A cool fix

From Ocean Navigator #102
November/December 1999
In a previous article (“Renewing the raw water system,” Issue No. 93, Nov./Dec.’98) I described how my Pearson 303 was running hot after about 15 minutes of operation at full power, but if slowed down to about 1,200 rpm it ran normally. In that article I described overhauling the raw water intake system. Unfortunately, this didn’t stop the overheating, so the rest of the system had to be examined.

Figure 1: Elements of a fresh water cooling system for a marine engine. Raw-water cooling systems pump sea water through the engine, eliminating the heat exchanger.
   Image Credit: M.D. Ryus/Ocean Navigator

Figure 1 is a diagram of the cooling system of the 2GMF.

For those people experienced with the cooling system of the automobile, you will see a similarity between this system and the average car. In the 2GMF, the heat exchanger takes the place of the radiator. But there is an interesting twist. In a car radiator the cooling water runs through vertical tubes with cooling fins on the out side. Air flows around these to cool the water. In the heat exchanger, tubes running horizontally carry cold seawater. This water cools the fresh water running in a jacket around the outside of the heat exchanger tubes.

A boat diesel’s freshwater pump, freshwater reservoir, thermostat, path through the cylinder head, and cylinder block are nearly identical to an automobile system. The big difference here is the mixing elbowthere is nothing on an automobile system that compares to it. It is actually a double-walled elbow with the outgoing seawater passing through the outside compartment or walled off area and the engine exhaust passing through the center of the elbow. The outgoing seawater cools the exhaust gases down to a temperature that can enter a rubber hose on the way to the water lift exhaust.

When I started troubleshooting the system, I decided to replace the things I was familiar with from working on automobiles.

Figure 2 shows the location of the fresh water pump, heat exchanger, and thermostat. In figure 2, No. 1 is the front of the heat exchanger, while No. 2 is the cover over the thermostat and No. 3 is the fresh water pump.Emergency spares

This boat is 14 years old and had not given me any trouble up to this point. So I decided that anything I replaced would not be a waste of money due to system age. If the part replaced did not fix the problem I saved it as an emergency spare. One reason I think the cooling system had given me so little trouble is that it was filled with mostly antifreeze to prevent corrosion. Thermostats are infamous for giving problems in automobile systems. The purpose of the thermostat is to open and close, thus controlling the flow of water. This maintains your engine at the proper operating temperature, necessary for efficient engine operation.

Figure 3 shows the cover removed from the thermostat (figure 3, No. 1).

I could see the unit was very clean and was probably okay. You should understand that thermostats are spring-operated devices so looks can be deceiving. One method for testing a thermostat out of the engine involves placing it in a pan of hot water. You can use a thermometer to note the range of temperature between the thermostat’s opening and closing. If this matches the proper range of the thermostat, it can be considered to be operational. Any time the thermostat cover is removed and replaced the gasket should be replaced (No. 2 in fig. 3). For voyagers, a complete set of gaskets for the engine should be carried. As I expected, after replacing the thermostat the overheating problem was still present. I next went to the fresh water pump (No. 3 in fig. 2). The freshwater pump works exactly like the automobile, pumping cooling water through the engine block and the cooling mechanism; in this case, the heat exchanger. After repairing the freshwater pump, I found that I still hadn’t run down the cause of my problem, but I was starting to collect a nice set of emergency spares!

The next place to check was the heat exchanger. I’ve talked to many boaters who had found their heat exchanger tubes plugged with scale after a few years in salt water.Heat exchanger checked

Figure 4 shows the heat exchanger with its cover removed.

We’ve divided it into pie sections to explain how water passes through the exchanger. The raw cooling water flows into section 1 and at the other end is turned and flows back through section 2. The front cover will turn the water back into section 3 where it flows out the rear of the unit. If any of the cooling tubes in any section gets clogged, the exchanger will not work properly. Even with all that I had done, it turned out that the problem was most probably being caused by a clogged mixing elbow. The heat exchanger slides out, making it easy to look through the cooling tubes for blockage. Any tubes blocked needed to be “rodded”: A stiff cleaning rod is used to clean out the blockage. However, extensive blockage can’t be removed with rodding; this could damage the cooling tubes. The best approach at this point is to take the unit to a radiator shop and have them boil out the blockage using a special chemical bath.

By performing a quick visual inspection, it was easy to see that my heat exchanger had no external corrosion. And after sighting down the tubes I could see that none were blocked. I replaced the unit with new gaskets and, of course, still had the overheating problem. If you get this far with your system be sure and replace the end caps in the same orientation as they were before you took them off, since they need to be right to properly direct the water through the heat exchanger.

At this stage of the process, I had to admit I was getting a little frustrated. I had reached the limit of my knowledge of cooling systems. I then called my Yanmar parts distributor. Since I had done my homework thoroughly, they were glad to talk to me and suggested the mixing elbow was clogged in the raw-water jacket section. This could reduce the rate of water flow sufficiently to cause overheating.

Figure 5No. 1 is the mixing elbow.

The water hose that takes water out of the rear of the heat exchanger and inserts it in the outside jacket of the elbow is shown as No. 2 in figure 5. No. 3 in figure 5 is the exhaust coming from the engine manifold and going to the outside of the elbow. In the end, after doing a considerable amount of work, it was a new mixing elbow that solved the problem. I have since motored for an entire day on the Intracoastal Waterway without any repeat of my previous overheating problems.

By Ocean Navigator