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The Gentleman's Express: Tech-Tips from the JIOC

Table of Contents

Thermostats and Gaskets

Author: Lucas Service

The following are a list of the most-often asked questions, with answers, about thermostats and gaskets in the coolant systems of our automobiles:

Q. What is a thermostat?

A. It is a valve that controls the amount of coolant flowing through the radiator.

Q. How does the thermostat work?

A. The heart of any thermostat is its ‘heat motor’. The heat motor, sometimes called the pellet, moves the thermostat valve in response to changes in coolant temperature. It is a small pressure vessel filled with a specially prepared expansive wax. When engine temperature reaches a predetermined point, the wax begins to melt and, asit does, it expands. This in turn forces a piston to move outward forcing open the thermostat valve. When the engine cools, the return spring, found on all thermostats, forces the piston back into the heat motor, which closes the thermostat valve.

Q. How long does a thermostat last?

A. There is no way of predicting the life span of a thermostat. Peculiarities of an engine, the general condition of the cooling system, use of cooling system additives, method and extent of vehicle operation will all affect the life of the thermostat

Q. How can a thermostat fail?

A. Several things may cause failure. First, they can like everything else, wear out. Premature failure may occur through excess pressure within the heat motor from abnormally high coolant temperatures, caused by such things as belt slippage, a defective cap or any one of a number of other things. Excessive pressure can cause wax leaks and mechanical failures such as cracking of a part. Wax leaks can also be caused by a perforation in the rubber boot, or improper crimping of the heat motor cover and cup. High internal pressures that build up during its operation could gradually force wax out of the heat motor. These failures should be detected by the over-temperature warning light before engine damage occurs. Though thermostats are often blamed for cooling system failures, it is rare indeed when they are actually the base cause. Each Lucas thermostat undergoes a 100 percent final check in addition to numerous ‘in process’ quality inspections.

Q. What happens if a thermostat fails?

A. Failure can occur in two ways. It can fail with the valve closed, resulting in engine overheating, or it can fail with the valve open resulting in engine over-cooling. One caution should be observed when the red indicator light comes on, signalling an over-heating problem. The vehicle should be immediately stopped to determine the cause.

Q. Is a thermostat the only cause for overheating?

A. No. There are at least 15 different causes for engine overheating that do not involve the thermostat, such as:

  1. Leaking seals and fittings.
  2. Leaking radiator, oil coolers and headers.
  3. Rust deposits.
  4. Clogged air passages in the radiator.
  5. Worn or loose fan and/or waterpump drive belts.
  6. Defective or wrong pressure cap.
  7. Air in the system.
  8. Cracked and leaking hoses.
  9. Improper coolant fill.
  10. Gummed-up engine passages.
  11. Compression leaks.
  12. Eroded waterpump impeller.
  13. Loose hose clamps.
  14. Obstructions to flow of cooling air.
  15. Improper ratio of ethylene glycol and water.

Q. Is it a good policy to replace a thermostat whether or not it needs replacing?

A. Although it is designed to give long life, when there is reason to open up a cooling system and the thermostat has been in operation for over a year, it is good insurance to go ahead and replace it at that time. When in doubt - replace it.

Q. When replacing the thermostat is it necessary to replace the gasket?

A. Yes. And under no circumstances should a gasket be reused as it will offer a potential leakage path for coolant and cooling system pressure which can cause boiling at lower temperatures.

Q. Why not have adhesive on both sides of adhesive backed thermostat gaskets?

A. Adhesive is put on one side only for the purpose of holding the thermostat centered in place in the counterbore of the outlet housing as a subassembly. The adhesive itself does not do the sealing. This is done by squeezing the gasket between the block and the housing.

Q. Can we reuse a thermostat taken out for a summer/winter changeover?

A. It is not a good idea to reuse them. Thermostats left out of the engine for any period of time after having been in operation should not be reused. The coolant tends to change the original rubber lubrication characteristics of the thermostat, such that if allowed to completely dry for an extended period of time, there would be the possibility of piston seizure.

Q. When selecting a new thermostat, how is the correct temperature determined?

A. The original equipment manufacturer’s recommendations should always be used. For correct selection, refer to the manufacturer’s Shop Service Manuals. Since 1968, all passenger cars made in the U S have been designed to use thermostats in the 190 - 195 degrees Fahrenheit range. (Use 160 degree units on our Jensens:....TECH).

Q. When replacing a thermostat from an engine with two or more thermostats, should all be replaced?

A. Yes. This is the only way to insure that all the thermostats in the system are fully operable.

Q. When replacing a Heavy-Duty thermostat of the top bypass valve design, should the seal also be replaced?

A. Yes. because it is virtually impossible to remove the old thermostat and install a new one without damaging the seal to some degree. This, coupled with seal wear which takes place during operation, can cause an unsatisfactory installation. In fact, the seal is often the cause of the problem when overcooling is experienced.

Q. What is the most common cooling problem?

A.Boiling. Regardless of cause, the most common problem is internal boiling to some degree which is recognized as an engine overheating problem. Any one of the previously mentioned causes of overheating might be the actual cause of the problem.

Q. Should an engine ever operate without a thermostat?

A. No. The thermostat serves the very important function of getting the engine operating temperature up to design specification quickly. With no thermostat, the engine will usually be over-cooled, resulting in the formation of acids and sludge.

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