[C38] How do I tell if my heat exhanger is not working?

[1derful at comcast.net]

Hello, 
  
My Catalina has been overheating.  Universal Diesel 5424. 
I removed the thermostat........it was corroded and wrapped with debris. 
I removed and cleaned the internal water pump.  Also lot of debris. 
Put hose in engine.  Water didn't flow but eventually it did.  More debris. 
now when I run the engine it is cool until I put on the radiator cap..........then it gets too hot and loses water. 
  
Any thoughts? 
  
Patrick 
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