Is this supposed to be there?
 

https://photos.app.goo.gl/AyNySycWWFoKc6659

https://i.ibb.co/WWSPG2y/20231105-094710.jpg
From what i understand its supposed to provide fresh air into crankcase. Clearly its not. So WHY would it be in there?

No its not supposed to be blocked off. The hose is for the crank case ventilation and any fumes are routed to the inlet manifold to burn off. I suspect there were some vacuum issues and this was an attempt to remedy that. Which part of Dublin are you in?.

Is it a transit plug that wasn't removed when it was fitted by any chance?

Dublin USA? 1800E/1800ES? Check the layout and routing of all of the hoses in your PVC system. No it shouldn't be there. Looks like someone was chasing vacuum leaks. Hose should go to the middle of the inlet manifold.

That will help get through any emission tests if it’s required for that age of car

Guilty as charged LOL Indeed its Dublin CALIFORNIA USA (I am quite found of the original one though) and car is '73 1800ES.

I gathered same - Hose should not be plugged. It baffles me why someone would just straight up do this?! And that little cap seems to be so weirdly perfectly fit.

Hose indeed goes into middle of the manifold. However another peculiar thing is that plugged hole there. I seen other motors and its entirely not there. So i suppose it is OK for it to be plugged but why is it even there?? Mystery!

https://i.ibb.co/5MfcnPN/20231029-172835.jpg

Original design just vented through the cap to atmosphere, trying to reduce emissions they plumbed it into the manifold to burn the oil/water vapour I’m not sure how they control this additional air input into the intake you would think it would create a lean mixture
On old engines that get more oil than water out of this breather it can be a problem to pass the emissions test so they get stealth blocked for the test, perhaps they forgot to remove it

25(a name would be nice!)

Welcome to this forum!

In the Positive Crankcase Ventilation Sys of the your 1800ES/D-Jet...
https://www.sw-em.com/pcv_b20_fi.jpg
Source: https://www.sw-em.com/pcv_diagrams_a..._Configuration

...that hose is the extraction path for Crankcase gases mixed with fresh air...so if intentionally blocked (sometimes, if not periodically checked and serviced, it is unintentionally blocked by a clogged Nipple [the intentional Orifice Limiting in the extraction path], I propose the plug is someone's stealth defeating of the PCV Sys), it will result in the direction of flow becoming reversed, with oiled air being pushed into Intake Manifold (by positive Crankcase pressure which can occur under certain conditions), making an oily mess of the IM, possibly resulting in increased oil consumption (if a high mileage engine), even oil expulsion from any handy point of exit the Crankcase oil can find (typically Valve Cover or even Main Seals). A pressurized Crankcase is BAD!

I recommend you restore the PCV Sys to factory config by removing that plug and allowing the PCV Sys to function in the normal manner (especially in Cal, where your state inspection undoubtedly requires it to function)...but accompany this action with checking and verifying Flame Guard and Nipple are not clogged and free and clear to flow, and all PCV hoses are snug on their fittings with no leaks to atmosphere.

Good Hunting and greets from Connecticut!

Ron. Is nipple #3 wide open or restricted in size like the ones on the carb engines?

Derek;

I believe at the nipple is where the flow restriction is located...1.5mm comes to mind, and I don't know if its is part of the fitting, or a separate insert with the calibrated orifice.

Cheers