you may have enough trapped air in the upper coolant passages so as to
interfere with getting the thermostat open.
the 3800 engine has a bleed screw on the thermostat housing because
air pockets form in the upper cooling passages of a newly filled Buick 3800 motor.
said bleed screw helps get the air out, but only after thermostat opens.
if there is no coolant or not much hot coolant contacting the thermostat, it has a hard time opening
so as to allow coolant to flow. [ and that coolant flow is what gets the air pockets out of the system]
what is not good is that where an air pocket is located, the head can form a hot spot because
coolant against metal is what removes harmful heat.
a 1/8 inch hole drilled at the edge of the thermostat helps the coolant system fill naturally and
allows the newly introduced coolant to displace air if one has patience and adds coolant very slowly.
the Factory manual directs the mechanic to get the engine warm enough to make pressure in the
cooling system, then crack open the coolant bleed screw in order to release air.
with a drilled thermostat, not much air escapes via the bleed screw, usually none.
fml