Back in 2022 I had an engine fire and burned up one of the valves in my opti harness. 2023 and 2024 saw me having opti issues. Last year I took apart the first "failed" unit, cleaned/reassembled and it ran fine. I'm convinced that blowby was getting in and causing issues, so would like to get a filter, check valve, orifice, whatever GM used instead of dropping $100 on an aftermarket harness. Does anyone know what components I'm after?

 

I was pretty much in the same boat you are. Swapped a 96 engine into my 93 and didn't get the vacuum hose to vent it, thought the atmosphere would be enough like the older style. Well it wasn't. The opti corroded within a few thousand miles. Realized I needed the hose but couldn't find a way to make one. After a week of looking I ended up buying the GM $100 vacuum harness. I foresight I didn't want to end up doing another opti clean because some aftermarket valve didn't work enough. Here's my money GM.

 

without the vacuum harness, the opti will quickly get condensation in it (500-1k mi)

These may work to make your own harness. Confirm you have the flow orientation correct

 

I still have the harness, all I need is to make sure I get the correct orifice and/or valves. It's not open to atmosphere, rather blowby from the engine is finding its way in.