Checking for Water in Your Fuel Tank

If you see this after taking a fuel sample, you’ve got a problem. Photo Tom Richardson

By now most boaters are aware of the headaches that can be caused by ethanol-blended gasoline (E10), chief among them excess water accumulation in the fuel. This, in turn, can lead to “phase separation,” in which the water-ethanol mix settles to the bottom of the tank, while the octane-depleted fuel sits on top. Even if the water level isn’t high enough to reach the fuel intake, the low-octane gas can cause engine problems, such as rough or inconsistent running.

If you want to make sure that your gas tank is free of water before launching your boat, there are two ways to go about it. However, before you proceed, remember to use extreme caution when dealing with gasoline. Or better yet, leave the job to a professional.

Routine replacement of the fuel/water separator filter on your fuel line can head off problems. Photo Tom Richardson

The best way to check for water in the gas tank is to remove the fuel line to the fuel-water separating filter and pump at least a liter of gas into a clear container (you will need to install a primer bulb ahead of the filter to do this). Allow the gas to sit for several minutes then check to see if any water has settled to the bottom. If the fluid is uniformly yellow or amber in color, you’re probably OK. If the fluid is clear, make sure that the entire jar is not full of water!

Another way to check for water is to remove the inspection port on the top of your gas tank, where the sending unit is installed. Apply an alcohol-formulated, water-detecting paste (e.g., Kolor Kut Modified Water Finding Paste or MDR 566 Water Probe Indicator) on the end of a wooden dowel and lower it to the bottom of the fuel tank. If water is present at the bottom of the tank, the paste should change color, although some marine service professionals warn that this method is not always reliable.

If you do find water in your gas tank, there’s good news and bad. The good news is that you’ve discovered it before an engine problem occurs. The bad news is that you may need to drain the entire tank and dispose of the fuel, especially if phase separation has occurred. To be sure, bring your boat to a marine service facility and have their staff assess the situation. Also, disposing of large quantities of gas is not something most people can easily do—at least not legally.

Aside from disposing of the fuel, you’ll need to figure out how the water is getting into the fuel tank in the first place. The culprit could be a broken vent or a bad fuel cap seal, but you’ll need to track it down or you’ll be facing the same issues down the road.

By the way, if you’re concerned about water in your fuel and your engine doesn’t have an internal sensor,  you can install an aftermarket sensor (West Marine/Racor Alarm Sensor)  (Entratech) that monitors the fuel for signs of water before it reaches your engine.

Note that water can also collect inside the engine during layup, due to condensation. That’s why it’s a good idea to run the engine at home or the marina before launching for the season. If the engine alarm indicates water in the fuel, drain the phase-separator tank and fuel filter bowl to check for water. Remove the water and run the engine again to see if more water is causing the issue.