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My heater heats up but quickly cools down when i turn the fan up all the way. If I leave the fan on two (out of three) the heat stays. The heat is gone within thirty seconds after turning up the fan. Any suggestions? Coolant is topped off.

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    Sounds like there may be something restricting the flow through the heater core. – dlu Nov 13 '16 at 23:08
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Coolant flow through the heater core is restricted, which means the fan is able to cool off the water in the heater core before it is replaced with warm water.

There are a few possible causes for this

  1. A water pump that is not pumping at full capacity. This can happen when the water pump impeller corrodes away
  2. Corrosion blocking the inside of the heatercore. The heatercore has small passageways, it can easily be restricted by the presence of corrosion.
  3. A kinked hose leading to the heatercore. This can also cause a restriction of flow.

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