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Old 09-19-2013, 04:52 PM   #1
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Onan Running Hot

Has anyone got an idea what would cause my Onan to run hot. I've checked both water systems; everything OK. It will run about 45 min. and then the HT switch (mounted on the block not the exh.) shuts it down. I took a temp reading in the switch area, it was around 250 deg.. Can there be a problem other than the water system causing it to over heat. BTW, I did removed the water thermostat; same thing. Any suggestions?
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Old 09-19-2013, 04:54 PM   #2
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fresh water circulating pump deteriorated? , fanbelt worn or loose?
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Old 09-19-2013, 06:28 PM   #3
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What model Onan??

Check the exhaust water flow. If it is normal flow amount and lukewarm, no steam, then it's the freshwater side of the cooling circuit. Fresh water pump or plugged heat exchanger.

If there is not enough flow at the exhaust, then check the sea strainer for blockage. Be sure and open the seacock and make sure you have flow in through the through hull. (Sucked up an eel one time and only his head was visible inside the strainer. Had to take the basket out to see it. He had a bad day and didn't survive the ordeal !!) If that is ok then check the raw water pump, impeller and bearings. Even a good impeller will not pump if the shaft and bearing are gone.
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Old 09-19-2013, 09:08 PM   #4
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Check your cap on your heat exchanger. Mine was spitting water out of the overflow and then it would over heat. Also check the hoses and make sure nothing is kinked. Let me know what you find out. I'm still trouble shooting on one of my onans....
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Old 09-20-2013, 02:15 PM   #5
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If the raw water flow is low (little or no water coming out the exhaust, or exhaust hose too hot to touch) then check for leaks on the intake side. I had to connect a garden hose into the system to find where a little water was squirting out. Of course, if water can squirt out then air can get in. And if the pump is sucking air, it isn't sucking as much water.
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Old 11-21-2013, 04:03 PM   #6
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Quote:
Originally Posted by CaptTom View Post
If the raw water flow is low (little or no water coming out the exhaust, or exhaust hose too hot to touch) then check for leaks on the intake side. I had to connect a garden hose into the system to find where a little water was squirting out. Of course, if water can squirt out then air can get in. And if the pump is sucking air, it isn't sucking as much water.
I have an Onan MDKC 4 & am looking for the zinc(s). Any help locating ? Jim
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Old 11-21-2013, 04:55 PM   #7
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250 is way too hot and can cause serious damage. If you have looked at your belts and they are not excessively worn, checked the coolant and it's ok, there is no water in the oil, oil in coolant or staining on the block--then this strongly suggests to me that the raw water cooling side is not working. Is the raw water seacock open, is the raw water strainer clean?

I am betting on the raw water side myself. If that stuff's ok pull the impeller on the raw water pump. It is the cause of many if not most overheats. It is a relatively simple task, requires a gasket , a new impeller and your time. You could merely inspect it--looking for cracks at the base of the fins, or excessively soft fins or even missing rubber, but the defects can be quite microscopic and still cause a problem. In MHO, replace the impeller to be sure you have eliminated the problem--they are cheap. If you find missing chunks of rubber from the impeller, try to get as many out of the heat exchanger tube as possible. They will clog the heat exchanger which will definitely cause overheating. If that doesn't solve the problem, remove the heat exchanger, take it it a radiator shop and have it pressure tested for leaks (pinholes)--those can be fixed usually--and have it "boiled" as you may have built up scale that is impeding flow and quantity. All of this stuff on the raw water side is routine maintenance which should be done periodically on all marine engines anyway and will improve the performance once you get it all back together. AND by all means, replace the zinc pencils while you have the thing apart--it's easier. If that doesn't fix it, then look into the other stuff.
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