year-round living on a sailboat in maine

Troubleshooting an Overheating Engine

Troubleshooting an Overheating Engine

Last month, with eyes toward the summer sailing season, we decided to put some time into investigating the mysterious coolant leak and overheating engine that had plagued our delivery to Maine from Annapolis. Without being able to locate the issue we became concerned that the head gasket may be compromised, allowing coolant to seep into the engine and exhaust to enter the cooling system. The oil didn’t show any signs of coolant interaction, so we were hopeful that this was not the case.

While browsing the internet I came across a neat method for testing the integrity of your head gasket without removing the head. There is a simple kit called a Combustion Gas Leak Detector (found here) that is able to alert you to the presence of combustion gases in your coolant via a chemical reaction. The kit consists of a double chambered “turkey baster,” and a bottle of detection fluid. Essentially you fill the two chambers with the fluid and set the tester into the top of your coolant tank in place of your pressure cap. With the engine running, squeeze the ball at the top of the device to draw in the air, or other gases, in the coolant tank. If combustion gases are present the fluid in the tester will change color as it reacts with the carbon dioxide. It is best to rev your engine through multiple cycles of high and low RPMs to be sure that you are testing for all conditions. Luckily, after multiple tests, we did not witness any color change. It was a relief that we would not, at this point, have to pull the head and replace the gasket. We have done this sort of work before, and enjoy it, but we have enough things on our plate right now.

The liquid did not change color, which means combustion gases are not present in the coolant.
The liquid did not change color, which means combustion gases are not present in the coolant.

At this point we felt pretty confident that the coolant was not leaking into the engine and decided to carefully inspect each step of the coolant loop. Though technically not part of the coolant loop itself, we started with the raw water pump knowing that a bad impeller would certainly lead to overheating. The impeller was intact and looked to be in good condition.

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Raw water pump and impeller.

Next up we removed the hoses leading to the Bowman heat exchanger and pulled the tube stack out. There was a good amount of surface rust and some corrosion so we thought maybe this was the location of our leak. We checked each tube in the stack for debris and found some old rubber bits of impeller. Let this serve as a reminder that when you change a damaged raw water impeller you need to go through the system and remove any little bits of rubber as they will impede the flow of cold saltwater through the heat exchanger.

We needed to test the tubestack for leaks, so it was time to get creative. After a quick trip to the hardware store we capped off one end of the stack with a rubber cap and used a rubber hose adapter to attach a shop vac to the other end. With the shop vac in reverse (blowing rather than sucking), we submerged the tubestack in a tub of water. With the tubestack pressurized under water, any leaks would be detected by the sight of bubbles escaping. We left the vacuum running for several minutes, slowly rotating the tubestack to make sure bubbles weren’t trapped between tubes. For video of this click here! Lacking the sight of any bubbles, we concluded this was not the trouble spot we were seeking. This is fortunate as the tube stack alone costs over $900!

The thermostat was our next project, and we fretted it as we had already spent many hours attempting to replace it during our delivery. The housing for the thermostat has a bolt on one side, which came out with little effort. But the other side is a stud and nut, which is conveniently buried under the elbow of the housing, making it very difficult to remove, particularly after years of corrosion. After several hours of battling the nut, I managed to shear off the stud and remove the thermostat. Since we were planning to replace the stud anyway, I was ecstatic when the stud broke. With the housing out of the way I was able to easily remove the broken stud, and replace it at the local NAPA. Earlier in the week we ordered two new thermostats and gaskets so that we would have a spare. When the thermostats arrived, we tested them in a pot of water on the stove to make sure they opened as intended. The new thermostat, gasket, stud and nut were installed so now, hopefully, service of the thermostat will be easier in the future.

Continuing through the cooling systems, we removed all remaining hoses and the heat exchanger  body itself. While the heat exchanger was out we took the opportunity to clean, sand and repaint it. We are planning to do this to several engine components over the coming year. It’s an old engine, so despite our efforts it’s still a rough finish. The cleanup and paint is more for preventative maintenance than appearance as we attempt to stave off corrosion. As we removed the hoses we found them to be severely cracked and found some spots that appeared to go all the way through the walls. Further incriminating the hose was a buildup of crystallized coolant. Aha!!! We have found the leak. We replaced the hoses and put the entire system back together, complete with new gaskets, to test our work. Fingers crossed!

Before
Before
After
After

With the boat encapsulated in shrink wrap, we weren’t able to go for a ride around the harbor to test the engine under load. So instead we double checked our lines, and put the engine in gear right at the dock. The goal was to see if the engine was able to maintain a cooler temperature, and also to see if we lost any coolant. After running the engine for 20-30 minutes under a heavy load, it was disappointing to see the temperature get quite high yet again. We waited for the engine to cool down and checked the coolant level. Despite the engine over heating we didn’t appear to lose any coolant. While we haven’t completely solved the problem, we have made some progress and are still optimistic we can solve the temperature issue.

What will we try next? The only remaining part of the cooling system to check is the water pump. If there is damage to the pump it may not be circulating coolant properly through the engine, which would allow the coolant to continue to heat without passing by the cold tube stack in the heat exchanger. These pumps rarely fail, but since the boat has been sitting a long time, it is possible. We also will be attempting to clean the air intake which seems to have a restricted flow due to a dirty screen. A lack of sufficient air is known to cause overheating, so this could certainly be playing a part in our troubles. Whatever the problem is, we will continue to monitor, tinker, test, and repeat until we discover a suitable solution.