The Offshore Voyaging Reference Site

10 Tips To Save Your Engine From Overheating

Deisel engines, as long as they are properly maintained, are fantastically reliable machines—in over 40 years I have never had one fail in such a way that I was not able to get it running well enough to get me home.

But overheat a diesel engine badly and all bets are off, to the point that not only might the engine not get us home, it might never be reliable again.

I know of at least three incidents where owners overheated a diesel, spent a bunch of money trying to fix it, and then finally had to re-power—a huge suck.

And each of those cases was avoidable. Let’s look at ten ways to do that.

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M Dino Kubik

John, i how do you suggest an exhaust elbow be cleaned?

Dick Stevenson

Hi Dino,
I always carrried a spare and would just swap them and do the chipping at some convenient bored moment off the boat during the winter usually watching a sports event. Dick

Wilson Fitt

My exhaust elbow was significantly constricted by carbon so I tried to chip it out using a hammer and cold chisel. I had little or no success on the carbon but did manage to pound a hole through the metal. I wouldn’t start on this job again without a replacement elbow on hand as backup, and if I had the spare I would probably just mount it, throw the old one out, and buy a new spare.


Darren Thomas

Hi John,
Who sells the sensor and water temp alarm to ship outside the US?

Dick Stevenson

Hi Darren,
I use a Borel raw water failure alarm. They are easy to test. (From memory, they go off at 160gegF.) When in the UK this subject came up and a friend reported getting one. Not sure it was a Borel, but believe it checked the same box.
My best, Dick Stevenson, s/v Alchemy

Petri Flander

Hello Darren, Vetus has one. It uses penetrating type sensor, and they also have a 52mm size alarm indicator to go with it.

Alex Borodin

I’d like to give a bit of defence to infrared temperature guns. I have tested my thermostat by heating it in a pot with black walls (i.e. not reflective) and my IR pyrometer showed exactly the temperature stamped on the thermostat when the latter opened. Well, within 0.1°C, actually, but that’s an acceptable tolerance for me.

Alex Borodin

Hi John,
I tried boiling some water in the same pot as before and test the IR thermometer and got disappointed. This time, with the water at a rolling boil, it showed pretty consistently ~93°C (+/- 0.2°), whether I “shot” the wall outside, the wall inside through the water, or straight down the bottom of the pot through the steam and water.

I don’t know what to make of it, frankly. I know that previously I also used it to measure the temperature at the water temp sensor and it pretty much agreed with the gauge. But today’s result is definitely way off. Maybe something as simple as a new battery would make it accurate again, maybe not, but reliable it is not. I guess a retraction of my previous defence is in order.

Michael Lambert

One thing I’d add is that our boreal is my first boat where the engine panel is inside the cabin. It feels odd to me to be so far away from the temp(and rpm until I learn the notes) gauge. On the list for the first winter is to get engine data on the mfd so one can keep an eye on the temp and learn the baseline.

Nat Smith


The last time I went looking for bits of impeller in the heat exchanger I decided that I would install a sea strainer downstream from the water pump.
The strainer would need to be able to handle the pressure and also not restrict the flow, but I am sure such strainers exist.

What do you think about this?

Petri Flander

Hello Nat, have heard that some manufacturer has wire screen on heat exchanger inlet to keep impeller bits away from pipes. Not sure of manufacturer, and if that is option or aftermarket part. Though not too difficult to DIY, if suitable bronze (?) screen would be available. Cheers, PF

Drew Frye

Very good stuff. Two more things, since engine coolants and heat exchanger maintenance were a big part of my life (I manufactured and formulated coolants, including research and development):

  1. You can get cleaning brushes from McMaster Carr, all sizes. Stay with something soft, the steel ones are for thick steel tubes. Typically they are ~ $10 and thread onto extension rods.
  2. Be VERY aware of seawater-to-coolant leaks. The condemning limit for chloride on the engine side is about 75-200 ppm, depending on who you ask. The manufacturing tolerance is 5-25 ppm, depending on the product. Considering that seawater is about 22,000 ppm chloride, that is a very small cross contamination indeed, just a few ounces. In fact, the prevalence of chloride in tap water is the main reason they no longer sell concentrate coolant, only pre-dilute. They could not control the quality of consumer-added water. In fact, the specs for coolant water are more strict than battery water, just to give you an idea.

