I was panning to mow my lawn today and got out the gas can (plastic, sealed) and there was a little under a gallon left from last year - Iknew I didn’t wantto put in the mower so I figured I would put in my car, which is about 3/4 full on a 12 gallon tank tank - thought it would mix in okay. BAD IDEA. My car is basically undrivable now. Choppy, no power, stalled once. Check engine light is on. I’ve got it idling in the driveway now hoping to burn some gas off, but a 2008 Yaris takes a long time to go through that much gas. What can I do? Is siphoning and starting over the only solution? How about an additive? Again, this is about 1 gallon of old gas to 10 gallons of new.
You might try adding a can of Seafoam Engine Tuneup to the gas tank. If there was any moisture/varnish in the old gas the Seafoam will absorb/disolve it.
Tester
Unbelievable…If you knew it was no good for your lawn mower, what made you think the Yaris would like it?? I would put a bottle of Stabil and a bottle of Chevron Techron in it and take it out and run it hard, hoping it clears up…
I plead guilty to extreme dumbness. I’m female - is that an excuse? 
Thank you for your replies - I’ll get someone to take me up to get some Seafoam/Stabil and some new gas to top it off. Is one bottle going to be enough? Should I get premium gas or does it matter?
Just dump one bottle of Seafoam in the gas tank and take it for a drive.
Tester
One bottle should work. Fill up and fresh gas, and take a somewhat long drive. Have a back up plan in case the vehicle stalls and won’t start.
I would put a bottle of Stabil …
The stabilizer might have prevented the problem, but it is not designed to fix it. Too late.
Should I get premium gas or does it matter?
Premium will be of no help at all. Premium only means it has a higher octane. That describes how fast it burns. High test burns slower, to reduce engine knock. In diesels it is the exact opposite. High cetane burns faster and easier, which is good in a diesel, but bad in at gasoline engine.
For future reference, if you put fuel stabilizer in your mower gas, it should keep fresh from fall to spring.
What was the capacity of the gas can that held the gasoline that you poured in the car? The reason I ask this question is that the larger the can, the more likely there was condensation and this moisture is what is causing your problem. I would put in a product such as Heet gasoline antifreeze. The gasoline antifreeze picks up the moisture in the gasoline and helps it burn in the engine.
I had some gasoline left over from last fall and although I knew it isn’t the best idea, I poured it in the gas tnnk of the lawnmower. The mower started right up and I have mowed the yard 4 times using this gasoline. I may have added some fuel stabilizer to the gasoline last fall–I really don’t remember. I was in a hurry to mow the yard so I decided to try just a little gasoline in the engine. I figured that if the engine didn’t run right, I would drain the gas tank on the mower and go buy some fresh gasoline. Since the mower ran fine, I decided to go ahead and use the gasoline.
Unless your old gasoline contained quite a bit of moisture, I can’t imagine it causing the problems in your Yaris when diluted down with 12 gallons of fresh gasoline.
If the mower gas had a few ounces of water in it, not very likely, that would cause the 10% ethanol mixture in the Yaris to separate, the water / alcohol settling to the bottom.