Heater blows cold?

First of all check the thermostat.If its checked ok or you have already replaced.
Then you have an "Air Lock" within the heater. One must purge all the air out of the cooling system as close to the highest point as possible after the system has been partially or fully drained. Most of the time one will find small BRASS Bleeder Valves located somewhere near the highest point. Open each one while the system is pressurized and let Air escape until some Coolant escapes. Sometimes it may be necessary to loosen a Heater Hose and bleed the air at that point. Remember that the system MUST BE PRESSURIZED to accomplish the Bleeding of Air. Many auto parts stores sell hand pumps with a pressure gauge for this purpose and leak testing. These pumps typically replace the Radiator Pressure Cap temporarily. The system does not have to be at normal operating temperature to bleed the air out. Once the Air has been bled you will have to refill the radiator to normal full and make sure that the Coolant Recovery Tank level is also brought up to recommended level. A quick check is to remove the Radiator Pressure Cap, Start the Engine, Rev the engine suddenly (A short Jab) and see if the coolant level falls rapidly in the radiator. If it does it is definitely an AIR LOCK situation.

This details will help.

Thanks.

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