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I have recently acquired a m25 engine. I am re powering a my C30 replacing a 5411. the engine starts up nice and easy , sounds good but after running for 10 minutes starts to over heat? Raw water flow is good, thermostat is good, coolant is topped up and seems to be blead properly. the problem seems to be on the closed cooling side of things. What should i be checking next?

thanks

SEAN
 

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Heat exchanger bundle fouled?

Hard to be certain that the fresh water side is 'bled' properly. I'd try running with the pressure cap off the next time and see if your level drops as the system 'burps' itself. If so this issue might go away.
 

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^^^ THIS

I have clients that bought an '85 O'day 35 with an M25. While teaching them ASA 104, we could not operate the engine at over 2500 RPM for more than 10 minutes, or it would overheat (+200ºF). We checked the coolant level and burped the coolant system, cleaned the sea strainer, cleaned the through hull (which had a grate - I have advised him to remove), and the problem persisted. It made for a long class, as we had to motor upriver on the way home. The only thing that we did not do is change the thermostat.

He contracted me to upgrade from the 2½" heat exchanger to the 3" heat exchanger, and re-plumb the hot water heater to draw heat from the bypass circuit instead of the main cooling circuit. After the upgrade, my wife and I met them at a nearby marina. They motored the entire way to meet us. After they settled in I asked how the engine temperature was. They told me that the engine temp never exceeded 160ºF, which is also not good (it may lead to carbon buildup in the exhaust elbow). I have advised him to replace the thermostat with a higher temp thermostat.
 

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I did a complete renewal of all of my cooling system on the M25 after I was experiencing the overheating at higher RPMs issue.
I did a complete flush of the system, and lots of gunk comes out of the heat exchanger for the hot water heater.
I replaced all pumps, thermostat, and hoses. Although I tried to be careful to not let any air stay in the system, I had a big air pocket any way. Normal bleeding techniques did not help. Engine would overheat rapidly, but coolant level in the reservoir stayed high.
I remembered reading some where about over revving the engine briefly as it began to over heat. As the temp gets real high, slam the throttle wide open several times for about 10 seconds. Shut off engine and let cool if it still overheats. After about 3 tries the temp dropped and the air bubble had passed. I was able to add more coolant. All has been well since.

Lots of good advice here. Check if you have the 3 1/2" heat exchanger and check your exhaust elbow too.
 

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There's a good section on the 25XP in the tech wiki at C34.org.

Diesel Tech Wiki
Cooling
Engine Air Lock - Engine Heating Problem - c34.org

The bottom link tells you how to get the air out without revving the engine (which would make me nervous).

Relocating the water heater to in front of the fuel tank where it is on newer models will make the filler cap will now be the highest point in the system and should eliminate the problem.

Something else too look at (isn't there always). Use a hump hose to connect the exhaust to the water lift muffler. It will prevent problems down the road. The hose is available from Catalina Direct.

 
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