Universal M25 Overheating - Puzzling symptoms - SailNet Community

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Old 12-08-2008
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Universal M25 Overheating - Puzzling symptoms

Hi there,
I have a Catalina 30 with a Universal 25. It overheats pretty quickly on startup.

When I feel the hoses going to and from the heat exchanger, there is no heat and flow it appears.

However, when I pull the thermostat out of the thermostat housing, the engine stays cool and I can feel warm water going into the heat exchanger.

The thermostat is new and opens just fine when in hot water.

So not sure what is going on here!

Any tips?
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Old 12-08-2008
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The thermostat isn't in backwards is it?
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Old 12-08-2008
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Water Pump?

Is your water pump moving enough water? Perhaps when you remove the thermostat the decreased resistance allows the pump to move more water and cool the engine better. I'd check the pump, if the thermostat checks out.

Paul
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Yeah, I thought about the thermostat being backwards. However, it only fits one way and also checked the engine diagram/parts manual.

This whole thing started with a water pump that was shot so have a brand new pump in there.

I do wonder if I have enough circulation/flow. What is the best way to check that?

Endre
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Do the thermostats come in different temperature ratings? Perhaps yours needs to open at a lower temp than the one you have.
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Old 12-08-2008
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E,

I had a similar problem with the Universal 25 on my Catalina 30. I started at the impeller and worked toward the heat exchanger. Disconnect a hose and turn the engine over with the kill switch on, to see if I had any flow. In my case the impeller was ok, but i had rock like deposits built up in the o/p from the impeller housing. I cleared those and thought I was home free and when I pulled the hose going into the heat exchaner, good flow. When I pulled the hose going out of the exchanger, hardly any flow. My Heat Exchanger was bad. I could not boil it out and ended up installing a new exchanger. Problem fixed. Good luck.
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If the problem was the heat exchanger, would it stay cool with the thermostat out? I think not.

If it truly is overheating (is the temp guage into the red?) then the thermostat is not allowing the primary coolant to get to the exchanger, or (less likely) the primary coolant pump is not beating the restriction of the thermostat.

Are you sure that it is truly overheating? Is it gurgling and boiling when you shut it down? Make sure you keep your primary coolant topped up until the problem is sorted. Be careful not to get poached by a steam escape from that primary coolant header tank opening cap. I have seen the aftermath of it once from a steam escape from a car, and it was a horrible injury. Be careful. Your primary coolant is pressurised, and very hot. Let her cool well down first.

I wish this one was local.

It's not in NE Scotland, is it? Perhaps not.
.

Last edited by Rockter; 12-08-2008 at 04:47 PM.
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Old 12-08-2008
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The approach suggested by Swadiver seems like to way to go. However, I would try another thermostat (and check the rating) before you start removing pipes etc. I would still be suspicious of it, since the engine ran cool when it was removed.....

Good luck

Stuart
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It is puzzling. It is overheating with the gauge in the red and lots of steam and gurgling right around the thermostat. But not in the header tank of course... It is really really weird. I will double check to see if I have the right thermostat.

Is there a way to figure out if the pump works well?
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Old 12-08-2008
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Which pump did you replace? The raw water pump or the fresh water pump on the motor?

Quote:
But not in the header tank of course... It is really really weird. I will double check to see if I have the right thermostat.
Now that is wierd. I wonder if the coolant isn't all the way topped off and there is a big air pocket between the engine and heat exchanger.
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