Sterndrives are tremendously versatile and rugged pieces of marine equipment, combining the advantages of inboard and outboard motors to give you tremendous reliability. However, these formidable engines are not invulnerable, and can encounter a number of teething troubles throughout their working lives.
One of the most common problems encountered with sterndrives is overheating. This problem can have a number of root causes, so determining which is affecting your engine can be challenging. Calling in the services of professional sterndrive repairmen is always the safest and most efficient solution, but you can also attempt to discern the cause of sterndrive overheating yourself by checking for the following problems.
Water pump impeller
Sterndrives used water cooling systems to draw heat away from vital components, so a faulty water pump should be your first suspect if your sterndrive is repeatedly overheating. In the majority of cases, water pump failure is caused by a damaged or faulty impeller inside the main pump housing; this impeller is responsible for pushing fresh water around the cooling system, and water left stationary inside water lines quickly heats up and loses its cooling potential.
The best way to determine if your impeller is functioning or not is to check the condition of the impeller yourself. Physical blockages caused by seaweed and other hazards may be the culprit, but the impeller itself may also have become degraded and require replacement.
If your impeller has significant corrosion around the edges of its blades, this may indicate that your impeller has suffered cavitation; this problem can damage other components in your pump, so sterndrive repair services should be called in to inspect the cooling system for more widespread damage.
Even if your sterndrive's cooling system is functioning perfectly, it will not be able to cool your engine without a functioning thermostat to activate it. If your sterndrive overheats frequently, check its thermostat to make sure it is not faulty, damaged or poorly adjusted. Faulty thermostats may be repairable in professional hands, but it is often simpler to replace them instead.
The exhaust fumes created by all but the smallest and weakest sterndrives contain significant amounts of soot and particulate matter, and these solid deposits can collect within the manifolds and risers of your craft's exhaust system. Manifolds clogged with solid matter will dramatically restrict airflow to the engine, causing it to overheat quickly. Overheating which is only encountered at high speed is often caused by clogged exhaust manifolds.
Clearing these manifolds is generally a fairly simple operation, as they can be accessed and removed fairly easily via the sterndrive's inspection port. Carefully knock any stubborn deposits of matter free from the manifold, and check for corrosion which may be feeding rust flakes into your manifolds. Damaged manifolds should be replaced as soon as possible, ideally professionally.