Id look at that pump, the mono valve, and do a flush while your at it. Those are the only 3 possibilities.

When you punch the throttle, you increase the rpms of the water pump, therefore increasing pressure. Something "breaks free" and starts to let the hot water through.

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Eugene

10 E63 AMG
93 300te 4matic
07 BMW X3
14 Ford F-150 Fx2