Can gasoline damage an engine?
Q. I'm suspicious that the local cut-rate gas station's premium gas isn't what it's represented to be. My BMW seems to run fine on it, but the "check engine" light comes on right after I fill up there. And if I fill up with premium anywhere else, the light goes out. My wife's minivan seems to run fine on their regular grade. I hate to pay close to 40 cents extra for premium at the branded station across the street.
Is this damaging my engine, and how can I tell if there's something wrong with the gasoline?
A. Any mechanic with a scan tool can tell you why the "check engine" light is on. There are even simple consumer-grade code checkers you can buy for a street price under $75. For a couple of hundred, you can get a scanner and actually see engine parameters changing in real time.
Is this gasoline damaging your engine? Probably not, because if the octane rating is too low, the engine computer will roll back the timing to prevent engine knock and, possibly, melted pistons. However, if the fuel has been adulterated with something else, there may be damage to the fuel system.
The common adulterant is alcohol. Alcohol will raise the octane rating of gasoline, but at the expense of fuel economy. I've seen both ethanol and methanol used to dilute gasoline. Ethanol used to be employed in concentrations as high as 10 percent to formulate oxygenated fuel. At one time such fuel was mandated by the EPA in some areas during months when there were high carbon monoxide levels in the atmosphere. This small amount and concentrations of methanol as high as 0.3 percent are acceptable, and specifically permitted by most of the car manufacturers.
Higher concentrations in vehicles not designed for alcohol can cause corrosion of metal parts wetted by fuel -- things like the inside of the tank and the lines, and the pintle valve in the injectors. High concentrations of alcohol also can make nonalcohol-rated plastic parts turn to Jell-O in a few months. I'm aware of at least two prosecutions of individuals who were caught selling tanker cars full of methyl alcohol under the table to unscrupulous gas station owners and fuel distribution brokers.
How to tell if you're trying to burn alcohol-bearing gasoline? Here is some simple kitchen-table chemistry. Get a tall, skinny vessel and add a measured amount of ordinary ethylene-glycol-based antifreeze to it. Fill to a level you can check easily, even if it's just a grease pencil mark. The vessel should be about one-third full. Sight to the bottom of the meniscus, not the high point where the liquid wets the wall. Add a similar amount of your suspect gasoline.
Cover the top of the vessel and invert it carefully about a dozen times. Do not shake. Allow the mixture to stand for an hour or so to let the bubbles coalesce. Any alcohol -- methyl or ethyl -- will migrate to the glycol layer on the bottom. If the meniscus stays at the same level, you have pure gasoline. If the meniscus goes up about 10 percent, you have legal oxygenated fuel. If it goes up more than 10 percent, you have adulterated fuel.
Be sure to dispose of your gasoline-alcohol-antifreeze mixture in an environmentally responsible manner, OK? Details on that are available at popularmechanics.com in the article "How To Dispose Of Hazardous Waste."
Q. My 2000 Oldsmobile Intrigue with a 3.5-liter engine will not start on the first crank. It will always start on the second crank and starts quicker if I hold the gas pedal to the floor. At first, it seemed to be temperature-dependent. It would normally start in cooler weather on the first crank, but after a year of intermittent operation, it now exhibits the condition almost every time on the first start. The vehicle runs well and restarts on first crank as long as the engine was running within the previous hour.
I vaguely remember that fuel will not be turned on until some timing pulse is received by the engine computer -- I think that's the problem. But I can't remember what it is that could keep that signal from being sent on the first crank. Any thoughts?
A. If it starts better with the accelerator held down to the floor, it's flooded. Full throttle during cranking puts the computer into "clear flood" mode and injects no fuel at all. The dry air being pumped through the engine lets fuel-wet plugs dry off and fire again.
My guess is a leaky injector dripping fuel into the manifold after the engine cools off. The cooler weather starts are clue: Cold engines require a richer mixture to fire. Sounds like the leak is getting worse. Get the injectors bench-tested and cleaned.
Mike Allen is a senior editor for Popular Mechanics magazine. Questions can be sent via e-mail to driveit@nytimes.com. 
