I have a 2005 Civic with under 100k miles and as of lately the radiator is losing about a cup of coolant to the reservoir tank and the reservoir tank is overflowing. My heat blows cold air at idle and won’t blow warm air until at least 40 mph. I’ve replaced the thermostat and radiator cap to no luck. This past week I talked to a mechanic and he suggested I poured bars leak or blue devil as he described the start of a bad head gasket. Poured it on and almost clogged the heater core. Took the car for a test drive and had the car running for a little over 3 hours. Everything worked like it should. Now a few days later I preheat my vehicle for 10-15 minutes and the heat is blowing ice cold air like my AC is. What can be the issue any advice or help is greatly appreciated

 

Has the system been bled?

 

System has been bled. Thermostat will be replaced again as engine over heated and coolant was lost. Thinking that liquid that was poured in the engine (Blue Devil) made the thermostat stick