ok, so I finished reflushing (emptying-starting for 10-20 min-filling radiator) 10 times (5 with distilled water to get all the old antifreeze out and five with 60/40 dexcool to bring it back to specs, first 2 times, chemical flush was added) purged the system the best I could, put new OEM thermostat-195 and a new radiator cap.
I just received a new OEM coolant temp sensor and compared it's reading to the old one and they both read almost the same resistance for the same temp (which was for both about 2.3 Kohms cold, and a little over 200 ohms at engine operating temperature). Then, I put little electronic resistors at the connector (with the sensor disconnected and the key at ON), and 100 ohms gave me a 3/4 gauge or about 245F temp, while 39 ohms gave me full gauge reading passed the dead dangerous zone, or about 275F. So, everything there looks fine.
I measured the voltage coming to the sensor (with the sensor disconnected), and one wire reads 5V to battery ground and 1.2V to the other wire, and then checked continuity between these two wires and ECM connectors and found their location at the connector and they were good with no brakes or loose contact or resistance (something that my Fluke shows as small fractions of an ohm).
My problem is that my engine is still, pretty much, in the middle between 1/4 and 1/2 the gauge which is about 192F, which is below the temperature that the thermostat opens, and the gauge never gets to the middle (210F) no matter what (all seasons, here in central PA it gets hot), but it does look unstable to me because it get's closer to the middle (I saw a little over 200F, once) when driving in traffic, while it approaches 1/4 (177F) when driving on a highway at night when there are no hills. That was the case for at least a year.
The normal behavor that I'm sure everyone knows is that it shouldn't fluctuate that much, and it should get about 20 degrees hotter (middle of the gauge, or 210F)
Are there any people who have ever experienced something similar?
Please help
Notes:
1. the car is a 2008 3.8 engine and has 120k miles, transmission just replaced 15k miles ago.
2. The temps of places on the gauge were measured with an OBD scan tool that was showing me the CTS reading while I was driving, that I fit in place of the GPS for about a week.
3. The cooling system was not perfectly taken care off through the years, but it was once flushed a couple of years ago at a cheap franchise and thermostat replaced, and then, in the last couple of years, green stuff was added and mixed with whatever that garage put in it.
