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!


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.

5 thoughts on “Troubleshooting an Overheating Engine”

  • Matt,
    I replied to your FB post, then decided to check out the blog (thinking about cruising instead of working). My family and I lived on our Morgan 416, Liberty, from 2007 to 2013, covering about 20k+ nm during that time from Texas to Bahamas to Boston to Grenada (with 4 seasons in the Bahamas). Liberty has a Perkins 4.154, and I learned a lot about the engine during that time. Still runs hot, and I never solved the problem although its been that way for at least 4 years. We sailed a lot, and motor sailed at lower RPMs, and rarely put ourselves in a position to have to push it hard – but when we did it would seem to run at about 190 and not actually overheat. I think 193 or so is the top of the range in the manual, or maybe 197, I can’t remember. So on one hand, its within range. On the other hand, for the first couple of years it ran at 185 or so without problems (during a time we had fuel issues, which kept my attention). Over the years I’ve replaced the fresh water pump (it was weeping, you’ll know when it needs replacing), several raw water pumps until I learned to not overtighten the belt (run it pretty loose; the way the pulley is set on that shaft on the Sherwood H5 pump it puts a lot of strain on the end of the shaft and will wear out bearings fast if overtightened), pressure tested the tube stack, replaced hoses, etc. MIstake I think was letting an old cruiser and boatbuilder convince me to use Barr’s Stop Leak in the header tank when I had a problem in the Jumentos back in 2011. Anchored at Racoon Cay, when I checked the coolant tank (did every time before starting engine), I found it a pint or more low on coolant and there was a crusty, foamy residue near the top of the header tank inlet. I topped the coolant off, we changed anchorages (less than an hour underway), and the next morning when I checked the coolant I found the same issue. I let the guy talk me into using the Barr’s, but without removing the thermostat (never even discussed it). Pretty much ever since the engine has run hotter – although the coolant loss stopped and we made our way back to civilization. I’d like to flush it, just never got to it (and since that time Liberty went back to Texas, back to Florida, up to the Chesapeake, back to the Bahamas, down to the DR, to the E. Caribbean, Grenada, and eventually St. Martin back to Texas – always nursing the engine and making sure it was not overheating).
    So, I will probably be watching this, and maybe will be inspired to finally do a flush on my engine and work at it. Unfortunately I learned to compensate, but I know its not good for the engine. I do run it up regularly for at least a couple of minutes to get RPMs up (and temps too).
    OK, back to work.
    SV Liberty, Kemah Texas

    • Thanks for all of that information! I will continue to troubleshoot and hopefully find a solution. For now we use it as little as necessary and continuously monitor. I’ll be sure to post any updates !

  • Matt,

    This overheating reminds me of my Bristol 32 with a Volvo Penta Diesel. I chased everything all over the engine that dealt with cooling. With thermostat open I could pull one of the coolant hoses to the heat exchanger and would get good flow of hot coolant. After a month of continually overheating I decided to pull the coolant circulating pump. On this Volvo its driven by the same belt that turns the alternator. Upon pulling the pump I found the bronze impeller inside was loose from the shaft and although the belt was turning the shaft at the proper speed the impeller was slipping and was not(just enough to make me think it was fine when I pulled a hose to the heat exchanger). After tracking down the only water pump in the country for that model Volvo and spending $500, I never had another issue with that engine. May be something to check. Good luck and keep the blog posts coming! Nate

  • Matt , I’m impressed with your efforts to find the cause of the overheating. I don’t believe it would be the circulating pump. You might try taking the hose off at the exhaust mixing elbow, start the engine and observe the flow. If that looks good I would investigate the elbow itself and make sure that hasn’t become plugged. There is also the remote possibility that the temp sending unit is bad. If you can find someone with an infered thermometer you could double check the temp.
    Good luck
    Don Eley

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