the bottom water pipe on my heater matrix going into the bulkhead has a small plastic box between the pipe that has a small steel rod that moves in and out what is this for sierra 2.8 xr4x4

 

its a valve that stops warm water going into the heater matrix until the engine has warmed up to a decent temperature

 

thanks for that does pulling the pin/rod out opens the flow then or its push in.

 

I didn't know the xr4x4 had these, my 4i certainly didn't, can't see why they'd need one as matrix is a full flow system, ie temperature is not controlled on the heater by a water valve but by a flap in the heater box.
tabetha