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I have half a full tank of gasoline (95 octane) for a 3S-GTE engine (fuel injected, four cylinder). This is on a car that has just been sitting still.
Can I safely run the engine on this gasoline?
If not, why not?
What are possible problems and consequences?
If the fuel tank was kept full, then there should be no issue using the fuel. The biggest problem comes from ethanol in the fuel absorbing water from the air. With a full tank of gas, there's no real room for air to be there, so little to no absorption occurs.
If the tank was not quite as full as you thought (either completely dry or completely full is the best way to store a vehicle), there are products you can purchase which will help the fuel. As said in the other answer, it is a good idea to get fresh fuel into the tank before you run it too much. Cutting the fuel with fresh will give you good results while getting rid of the old fuel safely. If there was only a 1/4 tank (or around that) without fuel stabilizer, you'd want drain the tank and run completely fresh gas in it. If you are in a part of the world which doesn't use an ethanol blend, you wouldn't have much to worry about. I've seen old, non-ethanol fuel, run without issue ... And I'm talking old fuel here.
Fuel can stay good for a long period of time if it can't absorb water. Fuel stabilizer can help if used prior to the vehicle sitting still, but does little if anything after the fact. If the fuel has absorbed water, there is a chance of corrosion and sediment which can plug fuel filters and injectors.
I would say yes, with a couple of suggestions. At the very least, I would go straight to a gas station and top off with fresh fuel. If you want to be even more cautious, you could add some fuel stabilizer (like sta-bil or similar) before starting it, and then head to a gas station.
EDIT: What I was thinking when I wrote stabilizer was really dry-gas to help combat the moisture from not having the tank full. In fact, if I'm not mistaken stabilizer won't help much with moisture even if added before storage.
Modern fuel injected vehicle fuel systems have evaporative emissions controls which essentially means that the fuel tank is sealed. Fuel can be stored for years without it going stale from evaporation like it will in a lawn mower or similar equipment.
Putting ethanol free fuel in the tank before storage may still be a good idea. I don't know how much moisture can be sucked in through the EVAP carbon canister and in to the tank. Ethanol can also settle out and cause the remaining fuel to have lower octane.
