Wierd Problem... Vacuum through Oil Filler on Valve Cover

[crunchie12268]

I just did a TFS 1 cam swap... I have a cobra upper intake and ported GT40 heads on the bottom. I fired it up last night and it idles at around 2000 rpms and then I unscrew the oil cap and I get some good suction... also the rpms drop to around 800. I plugged a vacuum gauge up to the vacuum tree and it read a constant 18 pounds of vacuum. I checked the PCV and so on and found nothing. I am wondering if it isn't my lower intake manifold gaskets? anyone have this problem... any advice would be great

 

[1988WHTGT]

Mine did that before and it was my lower intake gasket.

 

[crunchie12268]

thats what was in the back of my mind but I thought I ruled it out with the vacuum check. I guess if you think about it though it would keep a constant vacuum.

can anyone else second this?