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post #1 of 6 Old 05-12-2013 Thread Starter
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m25 Overheated

I had a bit of a scare the first time starting up the m25 on my new to me 1986 Catalina 30.

I started the engine and let it idle of for 10 or so minutes. (it never left idle for this problem) I noticed the temp gage was steadily climbing after the 10 or so minutes. Five or so minutes later I took a look and the needle was barred. I shut off the engine. There was water flowing out the back the whole time so the raw water pump was working.
Another Catalina owner helped me diagnose the problem. The first thing he did was bleed the valve for the antifreeze on the engine and just got hot air/stem. When opening up the cap you could see sum antifreeze but it was mostly air. We then determined the reservoir in the cockpit locker was empty.
He then check the oil and it was black (it needs to be changed) but mentioned it was good he didn’t see anything mixed with it.
So my question is two part.
Question one: Did I really heat my engine up that much or was the temp gage measuring the air in the system? If not what are the odds I damaged the engine?
Question two: It was on the hard all winter with anti-freeze added to the bulge to stop water from freezing so I can’t tell if it leaked into the bulge. But I assume if it wasn’t mixed in with the oil it leaked sum ware. Do any of you know a common point a leak can occur?

I did add new antifreeze and drive the boat under power to its proper dock and after the boat was tied up revved the engine in gear and the tempter was fine.
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post #2 of 6 Old 05-12-2013
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m25 Overheated

It is very likely that there is some air in the freshwater side of the cooling system. The temperature you saw is probably an elevated temperature of the air. The engine was probably hot but not as hot as indicated.

To get the air out, try the following:
1. Make sure the coolant cap is on and closed and the air bleed valve is closed

2. Start the engine, run until thermostat is open. You can tell the thermostat is open when the temp gauge registers above zero

3. Rev the throttle between idle and operating RPM (approx. 80% max). Do this back and forth slowly. This helps work the air through the system to the manifold where it will rise to the top. Keep an eye on the overflow container and fill it with more if needed as antifreeze/mix replaces air in the loop. If the engine gets hot, shut it down and repeat once cool.

3. Repeat process until temp holds steady. Probably will take 4-5 rounds

After some routine heat exchanger maintenance (removed, cleaned, etc.) my M-18 was heating up. The bleeding and coolant cap techniques weren't working very well. Another person at my marina with an M-25 recommended the approach above and it worked like a charm. It's a technique not mentioned in any of the texts/books.

If that doesn't work:
1. Check hoses, clamps, coolant cap, and bleed valve for leaks that might allow air
2. Check you heat exchanger for sufficient flow

I've read that many people upgrade the M-25 heat exchanger with a 3" model instead of the 2"

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post #3 of 6 Old 05-12-2013
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Re: m25 Overheated

If after you filled the freshwater side with antifreeze and the engine ran and the temperature was fine then you can conclude that the first overheating episode was due to lack of coolant. There should be no need to check for airlocks or a frozen thermostat.

You need to concentrate on what happened to the coolant over the winter. No coolant in the oil is a very good sign because it means the leak is external of the motor. First check to see if your coolant level goes down over a few days sitting idle. If it has not then check the levels before and after you have run it up to temperature. It's better to run it under load so tie yourself well to the dock and put it in forward or reverse and take it up to cruising RPMs.

The most likely spot in a 1986 boat is the heat exchanger but there are other possibilities. The best time to look for obvious leaks is while the engine is running at normal or above temperatures as the system is pressurized.

If you discover that the coolant level does not go down then you may have boiled off a lot of coolant when it overheated in the first place which was probably caused by a sticky thermostat. I would replace the stat regardless.

If the coolant goes down and you can't find the leak. First check to see if there is coolant in the exhaust water or raw water in the coolant. That's an indicator that your heat exchanger is shot and needs to be replaced. If all else fails you may have to call a mechanic who has a tool to pressurize the system and he will be able to tell you if you have a leak and/or help you track down the leak.

Good luck. Cooling systems can be a *itch.
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post #4 of 6 Old 05-12-2013
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Re: m25 Overheated

Whenever you change coolant in an M25 you need to BURP the engine. Some do it with the petcock on the top of the thermostat. The better way is to get rid of the air in the line to the hot water heater, this way:

Engine Overheating 101 - How to Burp Your Engine (Reply #6) Engine overheating problem (with Instructions "How To Burp!")

There is a lot of M25 engine information on our C34 website, the Critical Upgrades and the Tech wiki under engines


Catalina 34 - C34

Stu Jackson, Catalina 34, 1986, M25 engine, Rocna 10 (22#), Maple Bay, BC, Canada
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post #5 of 6 Old 05-13-2013 Thread Starter
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Re: m25 Overheated

Thanks for all the help everyone. When I go back to the boat I will se if the leaves are down and if so stat hunting to the leek. I will replace the thermostat also.
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post #6 of 6 Old 06-08-2013 Thread Starter
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Re: m25 Overheated

Well it looks like there is a very slow leek from the water heater. I se a bit of antifreeze in the forward part of the cockpit storage locker. I can't tell from where yet. It is so slow I don't know if I can pinpoint it. After three weeks and two days of motoring the level was down a quarter of an inch.
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