|
NOT to be reproduced or copied without the express permission of the Author |
First the system must be full ie. no air, only coolant (note coolant not water). More sophisticated systems use a fully de-aerating set up that continually bleeds any air trapped in the system and increases the coolant pressure in the head and block. The TR's system does not so it is important to ensure that it is correctly filled and the level maintained.
When the engine temperature is below the thermostats operating temperature the water pump causes the coolant to circulate through the engine and out to the thermostat housing. As the thermostat has not yet opened the coolant is directed down the bypass hose back to the water pump inlet. No coolant passes to the radiator.
As the coolant outlet temp increases it passes by both the temp gauge sensor and the thermostat sensor (a wax bulb). Once the coolant temp at the thermostat reaches its operation range, the wax expands and opens the thermostat against the tension of a spring. Coolant is then allowed to flow into the radiator top tank, down the various tubes into the bottom tank where it is collected then directed back to the water pump through the lower radiator hoses and pipe.
If the coolant temp continues to increase, the thermostat opens further allowing even more coolant to flow through the radiator. As the coolant outlet temp from the engine drops, the thermostat will start to close and restrict coolant flow to the radiator. In this way the thermostat attempts to control the engines coolant temperature.
All sounds easy so why all the problems. The reasons are many and varied. First lets assume that it is a standard system and look at the various components.
The radiator works as follows, the coolant enters the top tank and is distributed across the top of the core and then travels down the individual tubes to the bottom tank. Here it is collected and directed to the lower radiator outlet. As the coolant passes through the tubes, heat is transferred to them and then to the airflow. The amount of heat transferred is directly proportional to dT (where 'd'=Delta) or the difference between the hot surface and the cooler surface. So as the ambient temp rises, dT would decrease and the heat transfer would also decrease. Also if the coolant temp increased, dT would increase and heat transfer would increase.
When the TR was designed, the most common coolant in use in Australia was straight water. This allowed corrosion to occur in the system and together with dirt from various sources (dirty water, casting sand, dust from the air drawn in when the engine cools, etc) would form a slurry in the various passages in the block and head. A thin coating of rust would form on all the wet surfaces and acts as an insulator. If the engine experienced an overheat (boiled) it would agitate this slurry and circulate it through the system.
The usual end result is that it would tend to be filtered out by the radiator and either block or restrict some of the tubes. This is the reason that once an overheat is experienced in an older engine, it would continue to experience them, even if the original cause was fixed, until the radiator was cleaned.
Putting a flushing compound through an older engine can also have the same effect, even if it is correctly flushed.
The only way to effectively clean a radiator is to remove it and have the bottom tank removed and the individual tubes cleaned.
Standard cores also have a hole for the crank handle. This effectively reduces the core by up to 20% (depending on how the hole is incorporated).
As a matter of interest, the coolant temperature drop across the radiator is only about 10 ° F(7 ° C).
The original coolant was water but today we use a coolant consisting of ethylene glycol (EG), corrosion inhibitors, and water. The ratio of the ethylene glycol to water recommended for the TR is 1/3 to 2/3 water.
The need for corrosion inhibitors is self evident, ever removed your thermostat housing or what was left of it, but why the ethylene glycol. The EG acts as an anti freeze, an anti boil, and as a wetting agent. Its anti boil characteristics is of most importance in Australia with the wetting caricteristics a close second.
If we looked at the individual areas around the coolant system, especially around the cylinder head, you will find areas whose temp is well above that felt at the thermostat (and temp gauge). The temp at the thermostat is only the average of the outlet temp. Some areas will rise above the boiling point of water. This localised boiling can cause cracking or distortion of engine components
To prevent the coolant from boiling, the system is kept under pressure, which raises the boiling point to about 225 ° F(107 ° C), but in today's environment this is not really enough. The addition of 33% anti boil raises the boiling point to about 235 ° F(112 ° C).
Another phenomenon is after boil, where heat stored in the various components soaks into the coolant when the engine is shut down. You sometimes can hear this if you only use water and/or your radiator cap is defective. This is normally experienced if the engine is shut down immediately after some hard driving and as such it is always a good idea to let the engine temperatures stabilise before turning off the cooling system by shutting down the engine.
Good engine coolant can reduce the instances of this.
