So went out laning yesterday. Car got to temperature on the road and then when laning the temperature on the gauge dropped. I've recently changed the lower coolant rail and bled the system following how sierra does it. I was in 2nd gear low range laning at about 1500 - 2000rpm.

My only theory is that with the colder temperatures and the RPM I was at with the interior heater on it was over cooling the engine.

Has anyone else had this?

I don't know about the d2 specifically but i suspect the temp gauge is actually an idiot guide eg cold/cool/normal/too hot settings with little or no analogue movement, so if the temp passes a threshold then it will move a larger amount quite quickly.
Use an obd reader or diag to get the real temperature

Frazzle said:

Car got to temperature on the road and then when laning the temperature on the gauge dropped.

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That's a classic sign of stuck open thermostat

sierrafery said:

That's a classic sign of stuck open thermostat

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New thermostat it is!