Preventing Catastrophic Engine Failure

We talk often about maintenance schedules and the recommendation for cooling water pump replacement every year or 100 hours - on average. Of course, water pump service life also depends quite a bit on whether the boat normally sits up for extended periods (impellers dry out and vanes take a “set”) and also the amount of sand and shell particles that are drawn through it – churning sand and mud during shallow takeoff is especially harmful.

The photographs included here are made looking down the cylinder bores of outboard engines – focusing on the top surface of a damaged piston and the remnant of a completely disintegrated piston in the other. In both cases the cylinder block was damaged beyond repair.

So, what causes catastrophic failures like these to occur? The primary cause is heat buildup in the powerhead due to insufficient cooling water circulation. And, more than likely, not from a single overheating event – but several or many! Eventually, if the engine is forced to continue running when severely overheated, a breakdown of lubricating oil will occur, which then adds to the heating problem.

As the powerhead overheats, pistons expand and rings seize. Proper running clearance between the piston and cylinder wall diminishes and the piston scuffs the cylinder wall – adding even more heat.

We strongly encourage annual service intervals, during which we inspect the water pump and thermostats to insure coolant flow. Four-strokes receive a complete oil change service, new oil filter and new crankcase oil. With two-strokes we pull and clean the oil tank. Condensation collects at the bottom of the tank and mixes with lube to form sludge that can restrict the flow of lubricating oil to the engine. Since two-stroke oil tanks are never completely drained in normal operation, the problem grows worse over time if not addressed during annual service. Sludge problems in four-strokes are far less likely given the complete draining of the crankcase during oil change.

Repair cost for the type of engine damage depicted here runs around $6000 and could have been easily avoided with proper annual maintenance.

Continuing with the lower unit photo – If your gearcase looks like the one in the image, you could be a candidate for a similar powerhead failure. The skeg on this gearcase has worn down 30% from running too shallow and plowing through sand and mud. Remember what was mentioned above about abrasive particles in the cooling water stream damaging the water pump.

Lower unit repairs are not limited to the skeg. The prop, gearing, and bearings are also subjected to exceptional stress during this type of operation. The powerhead, too, is subject to premature failure. Manufacturer’s warranty pays only for defects, not negligent operation. Insurance usually will not pay for failure of this type as it does not meet the impact or collision criteria.

Have a great summer fishing and boating season and thanks for your business!

Chris Mapp