Preventing Outboard Cooling System Woes

Summer is in full swing and the fishing is great. You are running across the bay or offshore at 4500 RPM, the outboard is running smoothly and sounds great. Life is good.

Next thing you know your tachometer drops to 2500 RPM, the engine is running very erratically, and the alarm is sounding. There is just no power when you throttle up.

The boat is brought to a slow stop and the motor seems to run well at idle, don't see a warning light on, water is coming from the overboard indicator, so you bring the boat to a complete stop and the alarm goes off. The engine is switched off and restarted - all sounds good and it idles smoothly.

Powering back up, you cruise up to 3500 and then 4000 RPM, and all is good. (Must have been a fluke).

Now, increasing the throttle slowly and building to 4500, you go about 500 yards and here comes the alarm again and now you see the overheat light. So the whole process starts over and you spend the rest of the day cruising under 4500 RPM. Seems that all is good so long as you do not push it harder.

What happened and what can you do?

The answer is likely salt or other mineral buildup in the crankcase and cylinder heads. Water can flow and exit the telltale indicator, yet the cooling process is inhibited due to the mineral buildup. Even a thin layer of buildup holds the heat in and does not allow the cooling water that is circulating to carry heat away while underway at higher engine speeds.

This is actually rather common on outboard motors over 5 to 7 years old with low or high hours and flushing the engine while the motor is running for at least 10 minutes at idle before placing it in storage is the best preventive measure. Salt buildup is released to a greater degree when warm to hot water is allowed to circulate through the cooling system during the flushing process.

An acid bath mixture run through the engine at your repair center will cut the mineral buildup and restore the engine to top condition at higher RPMs. The cooling system is the heart of your motor, a little maintenance and regular flushing pays off in the long run.

Have a great season and practice safe boating.
Chris Mapp
Coastal Bend Marine | Port O'Connor, TX | 361-983-4841