"Frank,here is my contribution
"Frank,here is my contribution based on my own experience.I presume you have a closed cooling system using coolant in the motor and raw water for the heat exchanger. I would first check that the raw water pump is pulling in pleanty of raw water, and flow increases with revs. This can be done by observing the water flow through the clear top of your raw water strainer located between the sea cock and the raw water pump. You do have a strainer I hope,Make sure your strainer is clean and not clogged.
If the flow is weak, your raw water pump may be worn, or, the impeller is in pieces, some of which may be restricting flow to the heat exchanger, or be lodged in the exchanger.
Can you see the raw water pulsing out from the exhaust? If the activity in your strainer is strong and water is going out the exhaust, chances are all that is ok. If not I would pull the heat exchanger and back flush it with a garden hose and see what comes out. Then I would take it to a radiator shop have a pressure test and acid bath done before re installing.
If everything is good in this area, check the internal motor coolant pump. You can do this easily by removing the thermostat and with the cap off the expansion tank, observe the flow of coolant with the motor running.
If your raw water system and your motor coolant system are ok, then go for the thermostat. Do you have a thermostat ? What temperature rating is it ?
To test the thermostat,remove from beneath expansion tank, place in a bowl of just boiled water and observe if it fully opens, partially opens, or is jammed shut.
I will possibly attract a lot of criticism for this next suggestion, but believe me, it works fine.
Go to an automotive store and buy a 160 degree thermostat to suit a Chrysler 318 motor ( and a couple of spare gaskets ).
Do not install, repeat do not install, this stat without the following modification or you will certainly overheat very quickly.
Carefully place the stat on top of a partially opened vice, and drill four (4) quarter inch holes in the flange plate. Keep centered on flange staying away from inner and outer edges and the stat mechanism supports, file smooth,do not bend or distort stat during this modification. Re install with gasket cement making sure that the gasket goes down first on top of motor, then the stat fitting neatly into the grooved bottom of the expansion tank.
With this modification the motor takes slightly longer to get up to running temp, but even on long hard runs, or at speed up to 4200 rpm, my temp never goes over 175.
These checks do not take too long and anyone who can use a wrench and screwdriver can do them.
All this is presuming that everything is ok with manifolds and risers etc. The best thing you can do before you start, or when you have finished is have the motor checked with an infra red temperature probe, and see where the heat is concentrated, usually this will lead straight to the problem.
Good luck, and please keep us posted on your progress."
