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Old 25-05-2020, 10:17   #1
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M25 Cooling Problem C34

We are having problems with our 1986 m25 universal Diesel engine cooling. Running at idle for 10 minutes causes a steady rise of temperature to 200 and still increasing (at which point we kill it). All connections to heat exchanger (part 299049) are cold to the touch after this period, even though coolant is hot. Replaced thermostat, no blockages in heat exchanger (gravity pulls coolant from manifold out other end of heat exchanger). Was able to pump coolant with siphon pump from “to water heater” hose to the thermostat cap. Hot coolant from thermostat not seeming to travel to heat exchanger even though no apparent blockages or air locks. Coolant comes out of the thermostat pet cock when bleeding. Possibly faulty coolant pump? Any other causes for this problem?
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Old 25-05-2020, 10:39   #2
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Re: M25 Cooling Problem C34

Have you disassembled the heat exchanger to clean it?
Does the replacement thermostat have a small hole in the outer ring? Sometimes they will have a loose rivet.
I have never seen a water pump stop pumping, but it’s possible. You ought to be able to confirm that by removing a coolant hose and cranking the motor, if the pump is running coolant will come out of that hose fast, just don’t disconnect the suck hose.
Of course don’t do this with hot coolant.

Did this just happen out of the blue, or did you do some work to it?
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Old 25-05-2020, 10:49   #3
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Re: M25 Cooling Problem C34

Engine Overheating 101 - How to Burp Your Engine (Reply #6) http://c34.org/bbs/index.php/topic,4....html#msg26462


Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too

http://www.c34.org/wiki/index.php?title=Diesel_Engine


How old are your coolant hoses?
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Old 25-05-2020, 16:48   #4
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Re: M25 Cooling Problem C34

Are you pumping water out the back of the boat?
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Old 25-05-2020, 16:50   #5
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Re: M25 Cooling Problem C34

I’m still wanting to know if it just started on it’s on, or after a coolant change? I’ve seen overheating from an air bubble if you will, but always only after a coolant change, I have never seen a bubble just appear.
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Old 25-05-2020, 16:51   #6
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Re: M25 Cooling Problem C34

Quote:
Originally Posted by sailah View Post
Are you pumping water out the back of the boat?
Has to be, heat ex is cold?
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Old 25-05-2020, 16:55   #7
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Re: M25 Cooling Problem C34

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Has to be, heat ex is cold?
Just trying to cover all bases
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Old 25-05-2020, 17:42   #8
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Re: M25 Cooling Problem C34

I have same year engine. After I changed my water heater my engine was overheating exactly like yours . Mainsail's Marine How-To had an article on burping/bleeding the air out of your water system. I ended up attaching a hose to the thermostat bleeder valve and holding the hose up high and filling with coolant - that immediately solved my problem.
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Old 26-05-2020, 02:21   #9
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Re: M25 Cooling Problem C34

Solved the problem. It was an air pocket that developed in the coolant pipes leading to our water heater. Disconnecting it from the system finally lead to temperatures at normal levels. Our temperature never exceeded 160 F afterwards. Thank you everyone for your quick responses. Feel free to ask any questions, I will be happy to answer.
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Old 26-05-2020, 02:22   #10
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Re: M25 Cooling Problem C34

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Are you pumping water out the back of the boat?
Yes, water was always pumping out the back.
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Old 26-05-2020, 02:24   #11
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Re: M25 Cooling Problem C34

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Originally Posted by a64pilot View Post
I’m still wanting to know if it just started on it’s on, or after a coolant change? I’ve seen overheating from an air bubble if you will, but always only after a coolant change, I have never seen a bubble just appear.
That was the problem! Heat exchanger heated up as soon as our temperature reached our thermostat threshold (160F) once we took the heat exchanger out of the system.
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Old 26-05-2020, 02:33   #12
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Re: M25 Cooling Problem C34

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That was the problem! Heat exchanger heated up as soon as our temperature reached our thermostat threshold (160F) once we took the heat exchanger out of the system.
This was after coolant was replaced. The problem is our boat doesn’t have an air release valve on our water heater for some reason. They are generally found towards the top on our boat (c34).
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Old 26-05-2020, 14:13   #13
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Re: M25 Cooling Problem C34

Quote:
Originally Posted by OntarioSailor View Post
This was after coolant was replaced. The problem is our boat doesn’t have an air release valve on our water heater for some reason. They are generally found towards the top on our boat (c34).

It would be wise to make sure you don't confuse the (domestic) water pressure relief vale which you describe ON the water heater, since that's not for the coolant loop. There is no high(er) spot in the coolant hose routing on our C34s other than the thermostat housing on the engine (and its hoses which I use to fill the heater loop to "Burp" the engine), and which I included in my first link in my earlier posts. The water heater is lower than the engine and was built under the galley sink in the Mark 1 boats (1986-1994).
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