Boat Fuel Systems: Explaining the Basics

boat fuel filter, fuel-water separator,

A spin-on style fuel filter separates water and debris from fuel flowing through.

Article by Andrew McDonald courtesy of Canadian Yachting magazine

Engines don’t need much to run (or to stay running). Break down even the most complicated gasoline engine — even one with modern technology like an onboard computer, diagnostics and electronic ignition — it still only needs three things; spark, compression and fuel. (Diesel engines differ slightly, in that they rely on high-pressure and compression, rather than a spark.)

One of the trickier aspects for marine engines in today’s world is fuel. An engine’s fuel system seems complicated — but when it’s broken down and explained, it’s easier to understand for most boat owners.

Fuel Systems 101

  • The fuel tank
  • Fuel Lines
  • Fuel Pump
  • Shut-off valves
  • Fuel Filter(s)
  • Delivery system

boat fuel tank, boat fuel lines, boat fuel hoses

A fuel tank with many hoses. The bottom hose is the tank fill. The middle hose connects to the fuel pickup on the engine or fuel filter and the top is the vent hose.

It works pretty simply: When the engine starts, a fuel pump (either electric or mechanical) draws fuel from the tank via the fuel lines. The fuel runs through a filter that removes water and debris and travels to the engine where it’s delivered to the engine cylinders (where all the work happens) via the carburetor or injectors.

Long-Term Problems

In all of these parts the main problem encountered can be fuel itself. In most boats fuel is used for half of the year and then is left to sit in long-term storage for the winter months. Fuel breaks down over time, and is susceptible to microbial growth, and loss of octane (the ability of the fuel to ignite.)

Secondary to this is the ethanol additive that is found in many readily available fuels. Ethanol is an alcohol that is added to fuels available at the pumps to help reduce emissions. This summer, the Biden administration has allowed land-based gas stations to make E15 or gasoline with 15% ethanol more readily available. This is a problem for boats because it’s illegal to run E15 in marine engines, so consumers need to be careful when purchasing at a gas station. Alcohol additives in fuel attract water, which dilutes the fuel, creating problems for boats. Alcohols also break down gaskets, diaphragms, primer bulbs and fuel lines.

marine fuel additive, Revive anti-ethanol

An example of a fuel stabilizer that should be added to the fuel tank (and run through the fuel lines and engine) any time the boat is in long-term storage.

The Solution

How do we optimize fuel and maintain our boat’s fuel system?

First: Purchase ethanol-free fuel when possible. When filling up at a standard gas station, choose high-grade fuel. At a marina, ask if they have ethanol free fuel available or use fuel from companies like ValvTect that has been specially treated to minimize the effects of ethanol like products.

Next: When storing your boat for long periods, add a high-quality fuel stabilizer to the fuel in the tank. This will slow the loss of octane and counteract the effects of any ethanol or water. For diesel fuel, add a biocide during periods of storage

With these two steps, we can be fairly certain that the fuel itself is in good shape. Let’s look at some other maintenance points.

marine fuel line, marine fuel hose, boat gas hose

A marine-designated fuel line is specifically labeled for its intentions.

Fuel Tanks: Try to prevent sludge or debris from accumulating in the fuel tank. If you suspect that there is any debris or old fuel, try to keep the tanks topped up with fresh fuel and don’t let the fuel level get too low. Inside the tank, a tube (called a pick-up) sits vertically like a straw, and draws fuel from the bottom of the tank. At the top of the pick-up is an anti-siphon check-valve that stops it from continuously running out of the tank

Fuel Lines: Check fuel lines regularly for breaks, cracks, loose hose clamps or other leaks. At the first sign of damage, replace them with an appropriate grade and diameter fuel line

Fuel Pump: Most are designed to be maintenance free — but watch for lack of pressure. A broken fuel diaphragm or a blockage in the fuel lines can cause an engine to starve and die off with lack of fuel (especially at higher speeds), or not start at all.

Fuel Filters: check fuel filters regularly and replace them at least annually. If fuel filters accumulate water and aren’t drained or replaced, water can make its way into the engine. If your engine is bogging or dying off as it runs, check the fuel filter first to see if it has accumulated water.

fuel shutoff, shutoff valve, fuel shutoff handle

A fuel shutoff valve is one of the best pieces of equipment for preventing accidental leaks.

Shut-off valves are a safety feature allowing the user to shut off the fuel supply to the engine, to perform maintenance or, in the case of multiple tanks, to switch between fuel tanks

Delivery System: fuel can be delivered to the engine via a carburetor, fuel rails and injectors. Each will deliver fuel in the correct proportion to air into the cylinders where the fuel is burned. The fuel is aspirated into the cylinders – its delivered as a fine mist. Any clogs or broken down fuel can block up passages in the carburetor or the injectors. Proper winter layout procedures and periodic carburetor and injector cleaning can keep the fuel delivery in top shape.

In summary: keep fuel fresh, limit use of ethanol, replace fuel filters regularly and perform periodic inspections and maintenance to avoid fuel problems.

Andrew McDonald is the owner of Lakeside Marine Services — a boat repair/maintenance firm based in Toronto. Andrew has worked in the marine industry for 12 years and is a graduate of the Georgian College Mechanical Techniques - Marine Engine Mechanic program.