MST MST Marine Fuel Systems 2 — Questions and Answers
Question 1: What is the correct procedure to test a fuel system for leaks after a repair?
- Pressurize with air to no more than 3 psi and check fittings with soapy water (Correct answer)
- Fill tank and visually inspect only
- Run the engine and smell for fuel
- Use an open flame near fittings to reveal vapor
Correct answer: Pressurize with air to no more than 3 psi and check fittings with soapy water
A low-pressure air test (max 3 psi) with soapy water applied to fittings safely reveals leaks without the fire risk of using fuel.
Question 2: Phase separation in ethanol-blended gasoline (E10) stored in a marine tank causes what problem?
- Water and ethanol separate from the gasoline and sink, leaning out the fuel mixture (Correct answer)
- The fuel becomes richer and runs better
- No problems occur with E10 in marine engines
- Only diesel fuel is affected by phase separation
Correct answer: Water and ethanol separate from the gasoline and sink, leaning out the fuel mixture
When water is present, ethanol absorbs it and separates from the gasoline layer, creating a water-ethanol mix that can damage engines and fuel systems.
Question 3: A two-stroke outboard engine is drawing in air through the fuel system; which component is the most likely cause?
- Cracked or deteriorated fuel line or primer bulb (Correct answer)
- Clogged fuel filter
- Stuck choke plate
- Faulty tachometer sensor
Correct answer: Cracked or deteriorated fuel line or primer bulb
Air infiltration in a two-stroke fuel system is almost always caused by cracked fuel lines, a deteriorated primer bulb check valve, or a loose fitting.
Question 4: What is the function of the anti-siphon valve installed on a marine fuel system?
- To prevent fuel from gravity-feeding into the engine when it is not running (Correct answer)
- To regulate fuel pressure to the injectors
- To allow air into the tank for vapor equalization
- To filter water from the fuel before it reaches the engine
Correct answer: To prevent fuel from gravity-feeding into the engine when it is not running
The anti-siphon valve closes when the engine stops to prevent raw fuel from flooding the engine or bilge via gravity flow from the tank.
Question 5: What color is the standard fuel fill deck fitting on gasoline-powered boats to distinguish it from diesel?
- Red (Correct answer)
- Yellow
- Green
- Black
Correct answer: Red
ABYC standards specify red deck fill fittings for gasoline to prevent misfueling with diesel, which would damage gasoline engines.
Question 6: When replacing a fuel filter on an EFI outboard engine, what precaution must be taken before disconnecting fuel lines?
- Relieve fuel system pressure by running the engine until it stalls or using the service manual procedure (Correct answer)
- Disconnect the battery only
- Remove the fuel cap to vent the tank
- No special precaution is needed for EFI systems
Correct answer: Relieve fuel system pressure by running the engine until it stalls or using the service manual procedure
EFI systems maintain residual pressure in the fuel rail; pressure must be relieved before disconnecting lines to prevent fuel spray and fire risk.
MST MST Marine Fuel Systems 2
What is the correct procedure to test a fuel system for leaks after a repair?