Your Wheels

Not Really Looking for End of the Road

February 28, 1985|PATRICK BOYLE | Times Staff Writer

Question: I have a 1976 Chevrolet that has 131,000 miles on it. It looks good, sounds and runs well and does not use any oil. How many more miles can I expect to get out of this engine?--H.H.W.

Answer: There's really no way of knowing how much longer the engine will last. You apparently know how to take good care of the car. If you continue your regular maintenance, the engine could easily go 150,000 miles and possibly even 200,000 before it needs major work.

As you have noted, oil consumption is an early warning sign of wear. This happens because the piston rings and valve guides begin to wear, allowing oil to get into the combustion chamber and burn with the gasoline.

A way to monitor the internal engine wear is with a compression check. The compression is a measurement of the pressure created in each cylinder when the piston is near its topmost position and the valves are all closed. This is the point in the cycle when the spark plug fires.

Depending on the make of car, a new engine will have compression of from 120 to 150 pounds per square inch, and it will be nearly the same in all cylinders. But if one cylinder has a broken piston ring or a burned valve, air will leak out as the piston rises, and the compression reading will drop.

The compression is checked by removing the spark plugs and inserting the end of a gauge into each spark-plug hole. Then when the engine is turned over, the gauge indicates the compression in the cylinders.

It's normal for the compression to drop off as an engine gets older because the piston rings and valves don't seal as tightly as they once did. But the compression in all cylinders should always be about the same. The lowest reading from any one cylinder should be at least 80% of the highest reading in any other cylinder.

For example, if the best reading is 140 pounds, the worst should be no less than 112 pounds.

If a mechanic finds a low reading in two adjacent cylinders, the problem could be a broken head gasket rather than worn engine parts.

Q: I have just bought a Volkswagen Rabbit convertible with the larger 1.8-liter engine and a close-ratio, five-speed transmission. The performance is splendid, but the gas mileage is quite poor. I'd much prefer the traditional VW gas mileage. Are there any changes that can be made in the engine or transmission to improve the fuel economy?--J.S.W.

A: As you have found, your car was designed for speed and handling, not for economy. The larger engine contributes to the lower gas mileage, but the close-ratio transmission is probably a more significant factor. In any given gear, the engine has to run faster with the close-ratio transmission than it would with a conventional transmission to maintain a certain speed.

For example, at 55 m.p.h. in fifth gear, your engine is running at about 2,800 revolutions per minute. In a conventional Rabbit at the same speed, the engine would be turning about 2,400 rpm. The faster an engine goes, the more gasoline it burns. The only way to significantly improve the mileage would be to install a different transmission, and that would probably only improve the fuel economy about five miles per gallon.

Patrick Boyle cannot answer mail personally but will respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, You section, The Times, Times Mirror Square, Los Angeles 90053.

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