Diagnosing High-RPM Power Loss

Spring is well underway and we are about to roll into summer. You’ve had your boat out for a few trips and lately you have begun noticing a slight power loss in the upper RPM range – above 4000 RPM – yet when you throttle back the engine and cruise at lower engine speed it runs fine.

You are likely experiencing the result of a boat that has been sitting for a period of time and debris carried by the ethanol in your gasoline is finding its way into the vapor separator tank (VST) pump inlet filter. This is not due to neglect on your part or a lack of changing filters.

The 10-micron fuel/water separator filter installed on the boat is the first line of defense and though you change it once or twice a year, which prevents water and most debris from reaching the fuel injection system, extremely fine particulate matter has made its way to the VST filter. The fabric of this filter contains such fine mesh that the accumulation of particulates that make it past the fuel/water separator will eventually collect and impede fuel flow.

Servicing this filter component is an important part of our annual service check list. This problem is not uncommon if you have neglected or are running behind in your regularly scheduled maintenance program. Yamaha, Suzuki, Honda and Mercury four-stroke outboards are the most susceptible. The cure is a matter of diagnosing and cleaning the element although in some cases replacement is required. Ethanol attacks the plastic, sometimes causing it to swell and become unusable. Here again we see another of the long-term effects of ethanol in motor fuel.

The recommended service interval for recreational boaters is once a year or every 100 hours of operation. Outboards that see greater use, multiple-trip-per-week recreational owners and fishing guides, should be serviced every 500 hours.

It is a good practice to squeeze the fuel primer bulb until firm before each trip to fully prime the system with fresh fuel. This will eliminate a false symptom of filter failure and keep you from having that unexpected moment on the water.

Have a great summer season and thanks for your business!

Chris Mapp