posted 08-13-2009 10:46 PM ET (US)
Gasoline that is 1.5-years old and that was not prepared for storage with a stabilizer will very likely be deteriorated. It will not be a good fuel.You do not mention the fuel tank size and how much fuel you have in it. You may need to discard most of the fuel. If you can get most of the fuel removed from the fuel tank, you can dilute the remaining old fuel into a much larger volume of fresh fuel. This should be acceptable. Also, check for any sign of water in the fuel tank. Sitting for 1.5-years may have allowed water to enter and separate from the gasoline.
For the purpose of getting the engine running after a long period of storage, try using a small tank--say a 6-gallon portable fuel tank--with fresh gasoline. Because your engine is fuel-injected, and there is some volume of fuel contained in the fuel rails, fuel-vapor separator, and in the electric fuel pump, you may have to crank the engine for a bit to get the fresh fuel to actually arrive at the combustion chamber.
The Mercury OptiMax motor needs a very strong battery for engine starting. If the battery voltage sags during cranking it can cause a no-start condition. Be certain your engine starting battery is at least a 1,000-MCA rated battery, that it is in good charge, and that its voltage is not sagging during cranking.
Modern engines use quite high voltage to fire the plugs, and they can often overcome fouled plugs. However, putting fresh plugs into the engine will be a good way to help it start. You might try getting it to run on the old plugs, burn off the crud that will probably be in the engine from all the old fuel and unburned gasoline that accumulated during cranking, and then put in new plugs once you get it to run. I suggest this so you don't immediately foul the new plugs trying to get it going. Of course, if it doesn't start, by all means try new plugs, too.