I have a 383 longblock I rebuilt for a customer. Popped intake back on and the pcv hose running to the air cleaner is sucking an incredible amount of air and making a super loud hissing noise. I have swapped the IAC from the port motor - no change.

Idles butter smooth at 650 rpms. I removed the iac and found the foam baffle missing - could this seriously create so much pressure? Not convinced -

Here is a video of what I mean:

You rebuilt a 383 (6.2) , what is the serial number?

I can’t see much…..

For the vacuum in the crankcase while idling…

Since it uses an external iac (ford type) air source, vs channel in tbody, is a iac hose hooked to a pcv by mistake ?

Does this engine use an actual pcv ? If so, is one being used and does it still have it’s innards.

I triple checked with port motor, hoses are routed the same, i even swapped the IAC over, and the pcv valve - no change.. port motor ribbed hose has blowby (Opposite!!) Despite having all the same connections-

Wondering if there is perhaps an internal leak on the intake gasket, perhaps an intake valve? Everything was redone - might try a leak down.. Ive never seen this before tbh







from the iac one goes to the intake, and another to the tb-

this is exactly what this one looks like -

The hose in the first vid, where does that hose go to?

Between other valve cover (no pcv / just a 90 degree) to outside of air cleaner - basically an open path from outside to crank case. All the suction created by whetever is getting sucked in right there. The whole crank case sucks - if i remove the oil fill cap same deal.

Im wondering if an over lashed intake valve would cause this (hang open)? or worse an intake gasket leaking internally - I sprayed along the outsides and no change noticed..

Stumped!

OK, ya have a intake vacuum leak big time
Pull the intake and see where the gasket is not making contact

Well, if you are 100% convinced pcv and such installed correctly, then it’s time to check intake gaskets. The plastic vortec type aren’t hard to crack. Also, if block and/or heads surfaced it’s possible intake faces (or china wall) may need some too.

This is the breakdown :

heads were very very lightly skimmed by my shop - i usually request them not to cos most heads are straight enough according to my straight edge..

Anyway yes I guess intake will have to come off 2moro I guess.. I sprayed around intake again but no sign of an external leak.. Maybe intake gasket just sat a pinch too low somewhere- I have some felpro thicker intake gaskets - hopefully I can find the smoking gun 2moro.

Another think i noticed is the vacuum gradually gets worse.. takes maybe 2-3 seconds to reach full vacuum, so its not like its right off the bat..

To confirm, yes hoses routed correctly, and I tested with the port IAC and pcv - no change.