Question: I own a 1986 Chrysler with a turbo engine and a 1982 Mazda RX-7. The Mazda owners' manual calls for an oil rated as SD or SE. The Chrylser manual calls for an SF. Life used to be so simple. What is the significance of these ratings?--O.N.S.
Answer: Oil is getting more complicated these days, matching the increasing complexity of automobile engines. The American Petroleum Institute--the API stamped on most cans of oil--classifies all motor oil according to its intended use.
As engines have been designed for higher performance and greater fuel economy, they have required improved engine lubricants. In some cases, the oils themselves have contributed to fuel-economy improvements by reducing engine friction.
In any oil designation, an "S" indicates that it is for a gasoline engine. A "C" designates a diesel-engine lubricant.
The second letter indicates its overall lubricating capabilities, based on the additive package and the refining method. The latest oils are graded SF and would be considered an improvement over SD or SE.
You can safely trade up to an SF for your Mazda, but you shouldn't trade down to an SE for your Chrysler. Both oils have a big package of additives, such as detergent to keep your engine clean, a viscosity agent to maintain the thickness of the oil and anti-oxidants to preserve the life of the oil.
But the SF has a more diverse additive package and includes a so-called friction modifier that is the latest in lubrication. It also helps auto makers achieve the mileage rating certified by the government.
Q: I have a 1978 Mercury Zephyr with a six-cylinder engine. My problem is that at times the engine sounds as if all of its connecting rods are loose. They rattle when I turn on the air conditioner. I have tried all brands of gasoline, even premium. A Ford mechanic told me to get on the freeway and drive hard for several miles, but that didn't work. What do you suggest?--E.F.
A: The engine noise you are hearing is unlikely to be your connecting rods. It's most likely engine ping, which is caused by the uncontrolled detonation of gasoline in the engine's combustion chamber.
The gasoline explodes, rather than burning evenly. Ping usually occurs when the engine is under a heavy load, such as when accelerating or going up a hill.
Under heavy loads, the engine advances the timing of the ignition, which tends to induce uncontrolled burning of the gasoline. The reason you hear it when you turn on the air conditioner is that the compressor also creates a load on the engine.
You should have the engine's cooling system, carburetor and ignition timing checked. If those are all set according to factory specifications, you possibly have a carbon buildup in your cylinder.
You might also try Chevron Techtron additive, which helps clear out carbon.
Q: I have a 1984 Plymouth Voyager. On a cold start, a huge cloud of white smoke comes out of the exhaust. The dealer can't find the problem. The car runs fine otherwise. --C.A.K.
A: White clouds from the exhaust are normally associated with water or condensation inside the engine or exhaust system. A certain amount of moisture and vapor is normal, but a "huge white cloud" indicates a possible problem.
You might check the coolant level in the radiator. If it is dropping, you may be leaking coolant into the combustion chamber through a bad head gasket. Your mechanic can pressure test your cooling system.