A little background: Currently in Las Vegas daily temps are hitting a high of around 112F. My car is in my sig, nothing special, not even warmed over really.

Here we go - The radiator is fairly young, only around 2 years old. It was recently flushed out, and has a coolant mixture thats only about 2,000 miles old. The water pump is also only a few thousand miles old. The hoses are new. The thermostat is a 180, it is new. The coolant temp sensor is stuck in open circuit! I drive about 20 minutes to work in the heat, and the car does NOT overheat. However, it hangs around 220-230... and when I turn it off at that point, it gets very hot obviously b/c the coolant stops flowing, and the fans are off, no wind etc. I would think that the cooling system (all newish OEM) could hold it around 200 whilst driving even in Las Vegas. So would an open circuit coolant temp sensor cause this significant a performance decrease in the cooling system?

 

The CTS has nothing to do with the actual cooling system. It reads the temperature of the engine so the ECM can adjust the timing, and A/F mix.

Sounds like the system is operating the way it should, as a stock system. The problem with these cars, especiallt the tpi cars is the run hot, for smog reasons. The cooler t'stat will help a little in the beginning, but once all the collant has gone through enough times it will just gradually strat to hold more heat.

The sequense the fans come on are horrible too. The drivers side fan is controlled through the ECM so its been programmed to come on around 230* and the passenger side fan is controlled through a switch in the block that i think comes on at like 220* or 225* i think.
Changing these settings will make a dramatic change in operating temps. You can change the switch in the block to have the fan(passenger side) come on around 180*. But that is only half. The other half is in the PROM. You can get a new prom from a variety of sources or make your own. Since your car is mostly stock, and i dont know how into you are goin to be as far as modding it, i would reccomend just gettin an off the shelf chip.

Places like Fastchips, Performance Resource, Jet,and Hypertech all make them for our cars and are around the same price range. However, Jet and Hypertech chips have mostly the stock program, so this wouldnt change a whole lot, except make the car run cooler. Hope this helps.