How to Test an Outboard Fuel Pumpby Chris Stevenson
Fuel pumps on outboard motors work a little bit differently than they do on automobile engines. The outboard fuel pump has a sensitive diaphragm that receives a suction signal from a piston cylinder so the pump can deliver fuel. If the cylinder has a leak, the pump pulse signal will not be strong enough to send the proper amount of fuel to the cylinder. Outboard fuel pumps rarely fail, and diagnosing a fuel problem involves checking other components that can be easily mistaken for a bad check valve or diaphragm in a fuel pump. A process of elimination will narrow down a bad fuel pump.
Turn off your main electrical cutoff switch that governs motor ignition. Unclasp the snaps on your outboard motor upper case and remove the cowl cover. Pull the spark plug wire boot from the tip of the spark plug. Direct the plug wire away from the spark plug hole opening, wedging it or taping it to another component. Use a plug socket and wrench to loosen and remove the spark plug.
Screw a compression gauge hose into the spark plug hole. Twist your throttle position wide open with one hand. Pull your rope start with the other hand so the engine turns over at least four or five times. Check the compression reading in psi (pounds per square inch) on the gauge. If you have an electric start motor, have an assistant crank the engine over while pushing the throttle wide open.
Refer to your owner's manual for the correct psi your engine needs. If the compression falls below 30 psi, per specifications, your fuel pump will not function adequately due to low pulse pressure. Repair the compression problem first. Unscrew the compression gauge hose and install the spark plug. Tighten the spark plug with a socket and install the plug wire back on the plug tip. Perform this test for multiple cylinder engines.
Check to make sure you have enough gas in the tank to be picked up by the sump. The sump pickup screen should be unclogged, as should the tank vent. Blow through the tank vent to make sure it flows freely. Make sure the fuel primer bulb has no cracks or leaks and has not become deformed in any way.
Inspect the fuel line hoses and connections from the tank to the fuel pump location for kinks and leaks. There should be none. Loosen the in-line fuel filter hose clamps with a screwdriver and pull the filter free. Blow through both ends of the filter to ensure there are no obstructions. Reinstall the fuel filter and tighten the clamps with a screwdriver.
Loosen the clamp on the pulse hose, using a screwdriver. The pulse hose connects to the fuel pump and goes to the intake manifold. Place a coffee can under the hose and pull the starter rope several times. You should see fuel discharge coming from the hose. For an electric start, have an assistant crank the engine over several times; remember to have your ignition cutoff activated and the plug wire removed. Lack of fuel discharge points to a bad fuel pump check valve or diaphragm.
Hook up a fuel pressure gauge to the pulse hose that comes out of the fuel pump. Use the pull rope or electric starter to crank the engine over. Read the gauge. A normal fuel pump will put out aproximately five to seven psi of pressure. Any reading below this will indicate a defective fuel pump check valve or diaphragm. Replace the fuel pump or purchase a rebuild kit.
Things You'll Need
- Boat owner's repair manual
- Electrical tape
- Socket set
- Ratchet wrench
- Compression gauge
- Screwdriver set
- Coffee can
- Fuel pressure gauge
Chris Stevenson has been writing since 1988. His automotive vocation has spanned more than 35 years and he authored the auto repair manual "Auto Repair Shams and Scams" in 1990. Stevenson holds a P.D.S Toyota certificate, ASE brake certification, Clean Air Act certification and a California smog license.