In the area where the CTS and the fitting for the heater hose is located there is a third threaded opening, what's supposed to be there? It's obviously in the coolant passage but my stock base has only the two openings but the Big Mouth I recently picked up (from a Vette owner) has this third port. Was there some sensor or something specific to the Vettes that went there or what?

 

All the MAF TPI systems whether in the Vett, Firebird or Camaro had a Cold Start Fuel Injection Switch that provided a ground path to enable the 9th injector during cold weather starts. Since your working with an SD system just screw a NPT plug in the hole and your good to go.

 

I didn't even think of the cold start system, and the previous owners car was an '87. He was nice enough to include the sensor in the small parts bag as well and it never dawned on me what it was. Thanks for the info.