Diagnosing heater core issues, a post repair checklist and why they fail

  • If the engine is cold, the heater core will struggle to heat. The heat from a heater core comes from the engine so its has to be warm to generate heat in the cabin.
  • If the coolant level is low this will reduce the heater core's temperature.
  • If the inlet hose is close to the engine temperature e.g. 190-210o, the thermostat is functioning. If the engine is cold and it does not warm up, then check your thermostat is not stuck open or lower than intended.
  • If the heater core outlet hose is 10-25o less, the heater core is shedding heat.
  • If the outlet temperature is 50-80o cooler than the inlet, then the heater core may be clogged and require flushing.
  • If the heated air is hot but airflow is not strong your blower may have failed.
  • If there is not much heat, the engine is running at the expected temperatures, and coolant levels are appropriated, you may have stuck blend door or a clogged cabin filter.
After a repair checklist 
  • The coolant level is correct.
  • The engine coolant thermostat is opening on closing properly.
  • The heater core is not restricted.
  • The coolant flow from the block to the heater core is sufficient.
  • The water pump is moving coolant properly, and the drive belt is turning the water pump at the proper   speed.
  • Air is trapped in the system and blocking water flow.
  • The heater controls (both air management and coolant flow) are adjusted properly.

Why do heater cores fail?

  • Lack of adequate cooling system service. Most automobile manufacturers recommend that the cooling system be inspected, flushed, and refilled every two years or 24,000 miles.  The inspection should include a PH test.
  • Inadequate coolant properties can cause severe cooling system damage.  Modern anti-freeze should contain additives, which cleans and protects the cooling system, and slows or stops corrosion.  The additives are designed for specific types of vehicles, so care should be given when selecting anti-freeze to put in you vehicle. Shelf life of the traditional glycol based antifreeze is limited to eighteen (18) months, due to the silicate inhibitors dropping out of solution beyond this time.  Refer to OEM specifications for further information.
  • Erosion of the metal, due to the action of the flow of the coolant through the heater cores.  This is usually present in aluminum heater cores.  The faster the coolant flows through the core, the greater the effects of erosion.  The use of an “inlet restrictor” (required on several OEM applications) may be used to slow the speed of the coolant, as it passes through the heater core.
  • Coolant contamination usually occurs when the coolant system has not been serviced properly.  The protective chemical additives will become used up, and fail to protect the coolant system.  This can lead to erosion causing an accumulation of debris that can block passages, or the acidic action that may attack the base metal.
  • The affects of electrolysis can cause a rapid failure of the heater core.