Low Compression Must Be Monitored
Question: I am the original owner of a 1972 Chevy Nova with a 350-cubic-inch engine. The car, which has 105,000 miles on the odometer, performs well. But during a recent tuneup, a mechanic told me that two cylinders had low compression. I know very little about technical matters, so I would like to know whether low compression is a serious matter. Can it be corrected? Does this expense seem justified in a car this age?--E.S.
Answer: The measurement of engine compression is one of the most important diagnostic tests that a mechanic can perform in assessing the condition of a vehicle motor.
The power produced by an engine results from the explosive force against the pistons of a compressed mixture of air and gasoline. If the mixture is inadequately compressed, then the engine’s power output drops, the engine may begin to use excessive amounts of lubricating oil and a host of other problems will develop.
Low compression is caused by a number of mechanical problems, almost all of them serious because they cost a bundle to repair. The pistons, rings, valves, gaskets and cylinders that determine compression are the guts of the engine, and replacing them often means that the entire engine must be disassembled.
Although you have been told your Nova has low compression, you still cannot be certain you have a problem that needs to be fixed. Every engine has a normal compression, based on the design of the engine. But the engine can perform adequately for thousands of miles on less-than-perfect compression.
On high-compression engines, for example, a variation in compression of 20 pounds per square inch is often acceptable. On low-compression engines, a variation of 12 psi is sometimes acceptable. You also need to know whether the two cylinders with low compression are adjacent, because that may indicate a less severe head gasket problem.
A motorist normally does not have an engine rebuilt just because of low compression. If the readings your mechanic gave you are correct, then they may indicate only that your engine is beginning to show its age and may some day need to be overhauled or replaced. The decision to replace the engine will be based on the condition of the car and its value. Your Nova is 16 and may not be worth the expense of a new engine.
If your Nova’s engine starts quickly, delivers acceptable power, does not burn excessive amounts of oil and provides reasonable gasoline economy, then you should probably do no more than monitor the compression during regular tuneups. If you keep the engine oil clean with frequent changes, you may slow down the aging process, as well.
Q: My 1987 Ford Taurus sometimes runs poorly and lacks power. It doesn’t do it all the time. A friend said Ford has issued a number of recalls, and maybe one of them is related to my problem. Do you have any information on this problem?--P.M.
A: Earlier this year, Ford said it would recall more than half a million Taurus and Mercury Sable cars equipped with 3.0-liter engines, to replace the fuel tanks.
Engine hesitation and lack of power occurs when the gasoline tank is less than one-quarter full. The shape of the metal tank can become deformed, resulting in fuel that is below the pickup tube. When that occurs, the engine does not receive adequate gas.
Ford has said it will replace the fuel tanks at no cost to owners under a warranty-extension program until Oct. 31, 1989. Owners who have already paid for the repairs will be reimbursed.
