>Arch if everything in the GMC or any car for that matter is working
>correctly the thermostat in the summer will be constantly cycling on and
>off.
Thermostats are analog devices. Normally they don't cycle. The opening
varies in proportion to the temperature of the coolant. They typically
start to open at 15-20 degrees below the specified temperature and will be
fully open at the rated temperature. The result is that, on cold days, an
engine will run cooler than the thermostat rating. On warm days, when the
cooling system is reaching its full ability to keep the engine below the
rated temperature, the engine will be running at the rated temperature.
When the engine is running at a temperature higher than the temperature
specified on the thermostat, the engine temperature is no longer being
regulated by the thermostat. At this point the engine temperature will vary
in proportion to engine load and radiator airflow. If you are seeing the
temperature cycle during normal driving I would guess that its caused by
the fan clutch cycling rather than the engine thermostat.
One thing to keep in mind about thermostats is that they where originally
(in pre-emissions days) designed to establish a lower limit to the engine
operating temperature. A lower limit was needed to keep moisture and fuel
residue from building up in the engine lubricating oil.
The upper engine temperature depends on the boiling point of the coolant,
coolant flow and air-flow. The boiling point of coolant depends on its
pressure which is controlled by the radiator cap.
Keeping the boiling point of the coolant high is very critical to good
cooling, especially at higher engine temperatures. There are hot spots in
the heads that are much hotter than the engine as a whole. If the coolant
is too hot or the pressure is too low, there will be localized boiling in
these areas. This boiling will cause localized steam bubbles in the coolant
and will further reduce the coolant efficiency in these critical areas.
The 195 degree thermostat is largely a result of emissions controls
requirements. In the early emissions days, emission controls largely
consisted of limiting the number of engine variables and adjusting the
mixture and timing to operate over a more limited range of engine
variables. Thus it became necessary to raise the engine operating
temperature and to limit the range of engine operating temperatures. If an
engine is run hotter and operates over a narrow range of temperatures, the
mixture can be set leaner and the engine will produce fewer unburned
hydrocarbons in the exhaust. As a result, manufacturers switched to the 195
degree thermostats and increased the cooling capacity of their cooling
systems. These changes where done so the engines would operate hotter,
leaner and cleaner.
Other changes made to help limit the range of the engine variables where
things like intake air pre-heat manifolds and heat riser valves. These
things where used control the intake air temperature range and to speed
getting the engine temperature up to the narrow range that the system was
designed to operate in.
Of course those of us who had to live through those early attempts at
emission control can remember how well this approach worked. It was
mandated that it work for 50K miles and we could all be pretty sure that by
50K miles, it was no longer working and things where no longer running so
good....
>... Next time you are out and
>running about in the summer time and the temperature has just come down
>as you say crack the radiator cap-------naw you will get burned. You
will be
>well above 190.
In most cases a radiator will "boil over" when the cap is removed even if
the engine is not above 190. When running you can get hot spots, especially
in the heads, where the temperature is a lot more than the overall engine
temperature. If these are still hot when you release the pressure, the
coolant can boil in the heads and the steam will push coolant out of the
opening. Thats usually what happens when you open a radiator at the side of
the road after an overheat. Its also why it is a bad idea to add cool water
to an overheated engine. The heads will be very hot and dry (because
boiling has pushed the coolant out). Adding cool water will shock cool the
heads and often times crack them.
>Even in the winter time I am not sure you cant crack the
>radiator cap----
Another source of "boil over" when the radiator is opened has nothing to do
with temperature. The cooling system has air pockets which become
pressurized when the engine is warm. Releasing the pressure from the
cooling system will allow the trapped air to expand quickly and push the
coolant out of the opening. This can happen even when the engine is
reasonably cool.
In conclusion, I run my engines at 160 degrees for a number of reasons,
mostly related to reliability. Cooler temps result in longer oil life,
better lubrication, fewer oil deposits, less valve erosion, less
susceptibility to knocking, fewer problems with heat-soak and cooler
transmission oil temperatures.
Sorry for the long post. I should be working on a mid-term but, for some
reason, writing about engines is a whole lot more fun
Guess I should get back to work...
Dave
Ann Arbor, MI.
73 Sequoia (26'/455/EFI/HEI)
