Technical Comments on Cooling

With summer once-again near, many people will experience cooling system problems and overheating to various degrees. It seems that very little helpful information is available on cooling systems and frequently most mechanics are baffled as well as the frustrated car owners. I will try to briefly cover most of the important considerations governing automotive cooling, which hopefully will clarify everyone's understanding of this important subject.

Dallas summer's high temperatures make it necessary to have our car's cooling system operating near 100% efficiency since many cars were designed for maximum temperatures less than we experience. Operating under stabilized conditions with a fully open thermostat (i.e. 60 MPH on the highway) it can be shown that the engine temperature will rise very nearly degree for degree with the ambient air temperature. Thus a car running 190' at 80' will increase to 220'F at 110', air temperature. This may be surprising to many of you, but is a normal characteristic. A fully opened thermostat occurs when the ambient temperature is no longer low enough to allow the thermostat to maintain the temperature at it's rating (usually 160 or ~ 180'F) and the radiator's heat transferability controls temperature.

The thermostat's purpose is mainly to maintain an operating temperature at the 160' or 180' rating when ambient temperatures are low, so the engine will operate more efficiently and the heater can produce hot air. A common misconception is that removing the thermostat or using a lower rated thermostat will allow a car which runs too hot to run cooler. This is not true and it is unwise to run without a thermostat as many engines are designed to have an internal passage blocked by an open thermostats is sticking partly open causing engine (and heater) temperatures to be too low. Less often a thermostat will stick closed causing very rapid boilover. Since the thermostat is very cheap, if there is any doubt as to it's condition, it is recommended it be replaced and the results observed.

The first step in optimizing cooling system performance is to be certain the engine performs at it's best. A thorough tune up should pay particular attention to being sure that ignition timing is correct not only initially, but at all speed and load conditions. Many older British cars will have the mechanical advance stuck and inoperative and/or the vacuum advance diaphragm ruptured. Retarded ignition timing will cause sluggish performance with overheating even if the cooling system is in top condition. The vacuum advance is mainly intended to increase part throttle fuel economy, but if inoperative will cause higher operating temperatures; make sure it works properly. An inoperative mechanical advance will cause very noticeable performances loss.

Many cars will still have cooling problems even when the thermostat and tuning are in top condition. A physical characteristic of the automobile engine is that it produces heat, increasing in magnitude with increasing speed and load, which must be removed by the cooling system. The heat produced by the engine is transferred to the coolant which is circulated to the radiator by the water pump. This heat is then transferred by the radiator to the surrounding air, the coolant recirculated and the process continued. Since the coolant is continuously recirculated it is important that none is lost through leakage. Coolant leakage is the most common cause of overheating and often is simple to correct. Replace any questionable hoses and make sure all clamps are tight. A noticeable water leak that is hard to find usually turns out to be a water pump seal. Although water pump seal and bearing failures are common, the water pump rarely is the cause of overheating and replacement is unwise if the bearing and seal are OK. An exception to this is the occasional disintegration of plastic impellers. When no water leakage can be found and water loss is still a problem the water is likely being lost through the overflow (or out the tailpipe, blown head gasket or worse), Since most modern cars are dependent on a high pressure system, often a bad radiator cap will cause water loss. If any doubt, replace the cap. Head gasket leakage usually worsens rapidly soon becoming intolerable. If suspected, combustion leakage can be detected by filling the radiator or tank to the very top and starting the engine while observing the coolant. If water gushes out as soon as the engine is started or if steady gas bubbles are evident, major engine work is indicated. Combustion gas detectors are available, but usually only verify the obvious.

At his point, if everything covered was put in order, some cars will still display an overheating problem. Before going further with any solutions it is important that "overheating" be defined more clearly. Some may disagree, but my definition of "overheating" is when temperature rises to a point when boiling occurs, coolant is lost from the overflow, and a stabilized condition cannot be maintained. Thus the "overheating" or boiling temperature varies significantly with combined effects of altitude, coolant solution and system pressure. With a 14# pressure system, 50-50 coolant water mix, and at sea level, the boiling point will increase from 212'F to 265'F. Most modern cars can operate safely at considerably higher temperatures than older cars which have non pressure or low pressure cooling systems. Addition of a 50-50 coolant water mix with no pressure will raise the boiling point to 226'F, thus giving a slight margin over 50% coolant for summertime as the heat transferability is seriously reduced causing higher metal temperatures and accelerated engine wear.

Finally we will discuss the major variables in the cooling system; the radiator and fan combination. The radiator is selected by the manufacturer to provide adequate cooling under most normally encountered situations, yet is not overdesigned so as to cost more than necessary. Usually the standard radiator is inadequate in extreme climates or under severs operating conditions and most manufacturers offer an increased or heavy duty cooling package. The fan's purpose is to provide adequate air flow through the radiator when stopped or travelling at low speed and has no significant effect at speeds over approx. 30 MPH. A significant point to remember is that a larger fan, electric fan, or fan shroud will not improve an inadequate radiator's performance at road speeds.

Radiator problems are best detected by driving the car at steady highway speeds and observing the stabilized operating temperature. If the operating temperature is too high and increases with increases in speed and load, the radiator is not adequately removing heat. If a radiator problem is indicated the only solution is to remove it and send it to a reputable radiator shop for cleaning. Fast flush and chemicals to clean a radiator in the car are at best only partially effective and usually a waste of time. The radiator shop actually unsolders the top tank of the radiator and physically cleans each tube so the radiator will operate near 100% efficiency when cleaned properly. The improvement in cooling after properly cleaning a radiator is often dramatic. Radiator cleaning should always be done when an engine is overhauled. An inadequate fan is easily detectable when a car runs excessively hot in slow traffic or at idle but cools to a normal operating temperature soon after reaching steady road speed. Often modern cars are equipped with a fan clutch intended to reduce power drain from the fan at road speed. A defective fan clutch often is the cause of overheating in slow traffic. Missing or broken fan shrouds often cause some air to circulate without passing through the radiator. Electric fans have become increasingly popular recently and work very well. Often the addition of an auxiliary electric fan is the best way to improve low speed cooling as well as improve air conditioner performance if so equipped.

I have tried to describe as many important facets of cooling system design and performance as possible in a brief summary. One of the most important things to consider is to define what is normal for the particular automobile you are dealing with, and if normal performance will be adequate for the use the automobile will be subjected to. If improved cooling performance is desired, non standard components must be selected carefully to give the desired results. Most cooling problems are very straightforward, but occasionally very weird or unusual situations are encountered. A thorough understanding of the basic operating principles is necessary to resolve cooling problems as well as any mechanical problem.

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