Cold starts--Temp goes up high and then back to normal

[Natedog]

Just got my 87 2.9 running again and when I start it up stone cold engine timeperature, the stock temp gauge goes up pretty high (but not into the red) before it drops back down to about the middle or a bit less. My previous engine, sending unit, thermostat, fan, water pump etc, didn't do this. I'm still using the same wiring and cab from before. Wires look good and are securely attached to the sending unit on the engine. Any recomendatons on what to do/check first?

 

[almostclueless]

thermostat in backwards

 

[Natedog]

No it was in correctly,but good suggestion. Will start it tomorrow morning and see how it is with a good used t-stat from my other engine.