This is also why there is no such thing as long-life coolant in a boat.Seawater contamination.

Jeff Sowell

Is there a tester that can be used to periodically ensure that sea water has not entered the fresh water side? Perhaps detecting salinity or chloride?

William Murdoch

I have a note on the endpaper of the log book that says that the exhaust puts out 2.5 qt of water every 10 seconds at 2000 rpm. It is quick and easy to measure. I just hang over the side with a plastic bucket and looking at my watch catch 10 seconds of water from the exhaust. The amount is a little less than the 1700 liters per hour at 3400 engine rpm that the service manual for my Yanmar 3HM35F indicates when ratioed down to 2000 rpm. It is a quick test of the sea strainer, the raw pump, its vee belt, the raw water piping, the salt side of the heat exchanger, the exhaust elbow…

On our 2017 trip from the Bahamas to the Cape Fear River and on to New Bern, that test confirmed that our slightly higher that usual engine temperature was caused by something wrong in the raw water system. I swapped out the pump and picked two impeller vanes from the piping the day before we departed when it was much easier to do than it would have been to do at sea.

Eric Klem

Hi John,

This is a good list and it is nice to pick up a few new tricks that should save me some time in testing.

The only addition I can think of is to actually run your engine hard for a decent period of time every so often. A lot of people I know never run their engine hard so don’t know if they have a problem which has gone undetected. I know of at least a few examples of getting into a bad situation and needing the engine to really perform only to discover that it is running uncomfortably hot. I would consider the bare minimum test to be to get to normal operating temp then go to full load for 15 minutes while monitoring, it takes a little while to reach steady state.


Dick Stevenson

Hi Eric and all,
Steve D’Antonio has a good article on the benefits of running your engine wide-open-throttle under load which can be found on his web site.
I do a WOT under load run at the onset of every season and check (and compare) the baseline temps I have collected over the years at various engine points with an IR thermometer.
My best, Dick Stevenson, s/v Alchemy

Dick Stevenson

Hi Eric,
My notes on the care and feeding of the engine are on the boat, but, from memory, I run my engine at 60% for 10 minutes every 4 hours of run time.
My best, Dick

Dick Stevenson

Hi John,
Good to be reminded of yours and Matt’s writing on the subject. From my experience, it takes a leap of faith to run your engine WOT under load for a period of time and the more support one reads for doing so, the easier it gets.
My best, Dick

Joshua Marieholm

John I got a Volvo d1-30 new engine but not alarm in the elbow exhaust raw water.. I can’t figure out how to install the alarm on the rubber exaust rubber any idea? …

Kevin Dreese

Fantastic article. First time I have read these recommendations regarding simmering sensors that don’t get routinely tested in normal use. Good concept to consider about other critical components outside the engine room that might impact a cruise (eg fridge, etc). These are the kinds of articles I love to read here; nicely summarized and for us newbies like talking to an old salt (compliment not critique of John’s age) about how to prepare the boat.

Ernest E Vogelsinger

When running my old but new-to-me boat the first two times I wondered why the engine never reached a decent working temp – it went to 55°C at most. After shopping for a matching thermostat I carefully opener the box to get to the old device – if only there had been something in 🙂 Someone simply removed the thermostat from the engine without replacing.
Now, with the new correct device, the engine heats up to its specified 85°C.

Michael Albert

I replace my impeller way more often than the service interval- usually every 150-200 hours.

Cheap insurance and it helps that my Beta 50 has the water pump in a super accessible location.

None of the impellers have cracks in them on removal but why wait until then?

