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,
I hope this email finds you well.
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.
I appreciate your help and attention to this matter.
Thank You!