Overheating and cooling
Impellers, heat exchangers, exhaust elbows, strainers and the raw water circuit — the system behind most marine engine failures in Australia.
How often should a heat exchanger be cleaned?
Periodically, at an interval set by your operating water. Silty, warm or weedy water means more often. It is one of the most-skipped maintenance items.
Full guide: Heat Exchanger Service Cost
Can a heat exchanger be cleaned in place?
Chemical flushing in place helps with light scale. A properly silted bundle needs to come out and be rodded.
What is the pencil anode?
A small sacrificial anode threaded into the heat exchanger housing. It protects the tube bundle, and on many engines it has never been replaced.
Can a leaking heat exchanger be repaired?
On some designs a tube can be plugged or the bundle re-tubed. On others replacement is the only option. It depends on the model.
How do I know if mine is blocked?
The classic pattern is normal at idle, overheating under sustained load, with adequate flow from the raw water pump. It needs checking against the alternative causes rather than assuming.
How often should an impeller be replaced?
At the manufacturer's interval — commonly annually or at a set hour figure — and immediately on any sign of reduced water flow.
Full guide: Impeller Replacement Cost
Can I replace a marine impeller myself?
On many engines, yes: remove the pump cover, extract the old impeller, fit the new one lubricated, replace the gasket. Access is the deciding factor.
What happens if a vane breaks off?
It travels downstream and can block the heat exchanger. Every vane must be accounted for, and missing pieces retrieved.
Why does my new impeller keep failing?
Usually a worn wear plate or cam inside the pump, running dry on a blocked intake, or a partially closed seacock. Replacing the impeller alone does not fix those.
Is it worth replacing the whole pump?
If the wear plate is scored, the cam is worn or the shaft seal weeps, yes — a new impeller in a worn pump has a short life.
Is keel cooling better than a heat exchanger?
For workboats and silty water, generally yes. For recreational boats, the heat exchanger arrangement is lighter, cheaper, and entirely adequate.
Full guide: Raw Water vs Keel Cooling
Do keel-cooled engines have impellers?
Not for engine cooling. Some installations still run a small raw water pump for exhaust cooling or a gearbox cooler.
Can I convert to keel cooling?
It is done, particularly on commercial conversions, but it means a hull-mounted cooler sized to the engine and usually an exhaust change. It is a design job.
Why does my heat exchanger keep silting up?
Sandy or muddy operating water, an intake positioned low, or an ineffective strainer. Frequent silting is one of the arguments for keel cooling.
Does direct raw water cooling damage an engine?
It shortens its life relative to indirect cooling — scale, corrosion and a low operating temperature. It is why almost nothing is built that way now.
How often should the exhaust elbow be inspected?
At the manufacturer's interval. If it has never been removed and the engine is more than a few years old, do it now.
Full guide: Blocked Or Corroded Exhaust Elbow
Can it be cleaned instead of replaced?
Sometimes, if the casting is sound. But an elbow blocked enough to cause symptoms is usually corroded enough to be near the end of its life.
Is stainless better than cast iron?
Stainless lasts longer here and many owners upgrade when replacing. Confirm the replacement matches the engine maker's geometry requirements.
How does a bad elbow put water in my engine?
Either by perforating so seawater enters the exhaust passage, or by restricting flow so water backs up instead of discharging. Both end at an open exhaust valve.
Does my gearbox have its own cooler?
Most inboard installations do — a small raw-water heat exchanger. It blocks like any other.
Full guide: Marine Gearbox Overheating
Milky gearbox oil — what does that mean?
Water has entered, usually through a perforated cooler. Address it promptly; seawater destroys gears and bearings quickly.
Can a fouled bottom overheat the gearbox?
Yes. Overload puts more torque through the box for the same speed, and heat follows.
How often should gearbox oil be changed?
On the manufacturer's interval, alongside an engine service. It is the most commonly forgotten item on the drivetrain.
How often should the impeller be replaced?
On schedule rather than on failure. Interval depends on water conditions and how long the boat sits as much as on hours — ask a local operator what they use.
Full guide: Raw Water Impeller Has Failed
My impeller looks fine — do I still replace it?
If it is at its interval or more than a few years old, yes. Rubber hardens with age even unused, and a set impeller pumps poorly without looking damaged.
What happens if I leave a broken blade in the system?
It restricts flow downstream, usually at the heat exchanger inlet, and the engine overheats again quickly.
I ran the engine with the seacock closed — is the impeller gone?
Almost certainly, even if only for seconds. Replace it rather than hoping.
Why did a brand new impeller fail?
Usually a worn cam or wear plate in the pump housing, or debris still in the system from the last failure.
My engine only overheats at high revs — why?
A partial restriction. At low demand the reduced flow still copes; under load it cannot. Usually a heat exchanger losing capacity or a suction hose collapsing.
Full guide: Marine Engine Overheating
It overheats in summer but was fine in winter — is that normal?
No. It means the system has no margin left. Warmer seawater cools less, so a marginal cooling system fails in the heat and hides in the cold.
Can a fouled bottom cause overheating?
Yes, indirectly and commonly. A dirty hull makes the engine work harder for the same speed, and the cooling system was sized for a clean one.
