Ralph,
  
  Tate's recommendation is a good start. While your problem could well be within the heat exchanger, there are a number of other things you might want to check first. There are several good references online to help diagnose the situation. Following are a couple of good ones...
  
  
  
  Also, be aware that the Perkins 4.108 used different heat exchangers through it's life. Mine uses a Bowman exchanger, but the engine is a 1987 model. I believe earlier models were different. And, unless you know the full history of your boat & engine, it's possible that the heat exchanger installed was not the original type.
  
  If you insert a photo of your engine/heat exchanger, it will be easier to tell what drawing/diagram applies.
  
  Jack
  
  2/11/14 one link removed per the email below... 
  
  Dear Webmaster,
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  I am the webmaster of www perfprotech com and I am working towards cleaning up the links that have been created on your website:
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  Please remove the above links, as well as all other links from www.westsail.org
  
  
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  I would appreciate if you can please address this and update me as soon as it’s done so I can update my records.
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  Thank You!