Troubleshooting Low Power Complaints

There are many mechanical or electrical things that can go wrong with any boat engine. Troubleshooting is the process of reducing the number of variables to a very small number of realistic possibilities that can affect the engine in question. The following list will help you find the problem.

Air Filter Obstruction-A plugged air filter will reduce power, and on boats a slight exhaust leak will quickly plug the filter. Just remove the filter and do a visual inspection by shining a light from the inside out. If you can’t see light through the filter it’s time for replacement.

Exhaust System Obstruction-A collapsed silencer or plugged exhaust system will lower power as well. Have your marine technician perform a back pressure test on the system to see if this is the case.

The Fuel Supply-The fuel supply system easy to forget. First of all, check your fuel level and make sure there is plenty of fuel in the tank. If there is no fuel-level gauge or the existing gauge is questionable, then you must dip the fuel tanks. And, while you are at it, apply a dab of water finding paste that is available from your fuel dock, on the end of the stick to check for water in the fuel. If your fuel level is satisfactory then move on to the fuel filters.

Check the fuel filters for the presence of water and to be sure fuel is flowing through the filter. If you have recently changed the filters, then check for air in the engine’s fuel pump and injector lines. This air may have entered the system during the filter change.

Another area to consider when power is low is fuel line sizing. Some older boats might have, for example, a one-half inch fuel pipe coming into the engine room. This line may have been enough to feed the original engines when the boat was built. However, over the years after the boat was re-powered and new generator sets were installed in the engine room, you may notice times when one or more of the engines seem to be sluggish. You may also notice that the lights brown-out when the main engine is also running because there isn’t enough fuel to supply engines. If so, it is time to upgrade the fuel supply line to what the factory specifies for each engine, plus a little extra. First calculate the cross section of the inner diameter of all fuel lines and then add them together. After getting the area of cross section needed to feed them all, go up to the next higher sized pipe to feed all of the engines adequately.

When upgrading fuel hoses, be sure to buy U.S. Coast Guard hose. See the following link:

Inspect your fuel return lines as well. Some engines will not run well with a plugged or restricted fuel return line. Since most vessels have a valve on the return line, it is also important to check to see that the return line valve is definitely open.

A less common problem, but one that does happen, is accidentally filling up with No.1 fuel oil when you really need No. 2. This will reduce engine power because there is simply less energy in a gallon of No. 1 fuel than in a gallon of No. 2.

Controls-Verify that the governor speed control is getting full travel when you have the speed control in the full fuel position in the wheelhouse, or on the flying bridge. The transmission control needs full travel as well, and if you have a trolling valve, verify that it is closed when accelerating the boat.

Air in the Fuel-If you begin to bleed the air from the fuel system and the air just keeps coming, then it is time to use the sight glass.

A sight glass lets you spot air in the fuel. The place to install a sight glass is between the low-pressure fuel transfer (i.e., lift pump) and the high-pressure pump or unit injectors depending upon the type of injection system on the engine. Be sure to check the pressure that your transfer pump produces and be sure that your sight glass is rated for this pressure. Most major engine makers can supply a suitable sight glass through their parts sales organization.

If you can see even one bubble of air coming in with your fuel, then there is at least five percent air in your fuel. Air in the fuel will seriously limit the power the engine delivers and can cause hard starting as well.

When you do find air in the fuel, the next thing to do is to examine all fuel lines, hoses, fittings, and clamps between your engine and its fuel supply-on the suction side of the system. When air leaks into the fuel are repaired, you’ll feel the difference.

Electronically Controlled Engines-Verify that no trouble codes have been stored. If you see a code for a problem, contact your dealer and get a technician to plug in his laptop to find the problem.

Line In The Wheel-When nothing else is showing up as a problem, try this: With the engine stopped, take a hold of the propeller shaft with your hand and feel how hard it is to turn the shaft. Usually you can turn a shaft as large as 2″ inches in diameter by hand. If it’s hard to turn, get a diver to see what is wrapped around the shaft.

Further In-Depth Testing-When black smoke (mechanical engines) and hard starting are involved it is time to ask your technician to check the compression and fuel injection timing as well.

Questions and Comments-Please let us know your thoughts on this article and suggest new topics you would like to see.

(Some of this material excerpted from “PRACTICAL BOAT MECHANICS”, by Ben L. Evridge, to be published this fall.)