ENGINE PING AND GASOLINE (from Chapter 7: Caring for Your Used Car)
If you notice your engine has started knocking it could be a couple of things. Your engine may be a few degrees out of time, indicating the need for a tune-up. Or, if it is a manual transmission, you may be driving in the wrong gear and lugging the engine, which is a no-no (manual transmission people, make sure you are in the right gear). However, if you're not due for a tune-up, the most likely reason for engine knock is poor- quality gas from your last fill-up. Engine run-on, where your car wants to keep running after it is turned off, is also a symptom.
Not all gasoline are created equal. An 87 octane rating at one station may not be the same as the 87 rating at another, and the same goes for the higher octane ratings as well. It gets involved, but the octane rating posted at the pumps is reached by adding together the ratio of the Research Octane Number and the Motor Octane Number and then dividing it in two. Because these ratios can vary without affecting the posted octane rating, the gas at one station may be much better quality than the gas at another.
Good advice here would be to avoid the el cheapo gas stations and stick with the nationally known chains. You will pay a few cents more at the pump, but you get more than that back through better mileage and an engine that will run smoother and last longer.
So before you pay to have your car tuned up because it's been pinging, suspect poor quality gas and do some experimenting. Start by trying the 87 octane rating at several reputable stations. If this doesn't clear up the knocking, bump up to the 89 octane rating and start over. Sooner or later you will fill up with a tank of gas that has cleared up your engine ping and Eureka! You have found your gas station. Stick with them. And it would be safe to assume that if the 89 octane rated gas at station X down the street that cleared up the knocking, then the 89 octane gas at station X across town or across the country, for that matter, will have the same effect.
Just as gasoline is different, so are cars. Some cars are more sensitive to the quality of gas, while others run fine and dandy on the cheapest gas in town. Usually this becomes evident when you have made a change in vehicle ownership. As described above, do some experimenting.
For fuel-injected cars, a quality grade of gas is imperative. Engines with fuel injectors usually require a higher rated octane than carbureted ones. Check your owner's manual for recommendations or call your car's respective dealer and ask the service department what they suggest.