Overheating: Dealing with Engine Temperature Problems

Overheating or overheat warnings can be disheartening at best. It’s easy to assume the worst, but with some methodical steps outlined below, you can go through your cooling system and systematically rule out issues, or hopefully, discover them as you go along. The golden rule when it comes to identifying an issue? Start simple.

Raw Water System

1. Is your vessel using a wet exhaust system? If so, check your exhaust as the engine is running, is water coming out of the exhaust outlet in a consistent way? If so, it’s unlikely to be the raw water system. Though, this doesn’t rule it out altogether and in point 4 we go in more depth on the heat exchanger/coolers.
2. If this seems to be fine, it’s now worth checking the seacock assembly and indeed the valve. Often overlooked, but in the excitement of getting on the water, did you remember to turn the water intake valve in the ‘open’ position? Take note of the hose too, does it look collapsed? Does the strainer look to be in good condition with no obvious blockages or debris? Take the basket out ensuring you turn the water supply off first, and check the condition of internal passages, the basket itself and give everything a quick clean. It won’t hurt to do this and will help remove the possibility of these items being the issue.
3. All of these appear fine? Now start looking at your raw water pump. Whether it’s salt water or fresh water, the very nature of it being raw means that it’ll have marine biology in it, growths, plants and vegetation and the odd carrier bag! The strainer should prevent 90% of this coming through, but it’s not impossible that it could make it’s way past. Opening up the pump means you can see if everything looks as it should. Take note of the image below. Check that the impeller is in one piece with all blades intact. It’s also worth noting that raw water pumps are either cast with a cam profile into the internal housing, or supplied with a replaceable cam plate that does the job of ‘squeezing’ the impeller blades to create a suction effect that helps draw the water through.
4. Heat exchanger and/or oil coolers and/or charge air coolers. This is where the complexity is at, and can cause some confusion as well as some apprehension.  Simply put, a heat exchanger is essentially just like a radiator on your car. The exception being that instead of air, it uses the water in which you are in, and the radiator part is on the inside in the form of a ‘tubestack’. You have seals that keep the two different fluids separate (raw water and coolant) and that is it! Just like a radiator, deposits and debris can block up the passages where your water flows. This is also the place to look if you happen to find impeller blades missing, as they invariably end up on the end of the tube stack. We referenced water flow from the exhaust being present as unlikely to be a raw water issue, it is also possible that a build up of salts, debris and other blockages could allow for water to pass in some of the tubes, but not all. So you’ll witness water flowing, but it may be that it’s not doing so efficiently resulting in your engine running hot. It’s also worth bearing in mind that the wrong cooler can also result in reduced flow rates. If in doubt, speak to an engineer/technician.

We could go in more depth, but ultimately, these are you main pointers to check. Obvious issues such as failed hoses and leaks to name a couple, are of course worth checking too!

Closed Loop Fresh Water System

So the issue isn’t with the raw water system? Now it’s time to think about the closed loop cooling system. But before we begin dissecting parts, lets use that golden rule of starting simple again.

1. Lets check the instrumentation and senders/switches. These are not infallible and can throw up issues due to a short, a bad gauge/buzzer or the switch or sender simply failing over time. This is where a Multimeter comes into it’s own. Checking resistances on the likes of temperature senders can seem like a complex task, but really, it is a fairly simple process. Check online by searching “How to check resistance on temperature sender”. YouTube videos can also help. If you can get hold of a thermometer also, that you know is accurate and/or calibrated, you can verify any readings with this method also.
2. So we’ve confirmed that instrumentation is working and not the fault, now we need to determine if the coolant level is correct. Open the header tank (usually a radiator cap or on some newer engines, are plastic caps) CAUTION: Do not do this when hot. Allow the engine to cool first! Is the coolant level in the header tank correct? If an expansion vessel is fitted, does this have sufficient fluid in it too? Again, if unsure, check with your manual/engine manufacturer.
3. Levels correct? Maybe an airlock? Fluids travelling through channels can become ‘stuck’ at points where you have a high point going back to a low point. Give the hoses a squeeze, you may hear some gurgling. this may be you moving the airlock. Some engines have a specific sequence to conduct the bleeding of a cooling system, check your manual or engine dealer to see if this may be the case. In some cases, running the cooling system with the pressure cap off, can help burp the system allowing fluids to drop and as they do, continue topping up. Before you do this however, make sure you are comfortable and know what you are doing. If in doubt, consult with someone who does.
4. Is the thermostat operating as it should? Simply put, the Thermostat’s job is to allow for the engine to come to operating temperatures quickly, when cold. And then help maintain a desired temperature once it has reached it. The majority of marine thermostats open at 71°C and will be fully open at 82°C. As a rule of thumb, do not assume that just because it fits it will function correctly.  There is a lot more to thermostat design than is often realised, so make sure that the thermostat functions properly by immersing it in hot water of known temperature.  The easiest way is to use a pan of water on a stove with an accurate thermometer.  Hold the thermostat by the flange with a pair of pliers and do not let either thermostat or thermometer rest against the bottom of the pan. The thermostat should open at the temperature marked on it and be fully open approximately 20°C higher.  Malfunctioning thermostats cannot be repaired, they must be replaced.
5. When was the coolant last changed? As coolant ages, it’s ability to perform reduces, likewise, if the system has a leak somewhere and cannot get up to pressure, the boiling point of the water rises and thus, increasing it’s running temperature.
6. Finally, if the above points have been exhausted, it may be a case of stripping down the heat exchanger(s) or parts around the engine to see if something internal may be at fault.

 

Again, this is some guidance in a nutshell and will aid you in pointing you in the right direction. But some caution, if you are unsure or don’t have the right knowledge in these systems, they can be dangerous and without proper diagnosis, more damage can be done. As always, if in doubt, consult with someone who knows how to determine the likely issue.