The Importance of Careful Engine Flushing

The Importance of Careful Engine Flushing
A quick way to discover whether your outboard’s internal cooling water passages might have accumulated mineral build-up is to remove the thermostat cover and thermostat…and have a look inside.
Some of the boat performance puzzles we are asked to solve can be rather difficult for operators to diagnose by themselveslike this one.

Have you ever been running along at upper RPM range, say 4500 or a little higher, and out of nowhere the over-temp alarm sounds?

You have water pressure on the gauge, the water pump was changed less than a year ago, the overboard indicator flow (tattle-tell) is strong, and the motor has been running great. So as a careful operator should, you stop the boat to look for debris clogging the water pick-up, and a few other common sense checks. The alarm resets during the cool down. You take off again running cautiously at 3500 rpm and everything seems fine. What happened?

Based on the above conditions, you may have just experienced the result of years of salt/mineral buildup acting as an insulator on the engine's cylinder heads and crankcase assembly cooling passages. Although the water is flowing correctly, the engines ability to transfer heat, meaning allowing the flow of cooling water to carry away the heat of operation, is limited to less than optimum. The higher the operating RPM and the longer the time of operation at that speed, the more likely this situation might develop.

This high speed overheat condition usually occurs when the engine is a little older, maybe five to six years old, not often seen in newer models; and the good news is it is very fixable. All that is required is what we call a good acid bath service.

This procedure is actually rather simple. Removing thermostats, re-securing the thermostat covers, removing the lower unit and circulating a properly diluted solution of muriatic acid through the cooling system will remove the layers of mineral buildup that are actually insulating the metal surfaces inside the engine that limit or prevent heat transfer and thereby greatly reducing the cooling system's efficiency.

This is not something we see every day, neither is it confined to operation solely in saltwater. Any hard water; i.e. water rich in minerals such as calcium or iron, can create the same symptoms.

After the acid bath, we flush with lots of fresh water, replace the thermostats and gaskets, inspect the water pump impeller, reinstall the lower unit with fresh lube on the driveshaft splines, and the prop. Now it is time to give the engine a hard, high-end workout. Absent any over-temp alarms you can be assured the problem has been solved.

I cannot stress enough the importance of a good cooling system flush-out following every outing. The best practice here is to use a muff-style flushing device, get the water flowing, then start the engine. Let it run at least 10 minutes at fast idle, keeping an eye on all your gages. This insures the thermostats will open as the engine warms and freshwater will make its way through ALL the cooling water passages to wash away harmful salt and other minerals.

Be safe on the water and have a great Fourth of July holiday!

Chris Mapp
Coastal Bend Marine | Port O'Connor TX
361 983 4841 |