When your outboard motor won't start, you may remember that most of the problems with gasoline motors result from the electrical system, rather than the fuel system. If the electrical system checks out clean, though, it's time to look at your fuel system. You can conduct a simple function test in your driveway with some help from a friend. Unfortunately, the more accurate form of the test requires you to have proprietary data about fuel pressure and flow rates; it doesn't appear even in the Mercury Marine Shop Manual. Keep it simple.
Look at the sight glass on the fuel pump. If there's fuel in the sight glass, the pump's diaphragm is ruptured. To avoid a fire on your boat, replace the fuel pump before doing anything else.
With an open-end wrench, disconnect the hose that runs from the fuel pump to the fuel rail which supplies fuel to the injectors. Set a five-gallon plastic bucket beneath the hose.
Turn the ignition switch to the "Run" position on the key way. This will cause the electronic control module (ECM) to turn the fuel pump on for two--and only two--seconds. Listen for the whirr.
Tell your assistant to lower the hose from the fuel pump into the bucket. Tell your assistant to let you know if fuel begins to pump into the bucket.
Continue turning the ignition switch until it reaches the crank/start position. The ECM will turn the fuel pump. Fuel will be pumped into the bucket, if the pump is working.
Mercury Marine's fuel pumps have a sight tube; if fuel appears in the sight tube, that means the fuel pump's diaphragm has ruptured and the pump should be replaced.