You may have air in the cooling system. Try burping the system. Open the radiator cap when cold and check the level. Add if it'* low. Start engine and watch temp gauge. Also watch the coolant in radiator, it should start moving when it gets up to temp. If not, put cap on and squeeze the top radiator hose. There may be a air bubble under the thermostat. doing this should force coolant back threw the thermostat. Once this happens the stat can open and the coolant can flow. There could also be air in the heater core. Once moving it should clear that as well.
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1997,1998,2000,2005 year confused Bonneville SSExi. Stock? ish.
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