Old ethanol gas
 

OK so here's my problem. Almost 10 weeks ago I filled my tank with 89 octane regular gas which has according to the pump 10% or less ethanol. Unfortunately I haven't been able to use my boat so it's had 140 gallons of this stuff sitting in it. My engine is a Mercruiser 496 HO and according to my owners manual any fuel over 87 octane is fine. This is my first boat in a while and the last time I owned a boat this ethanol thing wasn't am issue. My question is what should I do now? I really don't want to waste a 140 gallons of gas so do anyone have any suggestions?

You should be fine but to be safe I would add some Starton fuel additive to it (link below). I run the Startron with every tank of fuel no matter how long it has been sitting and my engines run great.

http://mystarbrite.com/startron/

N/a motors will be ok, i'd top it off asap with 93 or whatever high octane you can get. most of the ethanol has likely evaporated, which gives ethanol blend fuels a good amount of there octane, if it was supercharged I wouldn't do more than let it idle

Just run it!!! 10 weeks is not a huge deal and the octane will still be fiine. The only problem that you might have is some water/condensation in the fuel caused by the alcohol. If it starts sputtering after a few hour of run time, dump remove and dump the fuel water sep filter.

FWIW, last spring (2010) I filled my neighbors 38 Donzi (525's) to the top w/91. The boat did not get used at all last season, Last week I put the boat in the water for him with full tanks from spring 2010. It ran perfect, I did not beat it too hard because of the old gas but there did not seem to be any difference at all. I think the computer controlled motors compensate for old gas pretty well. As mentioned, carbed blower motors an entirely different story.