Checking for Leaks and Cleaning the Fuel Tank(s)


Well-Known Member
Jul 31, 2011
Wilmington, NC
Owners of Shamrocks and inboard engines sometime have to perform some diagnostics – a rough running engine, fuel smell in the bilge, etc. Speaking from experience sometime we perform many tests only to learn that a little oil on the choke pivot points is all that is needed to address an engine stalling out when the throttle advances above 1500 RPM – after unnecessarily replacing fuel filters, fuel pumps, spark plugs, etc... If you have read the forums, you have heard about potential causes of poor engine performance such as timings off, "carburetor needs to be re-built", spark plugs are fouled, fuel pump is weak, etc… If you have an experienced marine mechanic to promptly respond to diagnose and repair your boats propulsion system, you can probably stop reading this post because this is lengthy post.

Seasoned marine mechanics experienced with inboard engine and fuel systems are becoming difficult to find and Shamrock owners sometimes need to roll-up their sleeves and perform some of the work.

To check out the tanks and attempt to run the engine with a known clean fuel source, just use a clean ("new" if possible then no filter needed) portable boat fuel tank and a few fittings to connect the portable tank hose to the fuel pump supply hose. It helps to have some rags, bolts and clamps to temporarily stop any fuel from spilling out of the disconnected fuel tank hoses. You can temporarily add a barbed brass cutoff valve for convenience - don't want any unintended fire events. I always use non-ethanol 93+ octane and Marine fuel stabilizer. If the engine operates normally when connected to the known clean fuel source then start moving the portable tank hose connection upstream towards the fuel tank until you get to the fuel tank (by moving the tank hose connection to the upstream side of filters, etc.., eventually working upstream to the hose barb on the top of the fuel tank). Start and run the engine each-time you move the temporary hose connection upstream to the next the component, preferably from idle to wide open. If the engine runs rough with the known clean fuel source, you need to focus your attention on the engine.

Then if the engine is operating properly with the temporary clean fuel source flowing thru the boat hoses, filters, etc..., plug and clamp the boat fuel hose end that normally connects to the tank barb. Then connect and clamp the hose barb on top of the fuel tank to a clean (preferably new) temporary hose with a length of your choice and extend the temporarily added hose to a storage tank that is suitable to receive whatever fuel, sediment, sludge, water, etc... will be coming out of the fuel tank. Next check the o-ring on the fuel filler cap and add some lubricant to o-ring seal to be assured the cap forms an air tight connection when normally tightened. Now take off the vent hose from the thru-hull vent fitting. Get a bicycle tire pump (not an electrical spark producing air compressor capable of producing excessive explosive pressure) and gather the fittings and teflon tape required to connect the bicycle tire pump hose to the tank vent hose. At the end of the temporary hose connect a low air pressure gage (0-15 psig) that has 1-pound increments. The bicycle pump and your diligent manual pumping will eventually add 2-3 psi of air pressure into the tank. Pump the air pressure in the fuel tank up to 2-1/2 to 3 psig. Record the time. Come back in 2 hours. If the pressure is close to the previous pressure reading you have verified your temporary connections are good and the tank did not leak during the test. If pressure has completely fallen to zero, somewhere in the tank and/or your connections and filler cap o-ring there is/are small leaks. If you can never get the pressure to 2-3 psig then something is really leaking and if you are convinced your connections are good then the tank must be leaking. You may not see fuel in the bilge if the tank leak is in the top of the tank and the fuel level is low.

All of this requires very little removal of hatches, panels etc, except for temporary access to the vent hose thru-hull fittings. FWIW - on one boat when I accessed and observed the actual vent hose connection to the thru-hull fitting, I discovered a kink in one hose which explained why filling the tank had always been accomplished at a very, very, very slow rate. Once the vent hose length was adjusted and the kink removed, the tank filled as fast as the marina’s fuel pump could deliver the fuel.

Once convinced the tank does not have a leak, it is time to flush the tank. To flush out the tank remove the 0-15 psi pressure gauge and direct the end of the hose to the receiving tank to temporarily store the flammable liquid and transport the liquid to an environmentally acceptable disposal facility. The bicycle pump will safely add sufficient pressure to the tank to push the liquid substances in the fuel tank to your receiving tank via the tank pickup inside the tank, thru the tank hose barb and into the temporary hose until the only substance left in the boat fuel tank is below the fuel pickup in the tank. You will know the tank is close to empty because you will lose air pressure in the tank and not be able to maintain or add pressure with the bicycle pump.

With the boat tank as empty as the pickup will allow, add 2-3 gallons of clean non-ethanol 93 octane and marine fuel stabilizer to the boat tank and then pull the boat and trailer around and up and down some bumpy, hilly and curvy roads. Come back, pump up the pressure again with the bicycle pump and empty the 2-3 gallons of fresh fuel from the boat tank into a clean storage tank. Occasionally sample and observe some of the fuel visually in a clear jar.

If the fuel is dirty, dispose of the fuel and repeat the flushing, bumpy, hilly and curvy road, sampling process until the fuel is clean. If the fuel is clean, pour it back in the boat tank, undo all the temporary hose/pump stuff, re-connect the tank to the engine fuel supply system and the vent hose to the thru-hull fitting and fire up the engine using fuel from the boat tank. Once you know the tank can safely store and deliver clean fuel to your engine and your engine is running properly, repeat the same procedure on the other tank.

Let me know if you have suggestions for corrections or typos that may need attention.
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