Scanning your Engine Temperature

Engine Scan

Keep an infrared thermal device in your onboard toolbox to check engine temperature and more.

Finally, a long-awaited week cruising aboard your boat. Pack up the family and the dog to head down to the marina. Stow all your groceries, and start your departure list. The family is well rehearsed to disconnect the shore power and the dock lines to follow. Hang on and here we go!

While idling out of the marina, you study the engine instrument panels, watch the oil pressure gauges settle and the temperature gauges rise. All systems are normal, so you easily slide the throttles forward as the boat comes up on plane. Everything is good in the world.

Scan the horizon, the channel, the GPS chartplotter, and a brief glance at the instrument panel. Uh-oh! One engine temperature gauge is creeping up higher than expected. Time to drop the engine rpms before the engine overheats. Whew! The temperature slowly eases back down to an acceptable level. But why?

Possible Causes for Engine Temperature Creep

As you cruise along at this reduced rpm and slower speed, you start your mental checklist. I believe the diver cleaned the hull last week. I have new rubber impellers installed in the engine raw-water pump. The water pump belts are in good condition. So why is the engine overheating?

Now you must decide: return to the marina or continue at a slower speed to your planned destination, whichever is closer. Either way, check the engine temperature when you arrive in port. 

As you cruise slowly toward the dock, keep a keen eye on the engine temperature. In this instance, you’ve witnessed the temperature cooling at lower rpms, but you want to be sure it stays that way. 

A New Tool to Help Diagnose the Problem

Grab your earmuffs and handheld infrared thermal device, and head to the engine room. A handheld infrared thermal device is a simple tool to help diagnose and prevent unwanted temperature creep. They are available at most hardware and automotive stores and perfect for checking engine temperature. 

As with all the equipment aboard your boat, it is beneficial to know how to use this easy tool and understand its value. Just add the dots and record your results on a check sheet.

While your mate is driving the boat, you can start in the engine room. Verify that the sea cock valve handle is open. Then shine a flashlight through the strainer and confirm water flow. Now take your infrared thermal device (sometimes called a thermometer or an IR gun) and aim it at the raw-water pump impeller housing to confirm it is running cool. This location may show less than 100 degrees Fahrenheit. If it is hotter, then it is safe to assume the impeller may have shed a rubber vane.

The thermal scan shows the pump housing is cool as a cucumber. Next, start aiming the IR gun at the downstream cooling components including antifreeze, oil, fuel, transmission, and the air heat exchanger after the hot turbo charger. If any of these heat exchangers have reduced water flow because of calcium buildup or broken raw-water impeller vanes, this could be the cause of overheating.

A mechanic may confirm your diagnosis, possibly using his own IR gun and a high rpm sea trial—these heat exchangers needed a cleaning. Scale buildup prevents the heat exchanger from cooling properly. Best to remove them and send out for professional cleaning by boiling then testing to prove they hold pressure. 

Once certified and installed, the engine should be okay for high-speed rpm cruising without overheating. If you have a twin-engine boat, consider cleaning both engines and the generator at the same time.

Track and Trend 

Once you understand the cooling system, I recommend establishing a series of checkpoints so you can track and trend temperatures while underway. Make a chart and use a pen to mark an inspection point, checking the same location every time. 

Eventually, you may start to witness engine temperature rise as the heat exchangers age over the years and become fouled. This will be an indicator that the heat exchangers may need cleaning again.

Having an infrared thermal device in your onboard toolbox can also help read the temperatures on your batteries, air conditioner systems, water heater, and propeller shaft seals. Preventive maintenance is the best way to keep your engines healthy.  

Captains Chris and Alyse Caldwell are USCG 100-ton Masters and cruising coaches who offer consulting and boat training online or onboard your boat anywhere. The Caldwells help build your cruising confidence with training videos and through two-day seminars filled with tips for the boater who loves learning. Contact for more information.

Ask Captain Chris

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