GMC 248,
Yeah, this one had me "buffaloed" for a spell!
I even tried two different brands of t-stat (Stant & Robt. Shaw), with the same results.
This was in the winter and the weirdest thing was, even as the gauge was climbing for "Hot" , the heater was blowing cold air!
This struck me as odd, since the six is supposed to have a by-pass cooling system so that coolant can still circulate in the block while the 'stat is closed.
So anyway, it's behaving now; next time around, I'll drill a 1/16" hole and see if strikes a better balance between warm-up & "spiking".
