Produced from 1982 to 2004 in the United States, the Chevy S-10 has went through various changes including new engines, transmissions, body style and trim levels. Despite the six different sized engines used throughout production and the various changes to those as technology permitted the cooling system has always been reasonably the same. Naturally the fan clutch was eventually replace by an electronic cooling fan, and after 996 Dexcool coolant was used as opposed to the tradition Ethelyn-glycol used previously. Aside from minor changes coolant has always traveled from the engine to the radiator for cooling and back again, with a quick stop at the heater core when the heat was turned on, of course. The bottom line is that an engine that isn't reaching operating temperature will cost you gas mileage and will prevent you from having heat in the winter.
The thermostat on the S-10 in general is mounted to the engine and serves as a door that opens only when the engine reaches a certain temperature. A faulty thermostat that is stuck closed will prevent coolant from leaving the engine and will cause the engine to overheat. The exact opposite is viable as well; a thermostat that is stuck open will constantly allow coolant to flow, often causing the temperature to never reach over 100 degrees. As an inexpensive part, it wouldn't be a bad idea to drain the coolant and replace the thermostat, especially if it hasn't been done in a few years. If you have recently purchased your S-10 it is possible someone remove the thermostat all together, which creates excessive coolant flow and generating hot spots in the cylinder head. Remember just because the temperature gauge is low, parts of your engine could still reach hundreds of degrees and you wouldn't be the wiser.
If your over cooling is to the effect the engine reaches operating temperature and then cools dramatically causing momentary loss of heat, could be the result of too much cold air blowing through the radiator fins. If you live in a cold climate, your problem is intermittent and you have some front end body damage; primarily the radiator grill, this could likely be the problem. In excessively cold climates, even without front end damage, some motorists will block off part of the radiator with cardboard and a few zip strips to help prevent this occurrence. As a temporary fix you can use cardboard to help control air flow, but blocking too much of the radiator can cause your engine to overheat. If the cardboard solves your problem search local salvage yards or online to procure a new grill that will properly regulate the amount of air passing through the radiator.
Sometimes a non operational electric cooling fan or a faulty fan clutch may not even be noticeable in cooler climates as the engine is subject to cooler outside temperatures, but if the fan is always at maximum, you may once again be pulling too much air through the radiator causing the engine to over cool. Keep in mind in this scenario the engine will normally reach operating temperature and the thermostat will open before the fan will have an effect on the engine temperature. Anytime an electric cooling fan always runs there is a short to power in the circuit for the fan or the circuit for the electric fan relay.
Nine times out of ten extreme engine over cooling will be the result of a thermostat that is stuck open; it is, in general the cheapest fix and is more likely to fail and cause your over cooling problem. Always double check and verify that your coolant temperature sensor is also properly connected and working. Older S-10 coolant sensors were much like the 305 and 350 setup where a single wire slides over the connector and is often disconnected when replacing ignition wires or if the engine is raised for any reason. If the cabin has heat and the upper radiator hose gets hot after several minutes of operation there is a fault somewhere in the coolant temperature sensor circuit, including the gauge or sensor itself.
