Question: My car ordinarily burns regular unleaded gasoline. Will an occasional tank of premium help the motor in any way?--D.M.
Answer: Your question addresses a common and long-standing practice among many motorists who consider an occasional tank of premium a special treatment that will help their engines last longer.
The basis for this assumption is that a tank of higher octane gasoline, maybe because it costs more, will somehow reduce engine wear. The assumption is fundamentally incorrect, though an occasional tank of premium may help an engine in other ways.
An engine's design, which includes such factors as its combustion ratio and its operating temperature, is related to something called an octane rating. Usually, we think of an octane rating as something that applies only to gasoline, but the engine itself has such a rating that represents the minimum octane level that the engine requires for proper operation.
An engine needs a minimum level of octane to avoid ping, which is caused by uncontrolled combustion of fuel inside the cylinders. Excessive ping can severely damage the engine. The octane rating on the gasoline is a measure of its anti-knock or anti-ping properties.
If your car does not ping on regular, then there is no reason to seek a higher-octane gasoline. The anti-knock level of the regular in this case is adequate for the engine.
But as a car gets older, depending on how the car has been driven and cared for, it may need a higher-octane gasoline anytime between four and six years. That's because carbon deposits inside the cylinders raise the combustion ratio, which in turn raises the engine's octane rating. You may notice that your car operated fine on regular fuel when it was new, but pings on regular as it gets older. So, the higher-octane fuel is not something to pamper a new car with but rather help keep an older car running properly.
In addition, premium gasoline has some other selling points. Most premium gasolines have a higher-quality additive package put in at the refinery. The actual additives in a particular brand of gasoline are generally not disclosed by refiners. But usually they include detergents and other solvents that keep the carburetor and rest of the fuel system clean.
Q: I own a 1980 Mazda 626. I recently had both the timing chain and a new carburetor put in. From the time I drove my car out of the repair shop, I have had a problem with it dying off. The idle is erratic and sometimes the idle is way too fast. Other times I have to keep pumping the accelerator pedal to keep the engine from dying. I had the gas tank, lines and filters replaced. Did I get a defective carburetor?--K.K.P.
A: The carburetor on your Mazda has a reputation for being balky, but your problem goes beyond what is considered normal. A microprocessor controls the carburetor's idle speed, based on sensor readings it obtains from the engine. Your mechanic should check the sensors because they could be feeding false readings to the microprocessor.
You should also conduct a thorough examination of all the vacuum lines to ensure there is not a vacuum leak, which can cause the sort of symptoms you describe. You may also want to install new spark plugs to ensure that the engine is properly tuned, which may help in the diagnosis.
Finally, the carburetor may be defective. A blocked passage can cause the type of problems you mention. But a competent mechanic should be able to solve the mystery.