1996 Blazer LS
4.3 Vortec, automatic

Searched but only found a mention of the vacuum check valve & no pics or indication where to find it/what it looks like. Does anyone have a pic of the vacuum check valve that is shown in the vacuum line schematic?

Old symptoms: Intermittent 4wd engagement, rough running (suspect fuel filter here...). AC/heat vents all worked fine.

Engine swapped last month so may have something loose I can't see. Replaced pcv valve and rotten vacuum lines to accumulator tank. Had vacuum line from intake that was collapsing (weak, new aftermarket crap... replaced with 5/16" fuel line). Briefly ran much better but random misfire returned (again - suspect fuel filter for this) Also, since replacing the rotten & collapsing hose the AC/heat vents don't work - defrost/floor only.
Here's the kicker though: the new PCV valve is buzzing like a bumblebee as the vacuum oscillates in & out! This is a new noise that showed up after replacing the lines & the PCV itself. I'm going out to check the lines again w/ engine cool but wanted to run this by the forum.

I did not see any kind of inline check valve on the vacuum line so wondering if it's internal to another part or was missing... just need to know what it looks like & where it's supposed to be! Thanks!

 

Follow up:
Found a small I.d. adapter that may be the 'check valve' with my old vacuum lines. It necks down from 5/16" to a smaller line that appears to run thru firewall. Reinserted this thing and it appeared to help smooth out the engine a lot as well as quieting down the pcv valve quite a bit. Was this the mysterious check valve?

This adapter tube had nothing inside it, no check valve inside..... Just necked down to a smaller diameter but did seem to act as one.

Engine much smoother (still a little sputter), Heat/a.c. still stuck in defrost. Will try 4wd again tonight.

 

The only check valve I am aware of in the vacuum system is on the brake booster. 90 degree angled plastic piece that plugs right in to the booster.