Systems+Engines

Fuel and Water Don't Mix

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This wasn’t the first VHF call I had taken from someone seeking advice for an onboard problem, and the caller was clearly distraught. He had accidentally filled his diesel tank with fresh water. To make matters worse, when he tried to start the engine, fresh water had been sucked through the fuel system. Always interested in a challenge, I went over to his boat. Together we fixed the problem and had the engine running again in less than two hours. Here’s how we did it.

First you have to clean the tank. The easiest way is to remove either the tank’s inspection cover or the fuel gauge. When you’ve done that, you’ll pump out the tank, sucking the liquid from the bottom.

I used a 12-volt PAR-type pump that has alligator clips on the positive and negative power leads. For the suction side of the pump, I attached a 4-foot length of hose on the inner end and then used a hose clamp to attach a 3-foot length of rigid PVC pipe to the hose (see photo, page 80). This setup put the bottom of the PVC pipe on the bottom of the tank, where the water settles. Finally, I rigged the discharge end so that flexible hose went into a suitable container; I used a drum on the dock. When everything was ready, I connected the alligator clips to the positive and negative battery posts; you have to make sure the polarity is correct so the pump doesn’t run backward.

With the pump running, I slowly moved the PVC pipe along the bottom of the tank and into the corners to suction out all the offending liquid. Pure water came out first and then slowly changed to a diesel/water mixture. It’s important keep pumping until you’re getting only clean diesel fuel out of the discharge hose. At that point you can secure the pump and close up the tank.

The next step is to clean out each fuel-filter housing. If there are multiple filters—a Racor-type primary and a spin-on secondary, for example—always start with the primary filter (1). Clean the housing, install a new filter, and fill the housing or spin-on filter to the brim with fuel. Repeat the process at each filter element (2). When you’ve finished, there will be clean diesel running from the tank to the fuel pump. Minute quantities of air may still be trapped in the lines or tops of the filters, but the pump should be able to clear these. If not, following the procedures described below will purge all the air from the system.

However, the fuel lines will still have air and water in them, so you’ll need to bleed and prime them. The purpose of the next two steps is to minimize water penetration into the sensitive lap-mated internal surfaces of the injection pump. First loosen the fuel-line connection to the injection pump to bleed the air and water from the system before they make contact with the interior of the pump. If your engine has an electric fuel pump, you can turn it on by energizing the ignition switch. If you have a mechanical fuel pump, you’ll have to either use the manual finger pump, if there is one, or crank over the motor. When this is done, fuel will start to flow through the filters and fuel pump (3). Any water remaining in the system will (mostly) be caught by the secondary filter (4), and any water or air still remaining after the fuel has passed the filters will be discharged through the loosened fuel-pump connection before it hits the injection pump. Use rags and a coffee can or similar container to collect the water and waste fuel as it drips out. When the line is clean and there are no longer any air bubbles or water contamination, shut down the fuel pump and retighten the fuel-feed line at the injection pump (5).

If you believe water may have made it into the injection pump, proceed with caution. Purging procedures can vary between engine types, so always refer to the engine’s operating manual for the correct procedure to follow. While trying to start the engine may work, there is a chance that it may also damage the pump and require expensive repairs. Removing the pump and having it serviced by a repair facility may be the best approach.

The final step is to loosen the injection nuts connecting the high-pressure fuel lines that lead from injection pump to the injectors (7). This step will remove the last bit of water or air that can be purged from your system. Use goggles to protect your eyes because the fuel pressure into the injectors can be high. When the nuts are loosened, crank the engine over to push fresh fuel through the injection pump and on to the injectors. Loosen all the nuts at the same time; the engine will not start with the injector nuts loosened. When the engine is turning over, fuel and air will come out from underneath the loosened injector nuts, so you will need to have rags and a container to catch the liquid. When clean fuel comes out of the nuts, retighten the nuts one at a time. Be aware that there is a chance the motor may start and run erratically as you nip up each injector nut. If this happens, don’t panic. Stop cranking the engine and just let it run as you tighten down the rest of the injector nuts. As the last injector nut is tightened, the motor will begin to smooth out.

We had one hiccup during the last step. The engine did not start as we tightened up the last injector-line nut, so I double-checked the tightness of the injector lines and asked the owner to crank the engine. This time it began to run. As a final step, I again loosened each injector-line nut very slightly, one at a time—just enough to let any remaining air leak out. The engine ran a bit erratically during this process, and that’s typical. Once there were no more air bubbles coming out from around the injector-line nuts, I retightened them, and the motor smoothed out. The final step is to check each fitting you loosened while the engine is running; also check the new gaskets around the filters for leaks. I like to use a clean paper towel, which makes leaking fuel easy to spot. Tighten any fittings that show fuel leakage. At the end of the process, you’ll once again have a clean fuel system.


A former Coast Guard engineer, Chris Lab, his wife, Beth, and their daughter, Yvette, are in their fourth year of full-time cruising. Now in the Caribbean, they plan to visit Ecuador aboard their Passport 40, Aquamarine II, before heading across the Pacific to French Polynesia next spring. Follow their adventures on www.svaquamarine.com

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