Your description is fine and the details clarified the symptoms. Both temperature sensors should be on or near the thermostat housing. The single-wire sensor is just for the gauge. That won't cause overheating. It will just indicate it.
It's the two-wire sensor that plays the important role. It isn't a simple on-off switch for the fan. Instead, just like the gauge's sensor, it changes resistance with changes in temperature, thereby telling the Engine Computer what the exact coolant temperature is. Oxygen sensors do not work below about 600 degrees, but there is no means of measuring exhaust temperature. Instead, the computer knows that the O2 sensors are ready to send accurate data by the time the coolant temperature reaches about 140 degrees. That's when it adjusts the fuel / air mixture based on many inputs PLUS it fine tunes it based on O2 sensor readings.
The computer also modifies ignition timing and turns various emissions systems on and off based on coolant temperature. The last thing is the radiator fan. All Chrysler fans turn on at 210 degrees, then turn off at 198 degrees. GMs can vary depending on the car model, year, and engine size, but some don't turn the fan on until around 220 degrees.
Going to a 180 degree thermostat won't fix anything unless the overheating is caused by the old thermostat. In that case it's not the 180 degrees that is the solution; it's the new thermostat. You may have introduced more problems because the coolant won't reach the proper temperature for some things to occur. Also, the engine will be too cold so parts won't expand to fit properly, leading to increased wear. That mainly applies to the pistons which are only perfectly round when they're at the right temperature.
The temperature gauges can be notoriously inaccurate. The best way to know the exact coolant temperature is to use a scanner to view live sensor data the Engine Computer sees. If the engine is still over heating or even running at 200 degrees with a 180 degree thermostat, that would point to a problem that has not been solved. It has only been masked. The most common cause is a leaking cylinder head gasket. Combustion gases can sneak into the cooling system and pool under the thermostat. That will prevent it from opening because thermostats open in response to hot liquid, not hot air. Usually that leakage is accompanied by a loss of coolant over time.
There's also a story I can share about testing thermostats. A new coworker working next to me was asked to repair a trade-in car with cold air from the heater and the temperature gauge stayed on "cold". Those are the two classic symptoms of a thermostat stuck open. Since time is money, and since this is a well-known problem, the fastest way to diagnose it is to just replace the thermostat. The cost is so low that if that doesn't solve the problem, no one is going to fault the mechanic for a misdiagnosis. Instead, the guy did all kinds of tests that none of the rest of us understood. Three hours later he finally put the new thermostat in and the heat and gauge were fine. You'd think the car was fixed, but then he tied a wire to a soup can, hung it over a vise handle, filled it with water, and against numerous warnings from us, he heated it with the torch to see at what temperature the thermostat would open. THAT was the final straw that got him fired. There were other instances where he refused to listen to experienced mechanics, resulting in a real lot of wasted time. One thing he didn't realize is there's a wax pellet in thermostats that melts, and a spring that relaxes to ALLOW the plate to open. It doesn't CAUSE it to open. You need the push of the coolant from the water pump too. If you see a thermostat open when you heat the water to the specified temperature, you know it's good, but if it doesn't open, you do not know it's defective. It might be but it could still be okay. You have nothing to lose by trying to test your thermostat for yourself, but a professional would never do that when a customer, or the used car department, is paying the bill.
Another potential test that may provide valuable information is to disconnect the two-wire coolant temperature sensor while the engine is running. This will have different results for different GM cars but on all Chrysler products doing that will cause the Engine Computer to turn on the radiator fan. That will prove the entire system is working in an instant. The computer does that by default because with a disconnected sensor, there's no way it can know the coolant temperature, so it turns the fan on just in case the engine is overheating. Unplugging the sensor with the ignition switch on will set a diagnostic fault code and turn on the Check Engine light too. Depending on the system, the light will either turn off as soon as you reconnect the sensor, or it will stay "latched" on until you stop and restart the engine. Either way, the fault code will stay in memory. You can erase it with a scanner, with some simple code readers, by disconnecting the battery for a minute, or they will usually self-erase after a certain number of engine starts.
There is no other way to accurately test a coolant temperature sensor. You can unplug it and measure the resistance between the two terminals, but the acceptable range is very broad, and the resistance will change due to changes in temperature. Those sensors have an extremely low failure rate, (except for some Ford models). By far most problems with that circuit are related to stretched terminals in the connector or broken or rubbed-through wires. I worked with a teacher who insisted his student memorize each sensor's resistance for any given temperature because there were reference charts in the textbooks, and he knew how to teach what was in the books. No professional has those numbers memorized for every car, and they would only do that resistance test when all the common tests provide inconclusive results. Many mechanics go their entire careers without testing a sensor. They do voltage tests that eliminate everything but the sensor, then they just replace it. You have the luxury of testing stuff if you want to because you're not paying for someone's time.
Tuesday, October 1st, 2013 AT 12:07 AM