So can someone please confirm my thought process. Background: 7.4L L29 Vortec, completely rebuild engine. Full back story here: https://www.gmt400.com/threads/andy’s-1995-cclb-7-4l-tbi-to-vortec-swap.45295/ After I put about 200 miles on it, I ran it WOT and then coasted down. After that it wouldn’t idle down anymore to the desired idle speed. Confirmed that the IAC is completely closed which is most likely a vacuum leak. It’s now gotten so bad it threw a P0507 diagnostic code. I did a smoke test on it. Blowing smoke into the intake yielded no external leaks. Capping off the left side valve cover port and blowing smoke into the right side valve cover, where the PCV valve goes quickly resulted in an intake manifold full of smoke, billowing out the throttle body. With how I had things capped off, I’m thinking there shouldn’t normally be a path for the smoke to get from the crankcase to the intake manifold. If that’s the case, I think I’ve found my vacuum leak, it’s internally leaking, a bad gasket allowing extra air to be pulled through the PCV system. If all that is true, my next question is how/why would a brand new molded gasket fail? Obviously, the only way to get a certain answer will be once I get it pulled apart.