I have a 98 GMC truck. My problem is real simple to describe. When I start the truck up from cold, the temp gauge moves up like it should. But it goes to 210 degrees, and then comes back down to 160 degrees and stays there. What would cause this initial rise to 210 before settling on 160? Heater seems fine. I think I do have a slow water leak around the drain plug, but I am guessing unrelated.

Thanks,
dbteeitup

Drain coolant to a level below the temperature sending unit. Try removing and cleaning the unit (deposits may have built up) or simply replace it. Clean connections thoroughly and spray with WD-40.

I have a 98 GMC truck. My problem is real simple to describe. When I start the truck up from cold, the temp gauge moves up like it should. But it goes to 210 degrees, and then comes back down to 160 degrees and stays there. What would cause this initial rise to 210 before settling on 160? Heater seems fine. I think I do have a slow water leak around the drain plug, but I am guessing unrelated.

Thanx,
dbteeitup

Before the thermostat will open, the coolant around the thermostat needs to be at the specified temperature, usually about 195°F. Once the thermostat opens, coolant that has not achieved that same temperature swirls in and the temp gauge responds. Your thermostat may be sticking slightly.
But, if the system isn't completely filled and purged of air, that can have an affect on the performance of the thermostat and the sending unit/sensor for the temp gauge.
Any leaks should be fixed as soon as you can.

I like CaptainRich's insightful analysis.