This is a very simple and common type of pump known as a centrifugal pump. It utilises centrifugal force to move the coolant from the centre of the pump's impeller to its outer rim. This creates a low pressure at the water pump inlet compared with the outlet and causes the coolant to flow through the system.
Very little goes wrong with the pump unless there is some mechanical failure. The most common problem is a leaking seal. This is the seal that stops the coolant from leaking out around the water pump shaft. Seals simply wear out but leaks often occur because of foreign matter circulating the system or failure of the water pump bearings that allows the shaft to run out of alignment.
Water pump seal leaks do not always show when the engine is running. While running the spinning impeller creates a low pressure area at its centre (close to the shaft) and this can cause air to be drawn in and circulate the system. This can lead to air pockets and localised overheating that does not show on the gauge.
Another failure that can occur is the impeller spinning on the shaft.
The impellor spinning on the shaft is the only one that can cause an overheat problem and it will cause the problem under all conditions.
Water pump bearing failure is normally caused by an incorrectly tensioned drive belt. The ‘Bush Mechanic' wrote an excellent article on the in issue No 8 (Jun/Jul 03) of Sidescreen.
The pump must be removed to repair any water pump failure but is generally the last place you would look for the cause of an overheat.
The thermostat is basically a temperature sensitive valve. It is fitted between the outlet on the cylinder head and the radiator. It has a wax bulb that expands as the temperature rises. This in turn forces the valve open against the tension of a spring. The original thermostat had a shroud or skirt around the operating valve. As the thermostat opens this shroud would move to cover the bypass line and so restrict the amount of coolant that would bypass the radiator. It can be seen then that the thermostat continually modulates the amount of coolant that would flow down both the radiator and bypass line. This two coolant flows would then mix at the water pump inlet.
The temperature at which the thermostat starts to open is around 158 ° F(70 ° C) and is fully open at around 195 ° F(90 ° C). The manual gives directions on how to check thermostats and is fairly straightforward. Never place a thermostat straight in boiling water. It can cause it to fail.
As previously explained the original thermostat is actually two valves in one, with the shroud closing off the bypass line as the thermostat opens. These thermostats are not readily available and modern units are normally fitted. They do fit OK but do not have the shroud and as such do not perform the important function of closing off the bypass line. This allows coolant to continually bypass the radiator, even when the engine is approaching overheat.
In our generally warm climate we can overcome this problem by inserting a restriction in the bypass line.
This restriction is simply a plug with a ¼” bleed hole drilled through. Never simply block off the return line. Coolant must be allowed to circulate the system with the thermostat close to prevent any localised hot spots. Also the heated coolant needs to circulate around the temp gauge and thermostat so they can perform their function.
Remember, when driving the temperature will fluctuate between about 168 ° F(75 ° C) and 195 ° F(90 ° C) (the thermostat is not fully open till 195 ° F).
Thermostats are relatively inexpensive and easy to change. If in doubt, change it. Modern thermostats are a lot more reliable than the early ones.
A lot has been written about this much abused component and I recommend that you read the very good article by the bush mechanic…or at least read the manual on how to adjust it.
It is often over tightened in a vain attempt to overcome an overheat problem yet is very rarely the cause and can cause the water pump and/or the generator bearings to fail.
This is the only indication the operator has as to what is happening in the coolant system and it does not tell you much. All it indicates is the average temperature of the coolant exiting the engine. The unit fitted to the TR is fairly reliable but should you be chasing temperature problems then it is strongly recommended that the gauge be checked first. Ensure you have a problem before you go chasing it. This may not be easy if the unit has not been removed from the thermostat housing for some time. The quickest and easiest method is to use a contact temperature gauge on the thermostat housing and compare the results or use an infra-red one if you have access to it.
A little known fact is that the temperature gauge will not work if the sensor is not immersed in water. So we can have an engine with no coolant and cooking but the gauge may show a low temp.
These are used to transport the coolant to and from the radiator and act as a bypass line. They are very seldom the cause of a temperature problem and tend to either work or fail. It is a good practice to change the hoses every 4 or 5 years so as to avoid failure on the road. They do deteriorate over time.
There is nothing you can do here except keep them clean. Some commercial engines actually incorporate a bypass coolant filter in the system. Use a good quality coolant that incorporates a corrosion inhibitive package and this will help keep it clean.
