So I just completed the Extinct thermometer mod, changed out coolant, and changed out the two rigid plastic hoses that run from the expansion tank.

Everything seems to be running okay, except for a loud hissing sound that seems to be released pressure? I've identified where it's coming from, but don't know what exactly is going on. If I put my finger over it, the hissing stops.

 

Okay, looks like I figured it out. That was an elbow piece that had a narrow hose pop out. I turned the elbow piece 45 degrees and reinserted. Does anyone know what this is? Looking through Rave but can't track down.

 

that connects into the SAI system.

 

There are three vacuum ports in the area of the upper intake manifold shown in your pics.

The one with the nylon line that enters the manifold via the port with the red collar runs to the brake booster to provide vacuum used for the power brakes.

The larger one next to the first one connects to the Bank 2 valve cover and is a port for the crankcase ventilation system.

The third port is much smaller and is located below the first two. It was used as part of the Secondary Air Injection (SAI) system on trucks equipped with SAI. On my truck, there's simply a small, neoprene or silicone cap over it (pictured). In the pics of your truck, it looks like there's a small hose connected to it. Is your truck SAI-equipped? If so, there should be a line running to the SAI system. If not, it should be plugged, capped or otherwise closed off. Otherwise there'd be a substantial vacuum leak that would likely trigger lean codes (P0171 and P0174) and the Service Engine Soon light.

 

Where does the other end of the line connect?

 

You need to do the PCV mod.