auto maintenance February 08, 2018

What’ll It Be? Buying gas isn’t a big mystery.

By Douglas Flint

About once every two weeks, my wife informs me that “the car is broken,” meaning that the Low Fuel light is on. I must then perform the manly task of filling the gas tank (after all, she does all the cooking). I usually use the discount station because it’s very convenient. I do not concern myself with brand names and I use regular unleaded in our cars.

So what do you need to know about gasoline?

Most stations sell three grades or “octanes” of gasoline. “Regular,” typically 87 octane, is the cheapest if you can call it that at current prices. “Plus” is the middle grade, 89-octane gas, and “Premium,” 93 octane at the station I use, but maybe 94 octane at the brand-name stations, for a princely sum.

What’s the difference? Why is there a difference? And which should you use?

First, an oversimplified explanation of engine operation. Gasoline and air are drawn into the cylinders and compressed (squeezed) as the piston pushes up. At the proper time, the spark plug ignites the compressed fuel-and-air mixture, which burns (not explodes). The expanding gas drives the pistons down, producing power. As the fuel-air mixture is being compressed, it may get hot enough to ignite on its own, resulting in pre-ignition, which can cause a rattling or pinging noise from the engine, a reduction in power, and in extreme conditions, engine damage. The octane rating of a gasoline is its resistance to pre-ignition.

Most cars and trucks sold in the U.S. are designed for and run well on regular gas. If your car or truck runs fine on regular, increasing the octane will not improve your car’s performance, and may in fact cause problems with cold starting and performance, because of another often-ignored factor called volatility. Volatility is the ability of a liquid to mix or atomize into the air. Gasoline will not burn until it has atomized into the air, and atomizing into cool and cold air is much more difficult than warm.

There is an inverse relationship between octane and volatility. The higher the octane, the lower the volatility. Many mysterious hard-starting and cold-drivability problems are attributed by car manufacturers to volatility. Refiners adjust the volatility of fuel from summer to winter, but they cannot control all factors, such as temperature and humidity. Also, regional emissions requirements may affect fuel volatility. Often an early cold snap can cause a wave of cold-engine problems, until the fuel blend catches up with the weather.

Never pay a penny more than you have to for gasoline. If your car runs well on regular, or pings just a little, stay with regular. If your car begins to ping continuously, have it checked by a mechanic who specializes in engine performance, such as the ones at Wiygul Automotive Clinic. If everything checks out, go up one grade on octane. Unless you are driving a car with a high-performance engine, or a truck pulling a very heavy load, you shouldn’t need premium, and if you do, you probably have an undiagnosed problem. Also try periodically switching back to regular in case your ping was a temporary condition.

But my friend’s car manual says that his car runs better on premium!

His car is probably a high performance car, and by utilizing a “knock sensor,” a sensor that hears pre-ignition, the engine control computer can change the car’s timing and fuel curve to benefit from premium gasoline. Your 2004 Caravan can’t benefit and won’t, so save your money.

But what about purity and quality?

I watch many different-brand gasoline trucks pick up their gasoline from the same tank farm. Often a one-quart additive bottle is all that differentiates the brands of gasoline and gives them their individual flavors. Environmental laws assure that in-ground tanks at stations are well maintained and leak-free.

The old demon alcohol.

Without getting too deep into the political weeds here, federal policies have forced gasoline refiners to produce a fuel blend that contains 10% ethanol (ethyl alcohol).  Alcohol attracts moisture, is highly corrosive, and is generally not nice to rubber and plastic components. After the mandate went into effect a decade ago, mechanics started spending lots of time replacing fuel pumps.

Car manufacturers have done their best to shore up the fuel system components against alcohol, but unfortunately, most of the newer vehicles no longer have a replaceable fuel filter.

That’s why you should have a fuel system service performed every two years. This service will keep the fuel and intake system clean and free of harmful deposits, and will keep your fuel economy and performance at their best.

Running on empty.

One thing you can do, besides having your fuel system serviced regularly, is keep your gas tank at half full or above. The moisture that ethanol pulls into the fuel comes from the air that is in your tank. More gas equals less air and less moisture. In the winter in the Washington metro area you always have the possibility of the 8-hour nightmare ride home if it snows, and no one wants to be fretting over their fuel supply while stuck on the Beltway!

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