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.
