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I have not seen gas so bad that a car wouldn't run. That is one that was running and quit.
If it's sat for more than 6-12 months I usually toss a can of heat in the tank and run it damn near empty and put some fresh in it.
Testing? Um? Most professional fuel pressure testers have a purge or relief valve on them and running some fuel in to a clear container (Pyrex measuring cup) will give you a nice visible look at the fuel.
I did see a young kid fill the tank with E85 on a non flex fuel car. Everybody was scratching their heads until they did a fuel pump volume test and corn oil came out of the tank.
Old bad gasoline smells terrible. I've seen lots of cars that won't run because of the quality of fuel, but I use "recycled" gas in my truck, and have gotten pretty good at knowing what to burn and what's going to leave you walking. I go by smell mostly, if you can't see the bottom of a 5 gallon bucket through it, it's no good.
If the fuel is so old the car won't run, you'll smell the old turpentine stink coming out of the tail pipe.
There's a specific test to check for water in fuel that involves taking a small sample in a clear container, adding alcohol and noting the difference in volume while its floating on the gas, then stirring it up and letting it settle again. If there's water, the alcohol will absorb it, and will show itself as being a higher volume than first noted.
Or it's something like that, I'd have to look it up again...
Your sample should be clear. I've seen many colors ranging from red, orange, yellow and blue. Brown is junk, cloudy is junk.
Hmmmm, good question. I'll be honest, fuel quality is pretty far down on my list of suspects for most driveability concerns. Unless it's a diesel.
I've seen ethanol fuel in non-flex fuel vehicles, like Chris mentioned. Those cases always had skewed fuel trims that couldn't be explained by vacuum leaks, air measurement, or fuel delivery issues. The water test works. I've also cheated and used a fuel injection service machine to substitute a normal fuel source.
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