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Cooling System Basics


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With the high temps of summer, I thought it might be appropriate to restate some cooling system basics in light of all the concern about coolant temperatures.<P>Thermostat - the thermostat exists to help regulate coolant temperature. It does this by restricting coolant flow when the fan, water pump, and radiator are providing excess cooling. While warmer in the winter and cooler in the summer might sound better, for cost reasons, only one temperature thermostat is used. The thermostat cannot force the engine to run warmer or cooler except within the limits of the overall cooling system design (the size of the radiator, the flow rate of the water pump, the air flow of the fan(s)). Older cars with mechanical engine driven fans used mostly 180 degree thermostats. Most of these older cars also had engine driven fans (which wasted power when not needed), lower pressure systems and no coolant recovery systems. Newer cars switched to 195 degree thermostats to improve fuel vaporization in carbureted engines. Air conditioning (and its associated cooling system load) became standard on most cars. Heater cores were redesigned to take advantage of the higher thermostat temp to improve heater operation. This would matter primarily to people in colder climates. Coolant improved and was used also as a summer cooling enhancer, not just "anti-freeze", as the summer boiling point is also raised, allowing higher system pressures (radiator cap determines system pressure) and for the most part a maintenance free cooling system once the coolant recovery system was added. <P>In moderate climates (20 degrees F to 85 degrees F) the engine temperature should pretty much stay at the temperature of the thermostat (because the system is designed for the extremes). In very cold temperatures (below 0 F), the engine may have trouble reaching the thermostat temp, at idle or low speed, because all the heat being generated by the engine is being used to heat the vehicle (this is especially true in vehicles such as a large van with front and rear heaters - the heater cores actually serve as the radiator). In very warm temperatures (90 degrees F to 115 degrees F), especially with higher humidity levels, the cooling system will operate near its limits, at low speeds or high loads. The thermostat will usually control temp well when the vehicle is moving above 40 mph, even with the air conditioning load, because of all the increased airflow for cooling (most electric cooling fans are not even allowed to operate above 45 mph). When the vehicle slows or stops, the natural airflow drops or goes to zero. It is at this point that the vehicle depends on the fan or fan(s) for cooling. If the vehicle has been running at high speed or high load or both prior to slowing or stopping, there will be residual heat stored in the block and heads that will cause the engine temp to rise as soon as the vehicle comes to an idle condition. If the engine is immediately shut off completely, it may "after boil". This happens when the engine temperature exceeds about 256 degrees F, the boiling point of a 50-50 mix coolant. This occurs because there is no coolant flow or air flow, but still a lot of BTUs stored in the cast iron engine which transfers to the coolant and the underhood air. At that point the cooling system pressure will exceed the radiator cap's limit and the coolant will boil over into the coolant recovery bottle. As the engine cools it will flow back into the engine (assuming the recovery bottle was able to contain all the overflow). If the engine is not shut off immediately, but allowed to idle, the fan or fan(s) should be able to bring the temperature back down near the thermostat temp. If the engine is allowed to idle for an extended period of time (30-45 minutes), the engine temp may normally start to rise again, especially if the humidity is high, the temp is high, and the air conditioning is operating. Cooling systems are not designed to keep the system at the thermostat temp under worst case conditions, but at a minimum, they should keep the system temp below 256 degrees F for at least 30 minutes while idling in drive, at 100 degrees outside temp with maximum air conditioning load, if properly designed (max air conditioning load is with outside air, not recirc. If "MAX" AC is available and selected, that will normally force recirc mode). Slow speed stop and go driving will be somewhat less demanding than extended idling, and should normally only require the low speed electric 'puller' fan. Extreme conditions and extended idling should require the high speed fans (the 'puller' fan has a low and high speed, the 'pusher' fan is high speed only). Shifting to Neutral and increasing the engine speed will provide some increased coolant flow, but will not be as effective as in older cars because the fan(s) are electric and do not speed up with increased engine speed (although increased alternator output may slightly increase fan speed). <P>All this being said, older systems may slowly deteriorate due to internal corrosion and deposits in the engine, radiator and condenser, external debris on the radiator and condenser, or defective thermostat. Regular flushing of the coolant system and occasional cleaning of the radiator and condenser fins will help. With all the electronic controls, numerous other component failures can also cause problems (relays, sensors, corroded terminals) as well as collapsed hoses and gasket leaks. Any sudden significant change in performance is likely due to a component failure and/or a significant change in climate. A significant climate change in addition to putting more demands on the cooling system, also may cause failure in aged and corroded electrical components and connections.

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