Outboard engine troubleshooting is an handy skill. While a fuel system problem may make it impossible to start an the engine, outboard cooling system problems have the potential to destroy an otherwise healthy motor as overheating can lead to a blown cylinder head gasket, or even complete seizure of the engine.
With only a very few exceptions (such as Honda’s 2.3hp model) outboard engines are water-cooled. An impeller pump at the bottom of the leg above the gearbox pumps water up to the motor, which then travels back down the leg and exits under water. A tell tale hole under the motor itself or in the leg shows whether water is flowing as it should.
Often the only sign of a problem is when the water from this tell tale stops flowing – it’s important to monitor it all the time the engine is in use, even though it’s not normally in your line of sight. For this reason modern large and sophisticated engines are provided with a temperature gauge on the dashboard. Sometimes the tell tale can become clogged by salt crystals – if the flow stops or is weak the end of a paper clip poked into the hole may clear the obstruction.
Generally outboard engine cooling system problems stem from a lack of basic maintenance – the impeller should be changed annually and engines used in salt water should be regularly flushed through with fresh water to ensure corrosion and scaling is kept to a minimum. Problems tend to stem from these two sources – failure of the impeller, or a partial blockage of the waterways by salt and calcium deposits.
Some of the smallest outboard motors, with no gear selector, have the water pump right in front of the prop, so the impeller is relatively easy to change. However, with most it’s a more major undertaking that requires the gearbox to be split from the leg. For this reason, the importance of proper servicing should not be underestimated – overheating is the easiest way to kill an intrinsically robust engine.
Larger outboards may be impossible to fix at sea, but generally have a limp home mode in which the motor will run at a slow speed that reduces the chances of long-term damage being inflicted through overheating. Smaller engines used as auxiliary power for a dayboat or small yacht can often be lifted onboard for repair, assuming the boat carries appropriate tools and spares.
Outboard engine cooling systems: how to replace the impeller
Start by disconnecting the gearbox linkage: remove the round plastic disc two-thirds of the way down the leg to reveal the clamp that joins the two sections of the control rods – you may need to engage reverse gear for the clamp to become visible. Loosen the nut just enough to allow the clamp to slide up and down.
The gearbox unit is normally attached to the leg with stainless steel bolts, which gives plenty of opportunity for corrosion between the bolts and the aluminium casing. This can be acute if the threads were not liberally coated with barium chromate paste to electrical isolate the two metals when the gearbox and leg were last reassembled. If you encounter problems at this stage it's important to take a methodical and patient approach.
Start by removing any encrusted salt from around the bolt heads – a kettle of hot water will do this, with the assistance of a soft brush if necessary. When dry, give the bolts a liberal dowsing of easing oil and leave for 24 hours. This may be enough to free the bolts, but don't be tempted to use excessive force. Heating the aluminium casing around the bolt threads with a blowtorch will cause the casing to expand a little, thus loosening its grip on the bolts. Even if this enables the bolt to be turned only a few degrees, the process can be repeated several times until the bolt has been removed.
With the bolts removed, the bottom section can be slid away from the leg. The impeller is located on the top of the gearbox – remove the bolts that secure the water pump in place and slide it up the drive shaft. Any missing bits of impeller blade need to be recovered before reassembling the pump, as they will obstruct the already narrow waterways.
Outboard engine reassembly
When refitting the lower unit, the drive shaft may need a little bit of manoeuvring to get it into the correct position, and the cooling pipe leading upwards toward the powerhead must be fitted into the rubber connector on top of the bottom unit. Finally, don't forget to smear a little barium chromate paste on the bolts before refitting the gearbox.
After dismantling and reassembling the engine it's worth starting it up and checking everything functions properly before leaving the dock. On some engines it’s easy to reattach the gear control marginally out of position, meaning reverse gear can’t be selected.
As with any job, the first time you do it can be relatively slow, especially if the there’s electrolysis around the bolts securing the gearbox to the leg. But with practice it becomes a quick process to replace the impeller, so there's no excuse for not doing it every year.
For more, see Solve your outboard motor problems: starting, fuel, shear pins and How to winterise a four-stroke outboard: video. If you are considering buying a new outboard engine, see 10 top outboard engines.
Rupert Holmes has cruised and raced more than 60,000 miles, between 60 degrees north and 56 degrees south. He writes about all aspects of boat ownership and marine travel, including destinations, seamanship and maintenance, as well as undertaking regular boat and gear tests. He owns two yachts, one currently based in the Aegean and the other in the Solent.