SETTLING THE DEBATE
QUESTION: Thanks for all the great advice. I utilized your insight on a past issue for fogging fuel-injected engines. I fogged my MerCruiser 502 Magnum engine with Marvel Mystery Oil dispensed from a garden sprayer.

My friends and I, all owners of performance boats, debate regularly on whether to leave the fuel tanks low when winterizing or fill them up to minimize condensation. We all agree to put stabilizer in. I subscribe to the theory that there is nothing better than fresh gas when starting up in the spring. I usually dwindle the tank down to less than a quarter tank. My friends don’t necessarily agree with me. Can you settle our debate?

ANSWER: The concept of filling the tanks to minimize condensation during storage was the standard procedure in years past. It is still common practice on airplanes. The problem is that most automotive gasolines purchased at the pump today are not the same quality as they were when it was common practice to fill the tanks prior to storage.

It is likely that your fuel also contains oxygenating components such as ethanol, isopropanol and ether that are currently used to replace MTBE. These additives are injected into the fuel during the reformulation process. Ethanol, isopropanol and ether are polar solvents, which means they are water-soluble. When they are present, condensation can be absorbed into the fuel resulting in further deterioration. The components added during the reformulation process also have the tendency to evaporate more rapidly than the base gasoline, which has the same effect as lowering the octane rating.

Aviation fuels are required to be higher quality and do not deteriorate rapidly, so it makes sense that filling the tanks on airplanes is usually the practice.

I used to believe that leaving the tanks full of fuel was the best practice. About five years ago, I switched to leaving them as low as possible as a result of tests that we performed on fuel to see how it held up during long storage periods. The bottom line is that modern pump gasoline tends to get stale faster than we imagined.

When you bring the boat out of storage, I recommend filling the tanks with premium gasoline. Using the higher-octane gasoline will improve the overall quality of fuel when mixed with the existing fuel in the boat’s tank(s). In addition, using a quality fuel stabilizer can’t hurt. In short, you win the debate.

LONG WINTER
QUESTION: My boat was winterized last fall. If it does not get taken out of storage and used this year, will it be OK, or is there something I should do before winter?

It has an 80-gallon fuel tank that has about 15 to 20 gallons in it. Fuel stabilizer was added to the fuel when the mechanic did the winterization last year. It was winterized with antifreeze. The motor is a 1999 502 MPI.

ANSWER: I think it would be wise to at least get the boat running on the garden hose. You should also consider draining the fuel if it is going to sit for another year. It is unlikely that the fuel will survive another year without becoming very stale. Also, raw-water pump impellers that are not used regularly can deteriorate faster than when the engine and pump are used at least periodically.

The impeller vanes that end up being pressed on the closed side of the pump housing will many times crack and break off when not used for long periods of time. If the vanes don’t crack, they will at least take a set and lose their resiliency. The result is that the pump will be less effective.

It is always a good idea to service your raw-water pump and replace the impeller at the beginning of each boating year.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

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