Very strong vacuum through oil fill tube
 

My ls1 just started missing the other day and then feels like it is completely missing a cylinder now. I just checked my spark from the coils and they are fine. It used to have quite a bit of blowby but now there is such a strong vacuum you can hardly get the oil fill cap off and it will suck your hand to it. What is going on?

Heavy boost

no boost on this ride. ls1 intake, 241 heads, ls6 cam

How did my ex wife get in there?

Have you checked your PCV system and made sure it's not goobered up.

Any recent maintenance or mods doen to the car?

Also you may have some internal issues like perhaps a broken ring land.

If you think it's missing check your plugs or see if there is any miss history in the computer. Normally when a ring land lets go it pressurizes the crank case due to the combustion gases getting past them. #7 seems to be the most common cylinder to let loose.

Intake valve aint opening someplace dood..................

ya this car hasnt been driven very far one way for a while after the engine swap but the first time I drive it about an hour and a half and then it starts doing this out of the blue without ever getting on it hard. There is no smoke or knocking sound and now I started it up this morning and it wasnt missing, but I didnt get to go drive it (still on trailer).

do a compression test and a leak down test and you will find the problem! crankcase vacuum is good but now you have too much also check your pcv lines start there make sure they are install correctly

Sounds to me like the PCV valve is stuck open, and that's the motor sucking air through the PCV tubes. Does the idle change at all when you pop the oil fill cap off?

if you have only one hose connected to the intake behind the tb and the valve covers plugs and no breather then that is your problem

found a corroded connection on a coil and that stopped the miss, but I now have more vacuum than ever.

If You have strong vacuum in the engine that is what You want and the pcv could'nt be clogged . That is exactly the purpose of the pcv is to create a negative pressure in the crank-case not allowing oil to make its way past the piston rings and not much or no oil can leak out gaskets and seals in that condition , but yes it could be exessive which can pull some of the fuel and air past rings and eventually thin You're oil . Could'nt be pcv stuck open , there is'nt that much opening where pcv hose goes into tb

Should it be so strong that it sucks a hickey on your palm in a couple of seconds at an idle? And also the miss is back again.

Yep. Same thing my buddies car did when his rocker arm mount broke off the head on a intake valve.

PCV valve is supposed to be a volume metering device, not a pressure
regulator. If youhave a tight motor and no blowby it might well draw a
pretty strong vacuum.

But if it's flowing so much that it messes up idle, that's not right. Shake
it (PCV valve) and it should rattle.

I'll check that pcv valve but if its not that then this might just give me a better reason to put some 243's on it.

The main purpose of the pcv valve is to vent positive crankcase pressure(open) and retain negative crankcase pressure(closed) so that there will be no positive pressure to push oil past the rings

I had the same problem. Number 8 intake valve pedistal broke off. Pull the valve covers and it should be evident which is the problem.

Expecially if you have ported heads.

so I pulled the valve covers and the heads and there is no obvious damage anywhere. So now what? I'm going to go ahead and replace the lifter just in case one of them is the problem and put the 243's on now. Hope its fixed when I put it back together.