A very important part of the system, inexpensive yet often overlooked and abused. The cap incorporates two valves. The main valve is the system pressure valve and works by way of a spring-loaded rubber seal that sits on a seat which is part of the radiator filler neck. The original cap was rated at 4PSI and is classed as a long neck type. Ie the top of the filler neck is about ¼” further from the seat than the more common short neck type. If you hold a long and short neck cap together you can easily see the difference. The long neck one is the longer of the two.
The second valve is a non-return valve. As the coolant temp increases it expands by up to 12% of its original volume. This excess coolant escapes past the pressure valve in the cap and out the overflow pipe. As the engine cools and the coolant shrinks the pressure in the system falls below the outside air pressure and air enters the system via the overflow pipe and through the non-return valve. This is why the system always appears low when you remove the cap.
Whilst the original cap was set at 4PST it is common and desirable to use a 7PSI cap these days. Caps do not give a lot of problems and can be easily checked at most workshops.
Unless the engine has been built to withstand the higher pressures it is not recommended that caps of a higher pressure be used.
Caps can be the cause of cooling system problems if they do not control the pressure in the system. Remember the reason why we put the system under pressure is to raise the boiling point of the coolant to prevent localised overheating, the ones that do not show on the gauge.
A common problem is that people do not understand how the cap works nor that it is a long neck type and fit a standard short neck unit because that is all that is commonly available. This results in the incorrect pressure being held on the system. In an emergency a short neck unit can be used but should be replaced as soon as practicable and do not run the engine under high loads or high ambient.
They are small, inexpensive and can easily be carried as a spare.
The fan is there to provide airflow through the radiator when the forward speed of the car can not provide the flow required and is mounted direct to the crankshaft.
At about 30 MPH the air flow resulting from the forward motion surpasses that provided by the fan. If you only ever drove on the open road you could throw the fan away, as do most race cars.
Around town is where the fan is vital but the original fan is marginal at best and inadequate in today's operating environment. Even when new, TR's overheated.
You need everything working at its optimum and there is very little you can do to improve the situation. The fan is the weak link in the cooling system.
If you are experiencing a problem on the open road then the fan is not the problem but is most likely the one if the problem only occurs around town.
The radiator works by transferring heat from the coolant to the airflow through the core. The greater dT, the more efficient it is. As the air travels past the first row of tubes it starts to heat up and therefore the dT is lower when it reaches the second row of tubes and so on. The aim therefore is to provide adequate cool airflow through the core so that the dT remains high.
To provide airflow you need a pressure differential, ie a higher pressure in front of the radiator than behind it, the greater the difference the better. In the TR2 and TR3 the air inlet passage is well controlled by the airway formed by the front apron ensuring that all air that enters the grill passes through the radiator. The only hindrance is the tortured path the air must travel to exit the engine compartment, not much you can do here but it does tend to restrict the flow.
With the TR3A a cardboard baffle is used to direct the airflow inside the front apron. Unfortunately this component of the cooling system is often damaged or left out during repairs. This baffle must be installed to ensure the system will work satisfactory… not negotiable.
With the baffle removed the following occurs. Whilst the vehicle is stationary or at low speed the fan has to provides the required airflow. The air from the fan travels rearward and hits the engine, some of this air will recirculate back through the fan thus reducing the possible flow through the radiator but this is normal. Without the baffle some air will also recirculate around the sides of the radiator and back through it. This recirculated and preheated air reduces D T and the radiator efficiency.
At speed, all of the air entering the grill is directed through the radiator. The flow will depend on the restriction and pressure in front of the radiator as well as the pressure behind it. The engine and all the other bits and pieces in the engine bay that are either original or retro-fitted, restricts the airflow exiting the engine bay. This acts to increase the pressure behind the radiator above what it would be if there were no engine at all.
No problem because the system was designed for that effect. If we remove the baffle then things change. Now we have reduced the restriction in the front of the radiator by allowing most of the air entering the grill to pass around the radiator. The restriction behind the radiator has not changed so the
pressure behind the radiator builds up as the increased volume of air attempts to exit the engine bay. The combine effect of this is to greatly reduce the actual flow through the radiator.
It can be seen therefore that removal of the baffle can effect both low and high-speed engine cooling