One of the most common issues iv’e seen over the years is the good old engine overheat.
So because this is so common ill be covering what to do when your engine overheats, the step by step process involved in diagnosing the possible cause, DIY tips on how to fix the issue and how to prevent a possible overheat from happening in the future.
The process is slightly different from petrol engines to diesel so ill be splitting into two parts covering petrol engine overheat and diesel overheat.
OK when you have an engine overheat what this means is the engine has a pre determined temperature range it is safe to operate within and for whatever reason the engine temp has moved outside this range giving you either fault codes on screen, an alarm, a warning light or all three.
The first things to rule out are the simple stuff, check coolant level (closed cooled engines), check engine oil level, are there any hoses popped off the engine? has an engine belt snapped?, is the sea strainer clogged? (if you have one) is the ball valve for the water pick up open? (if you have one)
Before you turn the engine off you need to know under what circumstances does it overheat, is it only at wide open throttle?, at idle? mid rpm ranges?
This will be valuable information for diagnosing the issue later on.
No doubt you will have an alarm screaming at you but at this stage the temp gauge is what you want to be paying attention to, so if you get an overheat alarm pull back to idle, does the temp start to go down?
If the temp does come down you can switch the boat off then back on this will reset the overheat alarm on some systems, if not you will have to just mute the alarm or put up with it.
Then you should be able to idle home.
If the engine temp just wont come down even at idle then unfortunately you have no choice but to key off and get a tow home.
Obviously if you are in a position that is unsafe to be without power then you will have to get somewhere safe to key off as soon as possible.
Driving an overheating engine for prolonged periods can cause some pretty serious damage to your engine, even total engine failure.
So before i get into the specifics of each engine type the first thing to understand when diagnosing a overheating issue is the direction of flow by this i mean the path the raw water (sea water) travels from outside the boat into a water pick up in the hull or in the drive then through a sea strainer if you have one, into your engine water pump then depending on your engine it will either go through your closed cooling system, heat exchanger (radiator), any oil coolers if you have them then into your manifolds finally exiting out through your exhaust.
Or on raw water cooled engines (no heat exchanger or coolant) the raw water will go from your water pump straight into your engine circulate through the engine block, out through a thermostat into your manifolds and exit out the exhaust.
Petrol Engine Overheat
Assuming you have already done the simple checks mentioned above the next step is following the direction of flow and checking key components along the way.
– Water pick up
– Sea strainer
– Water pump
– Coolers
– Thermostat
– Manifolds
– Exhaust
Some of these components will need new gaskets/o-rings for re assembly but at this stage we are just trying to find something out of the ordinary.
So let’s break down these components and talk about what to look for
Water pick up.
OK this one is only something that we can check out of water so if that isn’t possible move onto the next step
What we are looking for here is anything blocking the water pick up from the outside this apply’s
OK this one is only something that we can check out of water so if that isn’t possible move onto the next step.
What we are looking for here is anything blocking the water pick up from the outside this applies to through hull intake grates as well as the intake grates on a stern drive leg.
Sea strainer.
This one is super easy if you have a sea strainer check the clear bowl for excessive build up of weed or other debris and if you don’t have a clear bowl type shut your main water feed valve off and pop the lid off & have a look inside.
Water pump.
I’m writing this Assuming you have an engine driven water pump unit not a drive impeller unit.
Remove the impeller cover plate first checking that for excessive wear then remove the impeller and make sure it’s not damaged in any way like missing blades, chunks missing or tears/slices
Coolers.
Most petrol heat exchangers are sealed units but if you happen to have one that has removable end caps i would be removing these and having a look inside
Otherwise pop the raw water hoses off the cooler and have a look inside for any blockages.
Thermostat.
Checking the thermostat is for raw water cooled engines only
Closed cooled engines will have the thermostat in the coolant side of the system.
To check if a thermostat is working first remove it from the engine put it in a container and cover with boiling water.
The thermostat should open up easily once hot and should contract once cooled with cold water.
Manifolds.
This is the most common cause for overheat Iv’e found.
Manifolds have a balance tube on the bottom of them this tube shares the water around each exhaust port water jacket if this tube fills with sand or corrosion one cylinder can get hotter than the others.
Also manifolds can get blocked up inbetween the manifolds and risers/mixers.
Exhaust.
Lastly I have seen engines overheat from blocked exhaust outlets and internally damaged muffler boxes.
To check these remove the flexible exhaust pipe and have a look with a torch to see if there are any blockages.
Also any burnt or melted exhaust components are a dead give away that an exhaust blockage is likely.
Diesel Engine Overheat.
OK before we get into the diesel list a good thing to understand is the main differences in a diesel engine to petrol engine in regards to an overheat situation.
Because diesel engines have compression ignition (they use compression instead of spark plugs to make the fuel go boom) they require a much more robust build than most petrol engines so that means a larger and heaver overall engine compared to a petrol engine at the same horse power.
