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This condition is most often caused by a restricted heater core. Although the coolant flowing through the hoses is sufficiently hot to heat the climate control air, the heat of the heater core is dissipated into the system air faster than the coolant can flow through to heat the core. Some claim success by back flushing the core to expel the contaminants. I have had very little success doing so. I have gotten improvement, but never really fixed one.
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Sorry for the late response. The amount of flow going through the heater core is very hard to determine and the apparent resistance can be deceiving. The core of the heater core is not only a group of tubes, but the tubes are filled with a mesh material similar to steel wool. It takes very little resistance to overcome the low pressure from the engine. On a properly functioning system the temperatures of the two heater hoses is typically fairly close with only a slight reduction in temperature. Typically when I replace restricted heater cores the coolant appears perfectly clean. When I cut open the core and then it has a chance to dry out, small particles consistent with the appearance of sand are found in the mesh material.
I suggest you try to confirm the core blockage as follows. Fully warm the engine to operating temperature. Shut down the climate control system so that no air flows through the system. drive the vehicle at slow speeds or run at an elevated idle for a few minuets. turn on the defrosters at maximum heat. If the heat appears hot at first, but then rapidly diminishes, the core is likely the cause. If however there is no heat a temperature blend door or sensor may be at fault.
If there does appear to be a restriction in the heater core to further confirm, you can try back flushing (flushing backwards) the heater core with a water hose. Observe the expelled water, especially in the beginning, for any evidence of contamination. This may also give you increased performance, but typically will not be 100%.
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Jake "THE AUDI DOCTOR"