrong here. My hunch is that once the throttle cracks open it will go away. When I turned the motor off and pulled the cap there was no vacuum present.
On a side note, I've never had more than a drop or two of oil in my catch cans no matter how hard I ran the car. My intake however did have quite a bit of oil in it when I removed it. I did a compression and leak down test and everything seems ok and the car made good power on the dyno for a stock motor in the past.
thanks,
-V

 

Your PCV is connecting the valley cover exit to the manifold, this is correct, and manifold vacuum draws crankcase air out of the engine and into the manifold.

If you are running anything other than a stock LS1/6 style throttle body, what did you do to allow air IN to the engine/crankcase?

What I'm getting at is most 90+mm TB's don't have an inlet provision like the LS1s did (hose from the TB into valve cover)...many use breathers on the oil cap, some tap into the airbridge as the air source.

You are correct, the more you put your foot into it, the less manifold vacuum, the less air is drawn in, but seeing a noticeable vacuum by pulling a cap at idle tells me you have no source of inlet air.

 

Thanks for the response. Stock throttle body with correct fresh air breathing. I do have catch cans installed in both the valley breather and the fresh air line. I will play around with it today and see if I find anything unusual.

 

I had someone operate the throttle for me today and the crankcase still pulls a lot of vacuum. Car drives just fine, but there is some oil in the catch can that the valley cover is routed into. I'm thinking of removing the vacuum source and simply dumping both the valley breather tube and the valve cover tube into a generic catch can then either vent it to the t-body tube or open to the atmosphere with a filter as I do on race cars. That won't pass smog, but for track use it would be ideal. I don't have a vacuum guage, but the cam I installed seems to have a very strong vacuum signal.

 

something is not right. If the crankcase is that tight, you'll probably blow the dipstick out at WOT....

 

The dipstick cannot blow out with so much vacuum. It would take pressure to do that which is the opposite of what I have.