[cycleguy55]

Perhaps everyone already knows this, but at the risk of offending those that do I'm going to write it for those that may not. The temperature rating of the thermostat is the temperature at which it starts to open, and it is most likely not fully open until the temperature rises another 15 or 20 degrees - so 195 or 200 degrees F.

As I wrote before, my engine runs around 190-200 on the highway, consistent with the proper 'fully open' temperature of my 180 degree thermostat, but will creep up above 210 when in traffic on a hot day. As soon as get rolling or even if I bring the revs up a bit (e.g. 2,000 RPM) the temperature will immediately start dropping. Clearly there is lower coolant flow at idle and the temperature climbs a bit, but 210 or 220 degrees is well within the boiling point of my system (60-70% ethylene glycol, 13 lb. radiator cap).

That lower coolant flow at idle and/or power consumption of the water pump at higher RPM is why many people switch from belt-driven water pumps to electric water pumps. Perhaps it also has something to do with the consequences of losing a belt 100 miles from home. I don't know, I certainly haven't seen the need - at least so far.

The temperature gauge gets its reading at or near the coolant neck outlet, so it's the temperature leaving the engine - likely as hot a spot as any, other than a spot reading right at the closest point to a cylinder.

On my engine the sensor to turn on the cooling fan is at the radiator inlet, as recommended by Flex-A-Lite, so it's probably reading pretty much the same temperature as the gauge. It's adjustable and turns the fan on when my gauge indicates about 205-210 degrees. Once the fan is on, the temperature climbs very slowly, and only reaches 220 if it's a very warm day (e.g. 90+ deg. F). As soon as get rolling or even if I bring the revs up a bit (e.g. 2,000 RPM) the temperature will immediately start dropping.

Maybe it's just me, but I'm curious about what a second temperature gauge sensor at the radiator outlet or water pump inlet would read. Is the temperature differential 10 degrees, 30 degrees or 50 degrees? Anybody know?