Universal M-25 Overheating

[barleycopse]

We have (had?) a problem with out CD-31's Universal M-25 engine overheating. It was usually hitting the 220 degree mark after about 10-15 minutes at idle in the slip. After changing the thermostat and flushing the heat exchanger, both of which made little difference, our mechanic removed the hot water heater and the cockpit locker expansion tank from the engine coolant loop. After a sea trial of about 1 hour under a load and the rpm's at over 2,000, the temperature stayed at 160.

Of course, we can use the water heater's A/C power when we are at a slip, but it would be great to have the engine heat water, too. Has anyone experienced and solved this problen?

Thanks for your help.

Cheers,
Don Cline

[Phil Shedd]

Don

I also had this problem on our CD31 . I also bypassed the hot water heater for the half season when we first got the boat. The next year I put things back the way they were but at this time I replaced the hoses. When I filled the expansion tank it seemed to take a lot of antifreeze however when I started the engine it did over heat.

The first thing I did was bleed the air out of the valve on the top of the thermostat cover. There seemed to be a lot of air in the block. After a while I checked the expansion tanked and toped it up. After that things seemed to work very well. The main thing is to get all the air out of the hoses to the hot water heater and the engine block. When there is air the water will not circulate.

This is our third season and all things are working well(knock wood) . There is nothing like having hot running water on board. Just plan civilized.


Phil Shedd

[Oswego John]

Phil,

In many cases, that's what happens. Another name for it is an air dam. Air wants to rise and can get trapped. Some people install an air vent, or bleeder valve in the highest places of their systems to eliminate the trapped air in the system.

This is similar to bleeding air from convectors in a home that is heated by circulated hot water. For it to be efficient, all the air must be bled off from the system and all it's branches.

Another time that you may find an air dam is when an engine seriously overheats and a cracked head developes. The compression from the piston forces air through the minute crack and an air dam builds up inside the water passage in the head and the air has no place to go in a sealed system. This pressurized, trapped air blocks coolant flow in that area and more heat developes in the area of the crack.

An air vent also helps drain coolant before winter layup.

O J

[Tom]

Anytime you open the coolant system you get an airblock in the top of the water heater since that is the highest spot in the coolant lool. It's easily fixed but the way to test it is just to put your hand on the expansion tank. If there isn't a block the tank will be almost too hot to test. If there is a block in the water heater it will be warm but you'll be able to keep you hand on it. You have to force the air bubble out of the water heater. This is done by pulling the forward hose off of the expansion tank. You hold your thumb over the bibb on the tank and then blow into the hose until you hear air bubble up into the expansion tank. This takes a mighty and extended blow and when it comes there will be a real gush and gurgling sound. Push the hose back on. You will have to add coolant to replace the bubble that you've now blown out. Now open the petcock above the thermostsat and bleed the air caught in the engine itself as mentioned above. It took me most of a week to figure this out the first time. P.S. don't swallow any coolant as it's poisonous you know. A little on your lips won't hurt you, just wipe it off. 95% + of the time this is the problem. Nobody believes it when you tell them, but try it and see and then don't forget to thank me here

[John Gross]

I have not had any problems with my M25, 500 hours in salt and fresh water. However, since the heat exchanger in the water heater is cast iron, the problem could be rust buildup inside the heat exchanger due to insufficient anti-freeze concentration. The rust would block water flow. If you can blow air through the heat exchanger, (vacuum cleaner) rust buildup is not a problem.

I have had trouble with the coolant cap on the engine allowing anti-freeze to leak out; thus lowering the coolant level and allowing air to enter the heat exchanger.

If the water heater heat exchanger gets air bound, the circulating water pump may not be able to push the air out of the heat exchanger and water will not circulate.

The heat exchanger is in the high point of the coolant loop, just under the expansion tank, and would require that the upper hose on the water heater heat exchanger be disconnected to bleed the air out. Note, I have seen a couple of CD 31's and the expansion tank was in different locations on both boats. My water heater and expansion tank are both in the port locker. I have seen the expansion tank in the starboard locker. However, the effect is the same, if the coolant level gets much below the expansion tank, the heat exchanger could get air bound.

The air binding problem can also occur anytime the cooling system is drained to replace a hose, water pump, or thermostat. Or anytime the coolant gets hot enough to boil.

[Terry]

The older M-25 diesel came with a small heat exchanger that gave this diesel alot of problems in the over heating end of it. This heat exchanger was refitted with a larger one from universal(Westerbeke)and you will most likely need a new mounting bracket for the new exchanger-- The newer styles after this model all are bigger such as the M-25XP & M-25XPB. Check with your area dealer.