Question: I own a 1979 Oldsmobile Cutlass Supreme with 53,000 miles on it. I have a problem with my automatic cruise control. It does not seem able to hold a stable speed, especially when going up or down a hill. I have checked all the hoses and they seem to be OK. The electrical contacts are OK. How does the thing work and what's wrong?--M.B.
Answer: The cruise-control system is both electronic and mechanical. It reads the car's speed off the speedometer cable. The speed is fed into an electronic system that stores in electronic memory the speed that you have set with your dashboard controls. The car's speed is regulated by a vacuum-powered mechanical system that is attached to the accelerator linkage inside the hood.
You should first determine whether you have a problem. Sometimes, a cruise-control system is not able to fully control a car's speed when going up or down a hill. When going up a hill, it's possible the engine does not have enough power to maintain the car's speed. When going down a hill, a car sometimes will coast to very high speeds even though the engine is idling.
You can check this by setting the cruise control while going down a hill. As the car continues to pick up speed beyond what you have set, shut off the cruise control and see if the car continues to accelerate. If it does, the cruise control is not at fault. Today's higher gear ratios in differentials allow higher downhill coasting speeds. Similarly, if your car loses speed while going up a hill when the cruise control is set, see if the accelerator is pushed all the way to the floor. If it is, the engine just doesn't have enough power to holds its speed.
If you do have a problem with wandering speed, it could be difficult to diagnose.
Sometimes, a binding cable will cause the speed to hop around, though this is not a typical problem. Usually a binding or sloppy speedometer cable causes distortions or oscillations in speed so rapidly that the engine would have a difficult time keeping up.
Your cruise control is operated off vacuum power, which is subject to such problems as leakage and worn diaphragms. A mechanic should be able to check these systems. The newer systems are all electronic.
Q: We have a new fuel-injected Chevrolet V-6 Beretta. The Chevy salesman said it was a good idea to treat the gasoline every 2,000 miles with an additive to keep the fuel jets clear. Is this good advice?--W.A.H.
A: You received some questionable advice that runs contrary to the recommendations provided by General Motors to new-car buyers and provided by General Motors to the technical community.
It is true that the owners of fuel-injected cars have experienced widespread problems with clogged fuel injectors on all types of cars. Fuel-injection systems work by directly squirting small amounts of gasoline directly into the air that goes into the engine. In a carburetor, by contrast, fuel is sucked in by rushing air.
Auto manufacturers have complained that the problem is caused by poor fuel without adequate detergents to keep the injectors clean. The petroleum industry lays the blame on the auto industry for producing equipment that is too sensitive to fuel quality.
Since the problem surfaced a little more than a year ago, the petroleum industry has responded by adding detergents to their gasoline that will keep the injectors clean. If you are using a fuel with detergents, and most of the major brands now have such detergents, you should not have a problem. Moreover, putting an additive in every 2,000 miles would be an expensive and time-consuming hassle.