Is it safe to top up coolant with water?
In an emergency, briefly. As a habit it dilutes the corrosion inhibitors and lets the engine rot internally.
Do I need a new engine after an overheat?
Not necessarily. A brief overheat caught early is often survivable; a sustained one usually means at minimum a head gasket, and possibly a rebuild. Get it assessed before running it again.
The alarm sounds but the gauge looks normal — which do I believe?
Shut down and check for water at the exhaust. The gauge and the alarm often use different senders, and the alarm is the one designed to be conservative.
Full guide: Overheat Alarm Sounding
Can I just silence it and carry on?
No. If it is genuine you will destroy the engine; if it is a fault you have disabled your only warning.
Is an exhaust temperature alarm worth adding?
It is the most useful alarm you can add. It detects loss of raw water flow before the coolant temperature has moved, which is meaningfully earlier warning.
How often should an aftercooler be serviced?
Most manufacturers specify an interval — commonly measured in hundreds of hours or a small number of years. It is frequently skipped, and it is one of the few maintenance items that can put seawater directly into an engine.
Full guide: Aftercooler and Charge Air Cooler Service
How do I know if my aftercooler is leaking?
Pressure testing is the only reliable way. Symptoms — power loss, white smoke, water in the intake — usually appear once damage is already occurring.
Does my aftercooler have an anode?
Many do. If yours does and it has never been changed, that is a strong reason to have the unit serviced now.
Why does an aftercooler matter more on a boat than a truck?
Because it is cooled by seawater rather than air. A corroded automotive intercooler leaks air; a corroded marine aftercooler leaks seawater into your engine.
Can a blocked elbow be cleaned instead of replaced?
Sometimes, if the casting is otherwise sound. But an elbow blocked enough to cause symptoms is usually corroded enough to be near the end of its life anyway.
Full guide: Exhaust Elbow and Riser Replacement
My engine was hard to start after sitting — is that related?
It can be. Water sitting on top of a piston makes an engine hard to turn and hard to start. Do not keep cranking it; get it checked.
How often does a heat exchanger need cleaning?
Depends heavily on water. Tropical, silty and shallow water foul far faster than cool clean water. Every few years is common; annually is not unusual in Cairns or on a river berth.
Full guide: Heat Exchanger Service and Descaling
My engine only overheats at high revs — is that the heat exchanger?
Classic partial blockage symptom. At low load the reduced flow is still enough; under load it isn't.
Can a blocked heat exchanger be cleaned or does it need replacing?
Cleaning usually restores it. Replacement is for a stack that is perforated or too corroded to hold pressure.
Does my heat exchanger have an anode?
Most do — a pencil anode threaded into the housing. It is one of the most commonly neglected items on a marine engine. Why sacrificial anodes matter
Could a heat exchanger fault look like a head gasket?
Yes, and it frequently does. A perforated tube pushes seawater into the coolant. Pressure test before authorising head work.
How often should I change the impeller?
More often than most people do, and on time rather than on failure. Interval depends on water conditions and idle time as much as hours — ask a local operator what they use for your water.
Full guide: Raw Water Pump and Impeller Replacement
Can I change an impeller myself?
On many engines, yes, and it is the most worthwhile job for an owner to learn. The critical discipline is accounting for every blade of the old one.
What happens if a blade is left in the system?
It restricts flow downstream, usually at the heat exchanger inlet. The engine overheats again quickly and the new impeller gets blamed.
My engine overheated on the first trip of the season — why?
Very likely the impeller took a set over the layup, or the strainer blocked. Both are found in a fifteen-minute check before launching.
Do I need to close the seacock first?
Yes. And a note for after the job: an impeller run dry even briefly is already damaged.
Why does coolant matter if it never freezes here?
Because the important job of marine coolant in Australia is corrosion protection, not antifreeze. Depleted inhibitors let the engine corrode internally.
Full guide: Marine Cooling System Repair
Can I top up with water?
In an emergency, briefly. As a habit it dilutes the inhibitors and shortens the engine's life.
My engine runs cold — is that a problem?
Yes. A thermostat stuck open causes bore glazing, carbon build-up and fuel dilution of the oil. It is worth fixing.
I'm losing coolant but there's nothing in the bilge — where is it going?
Most often out through the exhaust via a perforated heat exchanger tube, or expelled past a failed pressure cap. Both are cheaper than the head gasket people usually assume.
How often should coolant be changed?
On the manufacturer's schedule. Test it rather than judging by appearance — coolant can look fine and have no inhibitor left.
My engine is fine at idle but overheats at speed — why?
Classic collapsing suction hose or partial restriction. At low flow the system copes; at high demand it cannot.
Full guide: Raw Water System Repair
How often should I check the strainer?
Before every trip is reasonable, and after any run through weed or debris. It is sixty seconds and it prevents the most common overheat on the water.
My seacock won't close — how urgent is that?
Urgent. A seacock that cannot be closed means a burst hose cannot be isolated, which is a flooding risk. Have it serviced or replaced.
What is the white crusty deposit around my pump?
Dried salt from a slow leak — an early warning of a failing shaft seal. Cheaper to address now than after the bearing fails.