[Gpersi]

I just changed my risers on Crusader 7.4 Tiara 31 open for preventive maintenance once we put a load on it and got up to cruise. The freshwater side on the port engine would start to overheat  Then mechanic said change the raw water pump impeller's and thermostat we ran happened again the engine still started to run hot thought maybe it was low on cooling and coolant topped off. Ran the boat again with the hatch open and then observed coolant coming out the overflow and the hose going to the exhaust manifold was very hot about 187 on the heat gun. The mechanic is saying, maybe we need to have the heat exchanger sent out and checked out which I don’t have a problem doing I just don’t want to keep trying things and spending money if that’s not the case any ideas?

[ReelMagic]

Not sure if this helps but as the engine warms up and the coolant expands its normal for the coolant cap to allow coolant to go to the overflow tank. When the engine is shut off and cools down it normally sucks the coolant from the overflow tank back into the engine.. Ensure your cap is correct for the motor and not failing... If the coolant doesn't go back into the engine and you need to add coolant after the engine cools down I'd suspect an air leak somewhere and or the heat exchanger (could be the oil cooler as well)... Ensure the thermostat is seated in its housing correctly as a failed or not seated thermostat could cause the water flow to partially bi-pass the engine when running high RPM. M2C

[Medicone]

My suggestions:

1.  Burp the system.  Very often you will get a bubble after you flush or refill the coolant.  Your mechanic should know how to do this.
2.  Make sure you have the correct temp thermostat.  Find your engine serial number and look up the thermostat part in the corresponding parts manual.  Are both engines hot or is just one hot?  If just one, probably not thermostat.  Another way to find out if it is a thermostat issue is to just temporarily take the thermostat out and see if the issues remains.
3.  Make sure the engines were refilled with the correct coolant.  Was it correctly diluted or was it pre-diluted.
4.  Leak in the coolant system.  Check to see if all of the clamps are tight and on correctly.  If there is a pin-hole leak or a bad seal, this can affect the FWC coolant circulation and also create air lock.

Check what temp your engines are supposed to operate at.  If I recall correctly, the non-FWC cooled engines ran a lot colder than FWC cooled engines.  On my old boat (sold), the normal operating temp on my 8.1s was 170-190ish.  I don't know what the FWC 7.4s operate at.

Since the problem only presented after you changed the elbows (I don't think the 31 has risers, right?), I doubt it is the heat exchanger.  Since it sounds like only one engine is affected, I doubt it is a thermostat issue or incorrect coolant.  I would vote on the possibility of an air bubble in the coolant.  Maybe try burping the coolant system.

Good luck!
M1