that is a vacuum line. Since this engine is used in multiple cars, I believe on the Grand Am that connection is for the HVAC controls (as they are vacuum controlled). On the Monte, I think just a rubber cap, I don't think that is used for anything, but does need the cap to plug it so you don't have a vacuum leak.

 

I (regrettably) made this same post a long time ago. Unused vacuum nipple. Its for the rendezvous that uses the 3400. As stated above however, you need to cap that off.

EDIT: Wow I looked back on that post. That was... Difficult to read. Well if you cant laugh at yourself....

 

Thank you for your replies. No one that has looked at it has been able to figure it out so far. So just so I'm clear..cap on or off lol It seems like if I plug it the car will stall out. I always take my finger off before that happens tho so I can't be sure. Thank you so much again for your replies! I love this car and am happy to learn all I can!

 

Ok I was mistaken.. There is supposed to be a line there.. There is another nipple towards the BACK of the manifold that should be capped.

Sorry I cannot remember where thats supposed to go but all the diagrams im looking up says it goes to the transaxle....?

 

I did some google image searching. It confirms as I recall, unless GM did something unique, that vac port on the 2000-2005 Monte and Impala is to be capped off. That port is used on other cars with this engine, such as the Grand Am. Here is a pic I found and you can see the rubber cap on this one. I found a few other pics, but they are not as clear as this one.