Our engne runs hotter the faster I run it, why?

[Bob Robeson]

We have a 1986 Catalina with the stock Universal M25, 21 HP engine. The temperature is very RPM dependent. The faster I run the engine the hotter its temperature.At 1800 RPM it will run at 165 degrees al daylong.At 2000 RPM it climbs to about 180 degrees but will stay there at day.At 2100 RPM it climbs to 190 plus.Anything above 2100 and the temp just keeps climbing. I do not know how high as at 200 degrees, I have gone to an idle or even shut the engine off.At 2500 RPM it takes less than an hour to go above 200 degrees.Things I have done.1) Replace the fresh water pump and all hoses and the coolant.2) Replaced the raw water pump (impeller)3) Replaced the raw water strainer4) Cleaned the heat exchanger.5) Changed the oil and filter.None of these things have effected the temperature.Things I have been told to try:1) Replace the radiator cap.2) Replace the Thermostat3) Replace the heat exchanger. Ours is bout 2” in diameter.Any ideas of what I should try next.ThanksBobC-36#586

[Gary Jensen]

I recently sold my 1986 (36) Catalina. I had the same engine as you. Have you opened the valve that allows the water to circulate thru the heater, Its located above the water intake valve aft the engine. That did help me, however they do make a larger heat exchanger for the engine at about $250..THAT is suppose to do the trick!!!! good luck

[Justin - O'day Owner's Web]

Bob,I would upgrade that heat exchanger. Unless you're some place where the water is too cold for the engine to warm up properly, they can't be oversized. If you _are_ someplace cold, the thermostat should shunt water around the exchanger when its cold anyway.Justin - O'day Owner's Web

[John Van Wagoner]

I had the same problem in a 2 cyl Yanmar. I did all the things you did and then checked the mixing elbow. If you have a mixing elbow where the salt water exiting the heat exchanger mixes with the exhaust gases, you can have build up of scale and carbon clogging his small port. If this happens, the engine runs at normal temps at low rpms but heats up at higher rpms because you can't move enough water through the clogged port. A back pressure develops and the water passing through your heat exchanger is significantly decreased due to the pressure and the engine overheats. I discovered that this was the problem when the back pressure blew off the hose from the top of the heat exchanger. Get a big wrench and take off the fitting onto which the hose from your heat exchanger is attached to the elbow. the inside of this fitting will be clogged with scale and the elbow maybe rusted out on the inside. Replace or clean out both and your engine should run cool for another 5 years or so.

[Don Wilbert]

I had a Catalina 30 for 12 years before I got the Hunter37.5. I had the same problem. The heat excahnger is to small. You could consider the upgrade. However, I could run my 30 all day at 2800 rpm's and not hit 200. However she started heating up at 3000rpms.

[Chris Ward]

I have a 1984 Catalina 30 with the stock Universal M25, 21 HP engine. The original heat exchanger is undersized. It would cool the engine when the system was in tip top shape. I always had to keep an eye on the temp guage. As soon as part of the system worn slightly (impeller, dirty exchanger, etc.) it would overheat. Upgraded to the larger heat exchanger six years ago and have had no cooling problems since; even when the bay water is in the upper eighties during the summer.