Diagram of the cooling system of a Yamaha F40 outboard.
Diagram depicts the cooling water flow within an outboard.
Outboard Overheating Frequently Fatal to Engine
One thousand one, one thousand two, one thousand three … an overheated outboard can go from fine to fried in a matter of seconds … the silence when a hot engine stops running can be a sobering and expensive experience.
The major components of an outboard’s cooling system are:
- Water inlets (or water pickups), slotted openings in the gearcase that allow water to enter the cooling system
- The water pump, situated just above the gearcase on the driveshaft
- Water passages in the gearcase, midsection, and powerhead (the engine)
- Thermostat(s), heat-activated valves in the powerhead’s internal water passages
And here’s how the cooling system works:
- When the driveshaft rotates, the water pump’s impeller spins, drawing water in from the water pickups on the gearcase
- The water pump pressurizes the incoming water and sends it up to the powerhead
- Water flows through the powerhead’s internal water passages (or water jackets)
- The thermostat(s) stay closed until the engine warms up, then open to allow the water from the water pump to absorb heat from the powerhead
- The cooling water then exits the powerhead and is returned to the body of water from which it came. A portion of the water is discharged via a small tube (commonly called the tell-tale) located near the back of the lower engine cowling. This lets the operator know that cooling water is flowing through the engine.
Obviously, we’ve simplified the cooling system and its functions quite a bit, but these are the basics.
Q: What can cause an outboard to overheat?
- A faulty water pump is a major cause of overheating-related outboard engine failures. An incredible number of pumps are damaged by starting the engine with the gearcase out of the water. Running the engine – even for a few seconds -- without water can destroy the rubber vanes (blades) on the water pump.
- Debris – plastic bags, weeds, mud, or a combination thereof – clogging the water intakes on the gearcase won’t let sufficient cooling water into the engine.
· Trimming the outboard up too much while underway or in turns can cause the water pickups to ingest air – this causes a sudden loss of water pressure - not a good thing.
Q: What can I do to monitor the engine to prevent overheating?
- Check the engine temperature and water pressure gauges frequently. Make it part of your normal operating routine, similar to looking in the rear view mirrors when you’re driving a car.
- Get to know the needle indicator position or digital read-outs of the water temperature and water pressure gauges (if equipped) during normal operation. That way, you can easily tell at a glance if something is amiss.
- Periodically look at the outboard to ensure adequate water flow coming out of the tell-tale discharge tube
Q: How can I know if my outboard is overheating?
- Most late-model outboards have integrated warning/self-preservation systems that sound a buzzer or horn, as well as automatically reducing engine RPMs if the on-board computer detects that the engine temperature is higher than it should be.
- The temperature gauge (if equipped) will indicate a higher than normal reading.
- The water pressure gauge (if equipped) reading can be significantly lower than usual.
- Gradual loss of power, often accompanied by scary and unusual noises (even sometimes there won’t be any noise!) from under the cowling.
- Smaller outboards may not have sophisticated warning systems, so pay particular attention to the engine’s power output and flow of water from the tell-tale tube; if either decreases abnormally, the outboard could be running hot (overheating).
- Refer to the owner’s manual for overheating information specific to your outboard.
Q: What should I do if my outboard overheats?
- Stop the boat, turn off the engine and pull the safety lanyard out of its clip (to prevent accidental starting). Trim/lift the outboard out of the water and inspect the gearcase water pickups for obstructions (plastic bag, weeds, mud, etc), and clean the pickups. Trim/tilt the engine back down into the water, restart and monitor the temp gauge and the water flow out of the tell-tale. If the engine runs good, the temperature gauge reads normal, and there is a steady stream of water exiting the tell-tale tube, you might’ve solved the problem.
- On the other hand, if upon examination you don’t find anything blocking the water pickups, the cooling system may have an internal blockage or be damaged. Let the engine cool for a while, restart, and monitor the temperature gauge and the tell-tale discharge. If everything seems to be as it should, motor on back to the ramp and have the engine checked out. Do not to operate the engine in an overheated condition; call for a tow if necessary.
Q: How much water should be coming out of the tell-tale discharge tube? Should it be cold or hot?
- At idle, a trickle or dribbly flow from the discharge/tell-tale tube is fine. Increasing engine RPMs to a fast idle and higher should produce a strong stream of water exiting the outboard.
- The water coming out of the tell-tale is usually warm. If it’s hot, that’s often an indication of a blockage somewhere in the cooling system (and the flow probably won’t be as strong as it should, either).
Q: Can I change the water pump on my outboard myself? How long should it take? What would it cost if I had my dealer replace it?
Yes, changing an outboard water pump is well within the realm of even a moderately-experienced do-it-yourself mechanic; just make sure you have a good service manual, tools, parts and supplies necessary on hand before tackling the job, and set aside a few hours for the task. Make sure you diagnose and correct the problem…not just a symptom.
A qualified shop can typically replace a water pump quicker than a do-it-yourselfer, because that’s what they do – work on outboards – so all the parts, tools, supplies, and knowledge are readily available. The repair shop will methodically determine the root cause of the problem and correct it, as well as assessing any damage that may have occurred -- so the cost will vary based on the time it takes the technician to diagnose and fix the outboard’s maladies.
Your best bet is to contact your local certified marine dealer and discuss your particular situation with them, before dropping your boat off for service.