Fuel Supply Diagnostics:

A common problem as boats and engines age is fuel delivery from a built in tank to the engine. A popular test to determine if it’s the fuel system in the boat or the engine is to run the engine on a portable tank. The logic is that if it runs good on the portable tank, the problem is in the boat fuel system.

The 6 gallon tank test is helpful but it’s not conclusive. It’s easier to pull fuel from a tank sitting in the boat than it is from a tank under the floor because of less lift, and the tank in the boat likely has an anti siphon valve in the fuel barb on the tank itself. The anti siphon puts a bit of vacuum in the line to prevent a leak from emptying the tank into the bilge, but that resistance can overcome a borderline fuel pump on the engine. A portable tank doesn’t have this valve so it can occasionally mask a fuel pump problem.

The most reliable test of fuel delivery from a built in tank is with a fuel vacuum gauge and a piece of clear plastic fuel line leading to the gauge. With this installed between the tank and the engine you can easily determine exactly how much vacuum is needed to pull the fuel from the tank to the engine. Most engines tolerate as much as 3.5 inches of mercury vacuum without a problem. Once the vacuum gets over 4 we can expect problems. I’ve seen fuel actually vaporize at that much vacuum, and that’s where the clear plastic fuel line comes into play. Placing that line on the supply side of the gauge allows bubbles in the fuel to be easily seen. Bubbles can represent an air leak, and in extreme cases you can see as the fuel vaporizes inside the line from extreme vacuum. Just as water will boil at lower temperatures with higher altitudes where pressure is decreased, so does fuel. High ambient temperatures and alcohol content in the fuel increases the chance of this vaporization inside the fuel lines.

If you see a vacuum reading of over 3.5 it’s time to find out why the fuel supply is restricting the fuel flow to the engine so much. A clogged filter is obviously a concern and easy enough to bypass in most cases. Filters used in boats are usually high capacity and not often the problem, but be sure. Beyond that a restriction can be a pinched or collapsed line, but the more likely cause is either the anti siphon valve or the fuel pick up tube in the tank.

As mentioned earlier the anti siphon intentionally ads a bit of resistance to the fuel flow but occasionally they work too good or get sticky and need to be replaced. The amount of resistance depends on the depth of the tank with deeper tanks requiring more resistance. A good average would be something in the 2” mercury range.

 Most DIY folks do not have the fuel vacuum gauge, and many techs do not have one either but checking the vacuum needed to pull fuel from the tank is the only dependable method of determining the source of the problem. Short of that, I’d start with that anti siphon valve and replace it temporarily with a straight fuel barb to take that resistance out of the system. If it works like that many are tempted to just leave it out, but it is an integral part of the safety features in the fuel system and should be there.

The fuel pick up line in the tank is most often a stiff pipe, nylon and aluminum are the most common materials, and many of them have a wire mess on the bottom end to prevent particles from interfering with the anti siphon valve. Particulate that reaches the valve could hold it open negating the anti siphon function. Obviously trash shouldn’t be in the tank but things happen and it doesn’t take much to clog this screen. Most pick up tubes screw in a fitting on the top of the tank. If corrosion isn’t an issue it’s easy enough to remove the tube and inspect it. Check that screen to be sure it’s clean, and check the tube for cracks that can leak air. If you find trash on the screen it’s a good idea to clean that tank out completely before wrapping up this project.