David, I am not that familiar with the D3 but I am certain you have a blockage in your raw-water cooling system. Start at the sea chest and work your way along the entire raw-water cooling system ending at the elbow. Remove and clean any heat exchangers (transmission, engine cooling, and after cooler). Check all the hoses to ensure they are clear of debris and ensure the raw-water pump is clean inside. This will solve your issue. Somewhere in the system you will have an “ah ha” moment. It is best to do the entire system since if you have a blockage somewhere, the chances are you’ll have more downstream. Don’t stop at the “ah ha” moment.
This is also a good time to replace the seals/o-rings on the heat exchangers as well as any worn hoses.
Hope this helps.
With best regards,
Sailing Vessel - Cream Puff - SM2K - #275
Currently cruising - Tahiti, French Polynesia
main@AmelYachtOwners.groups.io [mailto:main@AmelYachtOwners.groups.io] On
Behalf Of David Crisp