Chances are, gas is not harming engine

Dear Mike: 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?

— J.B., Suffolk, N.Y.

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 that you can buy for a street price of less than $75. For a couple hundred bucks, you can get a scanner and actually see engine parameters changing in real time.

Chances are that this gasoline isn’t damaging your engine. If the octane rating is too low, the engine computer will roll back the timing to prevent engine knock and, possibly, melted pistons.

If the fuel has been adulterated with something else, however, 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 of ethanol and concentrations of methanol as high as 0.3 percent are acceptable, and in fact are specifically permitted by most 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, the insides of the lines and the pintle valve in the injectors. High concentrations of alcohol also can make plastic parts that aren’t alcohol-rated turn to Jell-O in a few months.

I’m aware of at least two well-publicized prosecutions of individuals who were caught selling tank cars full of methyl alcohol under the table to unscrupulous gas-station owners and fuel distribution brokers.

How can you tell if you’re dealing with 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 simply a grease-pencil mark. The vessel should be about one-third full, sighting to the bottom of the meniscus — the concave top of the liquid — not the high point where the liquid touches the wall of the vessel.

Add a similar amount of your suspect gasoline. Cover the top of the vessel and invert it carefully about a dozen times, but do not shake. Allow the mixture to stand for an hour or so to let the bubbles coalesce. Any alcohol — methyl or ethyl alike — will migrate to the glycol layer on the bottom.

If the meniscus stays at the same level, therefore, you have pure gasoline. If the meniscus goes up by about 10 percent, you have legal oxygenated fuel. If it goes up by more than 10 percent, you have adulterated fuel.

Be sure to dispose of your gasoline-alcohol-antifreeze highball in some environmentally responsible manner, OK? Don’t simply chuck it down the drain, because it will end up polluting the watershed somewhere, probably not far from where you live.

Dear Mike: I have a 2004 GMC Envoy with the standard FM radio and a single-disc CD player. When I went to upgrade the player with a non-GMC product — I wanted to do this because there are no GMC satellite/CD players with MP3 capability — I was informed that the radio could not be removed or replaced because it controls the air bags.

Is this true? Have you guys ever heard of anything so stupid? If so, is there a way around it?

— R.B., via e-mail

A: The air-bag system has its own dedicated wiring harness that has nothing to do with the rest of the truck’s wiring or the radio.

Maybe somebody is confused because the original radio has volume and station-changing controls built into the steering wheel, right where the pyrotechnics for the air bag live. The new radio may not be compatible with the air bag’s components, but that won’t affect the air-bag operation.