Slow Fill on EPA regulated fuel systems: 

EPA regulated fuel tanks and the supporting parts are one of the most misunderstood yet simplest parts of boats built in recent years. Because of that techs in the marine industry spend lots of time diagnosing problems or complaints, and often more time than is necessary. I will try to help explain the way they work and what to do when they do not function as we expect them.

First the most common complaint is slow to fill. The most common cause of this complaint is the tank is full by design, but it will take more fuel if the operator is persistent enough. In fact the bigger tanks may take another 10 gallons or so after the tank is actually full. Of course this doesn’t make sense, until you understand what is going on. So let’s cover ullage space first.

Ullage is literally an air space maintained in the tank to allow for expansion during a normal heating and cooling cycle of a 24 hour period. This air space is big enough to contain the fuel without it being pushed into the vent line, which prevents the fuel from spilling overboard and contaminating the environment. In some systems there is a charcoal canister in the vent line which could be rendered inoperable when fuel enters the canister, and it can even become a plug in the vent line which would make fueling difficult at any time.

This brings us to the next issue where ullage space is concerned. Most tank builders, or maybe we should say fuel level sender builders, do not allow for that air space in the design of the fuel level sender. Most senders are a straight shaft mounted vertically from the top of the tank. On that shaft is a float which activates sensors in the shaft which in turn send an ohms reading to the fuel gauge. This ohms reading is interpreted by the gauge as a fuel level. Introduce the ullage space and the float can no longer reach the top of the shaft, so the full tank will sometimes result in a ¾ or so reading on the gauge. Seeing that on the gauge encourages the operator to keep on filling. Unfortunately modified senders to allow for ullage are not commonly in use, but they are coming.

Now let’s move on to physical problems that could cause the slow fill situation even when the tank is not already full. The most common physical cause is a venting problem.  A pinched or kinked vent line can be the cause. Test for that removing the vent line at the tank and blowing through it. It should allow air to pass through that line with little effort. Some vent lines will have valves or canisters, but still should allow an easy flow of air since that is what happens during filling. Keep in mind some of the vents are in the fill assembly so best to remove the fill cap to be sure the vent outlet is open.

The least common cause of slow fill is the fill line itself. This is a big and tough hose that is hard to kink or crush so that cause is extremely uncommon, but there have been some cases in older hoses where the inside lining of the hose has bubbled out effectively blocking the incoming fuel. Again, this is extremely rare but when all else is right, remember this one.

And another surprise the EPA tanks threw our way; some of the fill systems will only accept fuel at the rate it comes out of the pump. So the dock caddies, jugs, whatever present a challenge of filling too slow. There are some special funnels out there in the aftermarket designed to overcome this challenge.

In summary, EPA regulations have led to a more complex system with mysterious connections, but the fill process is very much the same as it’s always been. When fuel comes in, the air needs to flow out. Keep this in mind and most of the mystery is irrelevant.