Short Trips Mean More Oil Changes
Question: I own a 1991 Honda Accord. It’s not unusual for me to drive fewer than 2,000 miles a year, because I use the car only to shop for groceries and take infrequent long trips to visit friends. The dealer wants me to follow a time-elapsed maintenance schedule, in which I change the oil every three months, even though my owner’s manual says the car can go 7,500 miles between oil changes. I was thinking I could get away with changing it every year. Who is right?--B.W.
Answer: There is no reason for the dealer or any mechanic to insist that oil be changed at specific time intervals without considering how the car is driven.
But you probably should have the oil changed every three months if you use the car only for short trips on city streets, particularly in cold weather.
Such driving, in which the engine never reaches its proper operating temperature, can cause condensation of water inside the crankcase and dilution of the oil by gasoline.
Water contamination of the oil occurs when moist air is drawn into the crankcase and condenses after the engine is turned off. Frequent short trips increase the amount of condensation.
The water in the crankcase is not harmful, but it can combine with sulfur, a byproduct of combustion, to form sulfurous acid, a weak acid that breaks down the lubricating qualities of the oil.
The fuel contamination is a separate problem. In the carburetors of older engines and even some fuel-injected engines, a richer mix of fuel is sent to the engine on cold starts. Also on cold starts, some gasoline seeps down the cylinder walls into the crankcase. A small amount of gasoline contamination is not harmful, but larger amounts dilute the oil and lower the viscosity--again compromising the oil’s lubricating qualities.
Another problem associated with driving a car only on short trips is that engine deposits tend to increase because the combustion chamber never heats up enough to burn off hard carbon that forms on the piston head and valves. Eventually, this can cause engine ping, because the deposits create localized hot spots inside the combustion chamber that cause the fuel to burn unevenly.
When you drive a car a long distance, the engine, coolant and oil all get quite hot and these contaminants are boiled out of the oil and soot does not form as quickly. If the weather is cold, you need to drive a lot farther, certainly more than 10 miles at freeway speeds, to reach operating temperature. In warmer weather typical to Southern California, even a five-mile drive on the freeway can get the temperature to a normal range.
If you drive mostly on long but infrequent trips, there is nothing wrong with changing your oil every six months or even every year, according to Texaco oil experts.
The final consideration is your car warranty. If the manufacturer specifically requires you to change the oil based on elapsed time, it would probably be worth doing so until the warranty expires. If the engine needed a repair covered under the warranty, a dealer might balk at honoring a claim if the oil changes did not follow the manufacturer’s recommendations.
