I have no heat unless I rev up the engine

Car: Audi, A4, 1996 - Back to Fix-It - Audi Repair Manuals

Q.Checked the heater hoses going to the heater core. One is warm the other is cold at a idle. There are no leaks in the cab. The coolant was just changed along with thermostat and water pump. Any suggestions?

Answer Does the engine temperature gauge get up to normal operating temperature? It should be straight up, or half way. If not the thermostat is stuck open or opening to soon and must be replaced. This is a COMMON occurrence on this year model.
If the engine temperature is correct your symptoms are consistent with a plugged heat exchanger or heater core. There is NO valve in this system to restrict coolant flow to the heater core
The amount of flow going through the heater core is very hard to determine and the apparent resistance can be deceiving. If you try and remove the heater hoses and check for flow, you will not be able to determine the "flow" as the coolant is "drawn" (sucked) through the core by the water pump. This way there is flow through the heater core even before the thermostat has opened. If you run water through the core or blow through it, it may appear not to be restricted, again "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 often 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 heater valve (if equipped), 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%.
Note: This condition and testing is based on the fact that the engine is getting up to proper operating temperature. Straight up or 12 way on the temperature gauge, 90c (190 f).

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I have no heat unless I rev up the engine