It's cooling just fine, but the heat is very weak and fades almost immediately (rendering the air temperature to ambient).

If I wait long enough before starting the fan, the heat is strong at start, but then fades away after a few seconds.

What I've tried

  • Checked the heating unit hoses that come from the main radiator - they are both hot, so the water system is not clogged
  • Checked the air temperature switch (one of those knobs you turn from blue to red - cold to hot) - it pulls a green cable that goes to a black box under the dashboard, to the right of the gas pedal. I think this box contains a fan as it started humming when I turned the fan on. So cable is fine.

So everything seems to be in place, but it's still not heating very well. I have one theory - the green cable that shifts between the hot/cold units is working fine - but something is wrong in the 'hot area' system - maybe something is blocking the air from passing through the heating unit, and instead the air is sucked from other places, i.e, holes etc?

ideas on how to proceed with this?


Car type is Daihatsu Sirion CZ 2001

  • Welcome to the site. What would really help here would be to know what the year/make/model of your car is ... I see you have Daihatsu tagged, but that really doesn't help too much. This will help with an understanding of exactly how the HVAC in your car works. Jan 3 '17 at 19:25
  • When you say both hoses are hot. Is that at the heater core or at the engine? There is usually a bypass valve between the engine and heater core.
    – rpmerf
    Jan 4 '17 at 12:11
  • @rpmerf There are two hoses that go from the engine compartment to the heating unit inside the passenger cabin, I assume it's IN and OUT of the heating unit. they are both hot so I presume the water goes IN the heating unit and then goes OUT back to the engine compartment
    – Shai
    Jan 4 '17 at 13:17
  • @Shai yes, those go to the heater core. Sometimes there is a bypass installed between the engine and the heater core that will allow the coolant goes from the IN hose to the OUT hose without passing through the heater core. This helps keep the heater core cool in the summer, but if the valve fails, you will not get heat in the winter. The symptoms you describe point to a lack of flow through the heater core. This can be caused by a clog in the heater core, a failed bypass valve, or a bad water pump.
    – rpmerf
    Jan 4 '17 at 16:00
  • @rpmerf the only thing that the temperature selector does is to pull the green cable that moves a flap inside the black box I described, how does that affect the valve you mentioned? either way, I assume it's kind of a mess to reach these parts (heater core, bypass valve) and I will need to take the car to the garage :\
    – Shai
    Jan 5 '17 at 7:46

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