Dick Stevenson

Hi Michael,
I do the same which generally means I start out the season with a new impellor. I can argue that it is cheap insurance, but that flies in the face of my complaints that impellors are expensive. This leaves me with the reality that I will go a little overboard (spend $$) in the expectation that I will not be doing service calls with an overheating engine at inopportune times: an indulgence that seems ok now that I am moving toward the mid 70s.
My best, Dick Stevenson, s/v Alchemy

Colin Speedie

Hi John

what the service book schedule suggests and what (in practice) manufacturers Agents suggest are two very different things. I asked a major, reputable Agent to de-scale our heat exchanger at its scheduled interval on a relatively new Volvo D2-55 and they actually tried to talk me out of it!

For those who have re-engined boats, one thing to think of is when you upgrade or increase the size of the engine, everything else must be done, e.g. increase the diameter of the raw water inlet, or add blowers to feed and remove added air to ‘now too small’ engine compartments.

Especially where there’s a turbo involved…..

Rob Gill

Hi everyone,

I am profoundly deaf in one ear and rely on my one good ear when sailing…but any loud sound is quite disabling for me as skipper, especially in any developing “situation” – so I need the alarm mute function.

The Borel single raw water alarm in the link above, and the one in the photo of MC’s cockpit, don’t appear to have the alarm mute button. Is there another way to accept / mute the raw water temperature alarm please?

The dual panel does have a mute button, but we don’t have a generator. The custom panel does have a mute button and appears, in the picture at least, to be able to connect a bilge high water alarm – I like the idea of having this alarm audible in the cockpit, thus possibly solving my issue.

Does anyone know please a good bilge hw alarm, tested to work with the Borel panel and having a generous amount of cable to work with?

Many thanks, Rob

Dick Stevenson

High water alarms:
Hi Rob, John and all,
High water alarms are, to me, essential safety equipment and are an essential early warning system much like smoke detectors.
Well before I ever noticed “marine” high water alarms on the market, I started using household sump alarms and still do. The advantages I appreciate (besides being a fraction of the cost) is that they are a compact unit self-powered with a 9v battery (beeps like a smoke detector when low on power: never happened to me when a new battery starts each season).
Installation is made easier as there is no electrical connection to the vessel’s power grid. The sensor dangles on a cord making height placement easy to adjust (the alarm is silenced merely by lifting the sensor out of the water) and allowing some latitude in placing the control box where the alarm sound comes from.
The alarm must be loud enough to alert people wherever they are on the boat with the motor running (making silencing the alarm even more of a blessing).
I place my alarm height pretty low as I have a pretty dry boat so I wish to know whenever water begins to collect. The sensor is below the height my bilge pumps are activated, which are also alarmed.
Testing is easy as I just monthly drop the sensor in a cup of water.
I have a bilge area dedicated to the stern of the boat where I have a second water alarm. If I did not, I might consider building a dam and alarm that area as, over the years, this area’s alarm has flagged a number of problems well before they became “real” problems: leaking coolant, failing seal on the rudder shaft, cracked raw water hose, and the like.
My best, Dick Stevenson, s/v Alchemy 

Rob Gill

Hi guys,
I suspected this would be the case John – I think I’ll get the dual engine model and just ignore the second input. We don’t have room in our cockpit panel for much else.

Dick, we have a bilge alarm at the nav station that comes on when the bilge pump activates (automatically) with a mute switch. I thought it would be good to have a separate backup alarm also mutable, in the cockpit. I like your idea for an aft sump alarm – do you have a link to the model of sensor / alarm you use please?

Many thanks, Rob

Dick Stevenson

Hi Rob,
No, no link and I am a bit internet challenged at the time so I can’t explore to come up with one.
My notes indicate that my high-water alarm is the “Watchdog”, but it has been in use now for decades. Google sump alarm or basement alarm and I suspect it will come up. I believe I got one of mine or both at Radio Shack. It has done the job, but I have no idea whether it is “the best”.
My best, Dick

Rob Gill

Thanks Dick.
Best regards. Rob

Malcolm Crow

We overheated our engine and damaged the plastic muffler due to an object clogging the fresh water input. Is there any way to know if you have created any serious engine damage from the over heating?

We have since looked into an inline flow sensor for the raw water input as well as an exhaust muffler temp sensor. Your article on other measures are a great list of items to check as alarms did not go off as they should have.