have a 01 3800 series 2 impala did a engine swap and now have a major issue the valve cover has negative pressure and if the filler cap is removed the car hisses and stalls immidately the oil that flows through the head also gets sucked down and the car runs for 5 min before it starts making a noise that sounds like a dry main bearing. the motor that was put in was good we didnt change anything but the lower intake up the heads stayed on also. any help would be appreciated. also the only thing different on the upper halves was the round tube on the intakes were different sizes.

Which round tube?

Also, verify your PCV valve is installed/correctly.

the metal tube in the lower intake that is vertical

i removed the pcv valve and did the shake test and it appeared to be good just to be sure I put the one in from the donor engine. no change

Ok, that'* the EGR stove pipe.

Did you check your PCV valve?

yes the pcv is good changed it with a new one now. ive had four of these engines now and never had this problem or heard of this happening

Do you remember if the gasket around the EGR stove pipe fit flush around it? This would be the upper manifold gasket.

What you have is a big vacuum leak in to the block. This would be either the PCV valve. You said you checked the valve and it worked fine. But is there a O ring? Without that it will leak around the outside. The EGR could also do it, but less likely then the PCV.

That didn't dawn on me until after I posted. That stove pipe being smaller than the gasket could allow a vacuum leak.

Like Dan said, look for that big o-ring where the PCV valve goes.

yes their is a o ring around it its difficult removing it so i know it has to be a good seal their. the stove pipe is confusing me I looked at the other engine I have as the donor and with either of the two stove pipes it appears that one end presses into the intake and the other runs into the throttle body and the upper intake gasket seals the upper to the lower intake and neither stove pipe seals around the intake plenum gasket assembly.