Understanding engine overheating problems

2 Oct


Why is the engine overheating? Our Yanmar engine’s shrill alarm was the jarring start to some stressful hours during the last five months, and we asked that question many times. The answer was not one root cause, but more likely a series of related events, as a domino effect of different issues cascaded. I’ve written about the painful side of this before, but less so the final diagnosis and fix, so this is for cruisers like Mark, Lynn & Rick, Gary, and others who reached out and asked to learn from our experience.

It all started when tried to fix something that wasn’t broken. We’ve had great performance from our Yanmar 4JH3/TE, and want to maintain it properly so it continues to serve Totem well. In April, it had a major service (5,000 hour) done while we spent time on Langkawi island, a pretty spot at the far north of Malaysia’s west coast. This was a big line item on the pre-Indian Ocean checklist we’ve been working through as our budget allows.

As part of the service, the heat exchanger was pulled for testing. Because of all the calcification built up on it, the mechanics struggled to get it out and in the process of removal managed to crack it slightly. It was brazed, and pressure testing showed it should be fine, but the end cap seating was affected. We’re pretty sure that the O-ring leaked coolant into the sea water. This was the probably cause for our initial trouble with coolant loss leading to overheating, which forced us to shut down the engine when we least wanted to (in no wind, but plenty of current, at a major port entrance where large cargo ships converged from the shipping lane of the Malacca Straits). Not fun.

What did we do? In the near term, we kept coolant topped up as we made our way south, but rate of loss varied. Still, we heard the overheat buzzer a few more times along the way. At the next opportunity, we had more troubleshooting help from the mechanics who performed the original service- and a lot of support via the internet from various cruising friends and dockside mechanics (thanks to Chris, CJ, Richard, Gary, Colin, Mark, Robert and many others for their thoughtful advice and ideas). Ultimately, the issue appeared to be resolved by getting the end cap properly seated on the heat exchanger again. We spent many hours afterwards running the engine and checking for coolant loss reassure ourselves.

Thinking our engine troubles were over we departed Puteri, somewhat delayed but still hoping to sail east and spend a few months exploring in Borneo and possibly the Philippines as well. Unfortunately, those plans were quickly scuttled. After a full day motoring across Singapore to line up for crossing the South China Sea, weaving through traffic with vessels more than twenty times our size, we ended the day not just with a pretty sunset but with the engine temperature gauge rising to point at red again. Didn’t we just fix this problem? It was pretty frustrating. Sure enough, the coolant level had dropped again.

Engine repairs

lots and lots of head scratching over the engine compartement

We topped up lost coolant, limped back to the west side of Johor, and called the mechanics once more. With the heat exchanger fixed, our problem was almost certainly the head gasket. With the head removed, the gasket showed four failure points. Was this caused by the overheating events? We don’t know, but it seems unlikely because it was not cracked or warped. The mechanics said that it’s possible that rust particles from the cooling ports under pressure could have caused or contributed to the gasket fail. This rust occurs when coolant is mixed with water. That’s not something we’ve done; if the prior owner did this, it’s percolated for some time before becoming the serious problem we experienced.

The mechanics we worked with were based in Kuala Lumpur, a half days drive from our location, stretching out the time for repairs. With the head gasket replaced, we were ready to put this episode of engine problems behind us.


Unfortunately, that wasn’t the end of it. Jamie was running the engine while we were in the slip at Puteri Harbour, the oil cooler sprang a leak. In just a few minutes, it put about a liter of oil out through the exhaust: it’s really fortunate that he was right there and paying attention, so the engine was shut down before the oil loss caused a bigger problem. It had been cleaned and tested as part of the 5,000 hour service, so why it failed so soon after is a mystery.

Thankfully, these are all in hindsight now. We’ve motored or motorsailed more than 400 miles back up the Malay peninsula, and the engine has performed flawlessly again. As frustrating as it was to spend months being relatively stagnant instead of out exploring the islands in Southeast Asia, this was the right thing to do. Had we not done the initial 5,000 hour service, it’s likely that the debris from the heat exchanger would have caused problems for us during the big miles we plan to undertake in the next year. We’d much rather deal with a service problem while here in Malaysia instead of 1,000 miles from help in the Indian Ocean!

Chilled out sailors know we love it when you read this on the Sailfeed website


  1. Ahmed rameez

    I have replaced the head gasket but the same problem is there. Having failed to correct this issue, I am trying to replace the yanmar to Volvo because I feel that yanmar provides a very small raw water pump when compared to heat flow in their engine.

  2. James Gifford

    Hello Ahmed! – That’s a lot of engine in a 30 foot boat – power boat? I make no claim to diesel expertise and you’ve covered the likely suspects. So, seawater flow is good? Other things that come to mind: correct coolant type and quantity, fresh water pump working well, internal blockages in the fresh water (coolant) flow, thermostat problem, prop is clean and sized correctly for engine/boat (over pitched can overwork engine – you should be able to get RPMs to about 95% of max rated, if not maybe prop?). Good luck!

  3. Ahmed Rameez

    I have a yanmar 4jh3 dte 90 hp in a 30 feet boat. The engine overheats at above 2500 rpm. Tried many solutions but none helped. Heat exchanger, impeller, mixing elbow, strainer and many other areas checked.But still this problem exists. I dont know what to do. Can you guys help me?

  4. Mark Roope

    Thank you for this post. Like you we had exactly the same symptoms and like you we tried everything else first. Now it looks like the only thing left is the head gasket which we will have to repair this winter.
    Strangely enough the boat next too us has also had the same problem, tried the same things and yes, it is his head gasket as well.

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