The following is an article by Art Ziesk which ran in the Texas Chapter News,
the newsletter of the Texas MG Register.
Mark Dement
'35 PA
'59 MGA
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 cars 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 190o at 80o will
increase to 220oF at 110o 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 180oF) and the radiator's heat transfer ability controls
temperature.
The thermostat's purpose is mainly to maintain an operating temperature at
the 160o or 180o 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 thermostat which
significantly increases cooling flow to the radiator. The most common failure
with 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
performance 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 re-circulated and the process continued.
Since the coolant is continuously re-circulated 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 disintigration 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 this 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" as 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 212o F to 265o 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 226oF,
thus giving a slight margin over the traditional 212oF danger point. Never
use over 50% coolant for summertime as the heat transfer ability 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 over designed so as to cost more than necessary. Usually the standard
radiator is inadequate in extreme climates or under severe operating
conditions and most manufacturers offer an increased, or heavy duty cooling
package. The fans purpose is to provide adequate air flow through the
radiator when stopped or traveling 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 beat
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.
Art Ziesk
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