Hello and welcome to Just answer.com, Super mechanic here. About your vehicle.
There has been a service bulletin for that type of concern. Here is what it states.
19 08 14
November 21, 2008.
Supersedes Technical Service Bulletin Group 19 number 05-03 dated April 8, 2005 for reasons listed below.
19 Coolant Leak or No Heat in Passenger Compartment From HVAC
Both concerns may be caused by a coolant leak from plastic plug.
Plug is installed in engine cylinder block near connection for thermostat housing (arrow).
Improved Coolant Plug.
Other than loss of coolant, a leaking plug can cause air to enter into the cooling system and reduce the effectiveness of the passenger compartment heating system.
32 degree F / 0 degree C ) (see ElsaWeb Repair Manual, Engine Mechanical, 19 Engine Cooling, "Cooling system, checking for leaks").
Tip: The leak from this plug usually occurs only when the engine is cold (below 32 degree F / 0 degree C ).
If cooling system is tested for leaks with engine warm or hot:
Tip: Coolant leak from this plug can be sometimes misdiagnosed as a leaking coolant thermostat housing. Always carefully inspect suspected areas to detect the actual source of the leak.
If the plug is found to be leaking:
Electrical connector of coolant level sensor must be unplugged.
It is not necessary to remove coolant hoses from coolant reservoir.
Ensure that the O-ring on lower steel tube is not lost when tube is removed).
Fastener torque = 23 Nm .
When refilling engine with coolant: