A Solid Case for Valve Adjustments
Question: I have a 1987 Chevrolet Nova with a 1.6-liter engine. Does this car need valve adjustments at regular intervals? I have received conflicting advice from mechanics.--C.E.M.
Answer: One of your mechanics is obviously making up answers. Your Nova has what are called solid valve lifters, which do need periodic adjustment. The precise service interval should be listed in your owner’s manual, but a typical maintenance interval on valve adjustments is 15,000 miles.
There are two types of lifters in cars--solid and hydraulic. Until recent years, most cars had hydraulic lifters. These are small cylinders filled with fluid that form the link between the camshaft and push rod or valve stem.
Hydraulic lifters automatically adjust so that a small or zero tolerance is maintained. The solid lifter has an adjustment screw, used to maintain a few thousandths of an inch of clearance.
The reason for the switch away from hydraulics, which offer the advantage of being maintenance-free, is that newer engines operate at a faster pace in terms of revolutions per minute. At high speeds, the hydraulic lifters can become pumped up and prevent the valve from fully closing. A solid lifter typical on small engines with overhead camshafts can rev quite high without floating or preventing the valve from floating.
Q: I am the original owner of a 1974 Corvette. After 10 years, it was necessary to replace all four brake calipers. The owner’s manual suggests Dot 3 brake fluid, but I’d like to switch to the new silicon fluid. Is there any problem with compatibility between the two fluids?--S.L.S.
A: If you decide to switch brake fluids, you will be in for either big trouble or the expenditure of a lot of money. Silicon brake fluid, better known as Dot 5, is totally incompatible with Dot 3 or Dot 4, the other two types of widely used brake fluid.
The major difference between all of these fluids is their reaction to water. Dot 3 and 4 readily absorb water and as a brake system heats up under intensive use, the brake fluid can literally boil, creating steam pockets that then cause spongy brake action.
Dot 3 has a 350-degree dry boiling point, meaning if it contains no water it will not boil until 350 degrees is reached. Dot 4 has a dry boiling point of 475 degrees. But as water is absorbed into these fluids, the boiling point can drop to 275 degrees or lower.
This is not a problem with silicon fluid, because it does not absorb water. Another major advantage is that the lack of water should help prevent corrosion in brake parts, such as master cylinders, calipers and wheel cylinders.
The problem for you is that switching fluids would require you to change every rubber seal, clean out the system with solvent and then blow the system out with air. So, you virtually need to overhaul the entire hydraulic system, a major expense that is not warranted by the advantage you gain from silicon fluid.
Q: My car overheated and the problem was found to be a faulty thermostat. Would you kindly set me straight as to the main purpose of a thermostat?--C.B.
A: A thermostat regulates the flow of coolant from the engine to the radiator, where the coolant dissipates heat. A thermostat opens when the engine coolant reaches anywhere from 160 degrees to 195 degrees. If it becomes stuck closed, the engine will overheat. If stuck open, the engine will not reach a proper operating temperature.
Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Do not telephone. Write to Your Wheels, Times Mirror Square, Los Angeles, CA. 90053.