That also means the engine accessories tend to be bigger and beefier than there petrol counterparts.
These reasons are why the diesel list will be slightly different to the petrol list, most of the key points on this list will be the same though.
– Water pick up
– Sea strainer
– Water pump
– Coolers
– Exhaust
Water pick up.
OK this one is only something that we can check out of water so if that isn’t possible move onto the next step
What we are looking for here is anything blocking the water pick up from the outside this apply’s
OK this one is only something that we can check out of water so if that isn’t possible move onto the next step.
What we are looking for here is anything blocking the water pick up from the outside this applies to through hull intake grates as well as the intake grates on a stern drive leg or pod drive intermediate housing.
Sea strainer.
This one is super easy if you have a sea strainer check the clear bowl for excessive build up of weed or other debris and if you don’t have a clear bowl type shut your main water feed valve off and pop the lid off & have a look inside.
Water pump.
Remove the impeller cover plate first checking that for excessive wear then remove the impeller and make sure it’s not damaged in any way like missing blades, chunks missing or tears/slices
Most medium to large diesel engines will require a special impeller removal tool to get the impeller out, due to the large size of the impeller.
Coolers.
The first step is to identify how many coolers you have since a lot of diesels have turbos bolted and/or superchargers bolted onto them they can have quite a few coolers to check.
Lets just assume you have at least a turbo attached to your engine, if this is the case you will likely have a heat exchanger (radiator), after cooler (intercooler), drive oil cooler and a fuel cooler all having raw water (sea water) passing through them.
The first step is to have a good look at the hoses and end caps for any signs of salty white corrosion build up around the edges, this is the first sign of a cooler starting to block up.
What you can do to inspect further is shut the main water feed valve off and pop raw water hoses off the coolers and have a look inside with a torch.
The coolers will have a brass or copper core that consists of lots of small tubes running the length of the cooler its these tubes we want to make sure are clear of corrosion build up and/or sand, weed.
Exhaust.
Lastly I have seen engines overheat from blocked exhaust outlets, mixers and internally damaged muffler boxes.
To check these remove the flexible exhaust pipe and have a look with a torch to see if there are any blockages.
Also any burnt or melted exhaust components are a dead give away that an exhaust blockage is likely.
Now you may have read through the petrol list and are now wondering why we are not checking manifolds, the simple answer to this is usually on a diesel they are cooled by coolant not raw water, and the same goes for thermostats.
By following these checks properly you should have a very good chance of pinpointing the reason for the overheat.
These steps are the exact same steps i would use to diagnose a overheat on a customers boat.
If you find that you get to the end of these checks and you still haven’t found the source of the problem then unfortunately it may be a good time to get a qualified marine mechanic on board to investigate the issue further.
DIY tips for fixing overheat cause.
Simply put, any component that is broken, melted or corroded beyond repair will need replacing.
That goes for components such as – hoses, manifolds, risers, mixers, thermostats, muffler boxes etc…
Some components can be cleaned out such as coolers with removable end caps, hoses that are simply blocked, manifolds and risers that are also just blocked.
Coolers.
To clean coolers remove end caps and clear all the small tubes by using a thin metal rod poking it right through the core pushing out any built up corrosion calcification or debris blocking the tubes
If you need to remove coolers from engine to get to both ends of the cooler make sure to drain the coolant first otherwise you will get a little coolant surprise when removing.
Once all clear replace coolant and any o rings or gaskets, re install end caps.
Once all done and back on run up engine and have a good look around any caps and hoses while running looking for leaks.
Manifolds and risers.
To clean out manifolds and risers you will need to remove them from the engine first then separate risers from the manifolds.
You should see on each manifold at the bottom on one end a blanking cap and at the other end an elbow, remove the blanking cap and stick a garden hose up it flushing the raw water side out the elbow end.
Check the water galleries in the riser and make sure they are also clear, you can flush them with water but most of the time its the manifolds that are the culprit.
Once you are happy that the manifold is all clear use a pipe sealing compound on the blank off plug and re install.
Now you can install risers and manifolds onto the engine, make sure to use new gaskets where necessary.
Preventative Maintenance.
The best preventative maintenance you can do is use your boat regularly, this keeps the onset of corrosion at bay by keeping the raw water moving throughout your engine.
It is when the raw water sits in your engine for extended periods that the corrosion starts to take hold.
I realize not everyone can use their boat regularly and the next best thing is to ether just run the engine once or twice a week and / or flush with fresh water or a corrosion inhibitor like salt away.
If your boat lives in the water running the engines regularly is going to be your best option.
Wrapping up.
An engine overheat is never a pleasant thing to experience, alarms going off flashing lights etc… its always a stressful time when you are supposed to be out having fun.
Hopefully you are just reading this article for the preventative maintenance but if you have been unlucky enough to experience an overheat this article should put you in a good position to diagnose, fix and prevent another one from happening in the future.
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