It could be sending unit or thermostat, but what i am wondering is I purchased a laser temperature gun when the engine is fully warmed up
where should I really point it to get an accurate measurement? .
Establish the relativity of the gauge reading by pulling the sending unit out of where it is installed (intake, behind the thermostat? ) and drop it in a pot of boiling water; the gauge will provide the observation of what 212° +/- will look like.
As far a the infrared temp-guns, these are not terribly accurate, particularly as their implied values are sensitive to the reflective nature of the surface being targeted.
Scott.
Adding to this, you want to check the resistance as the temperature increases. I had a bad sender that read 11 ohms at 172 degrees in a pot of water on the stove. (reading high temp on gauge), The new one I bought showed 18.5 at the same temp and was right on the money on the gauge and went to around 35 ohms at 180 degrees. Reading through an old email discussion I had with Jim Cowles about this, I bought a Duralast version of the smaller 65 sender - TU22 part number that was manufactured by Wells Vehicle Electronics. At the time, they also had the engineering specs online showing the expected resistance at temp. The engineering drawing is no longer online that I can see but this is their sender and I would buy it again if needed
https://wellsve.com/parts-search/?partNumber=TU22 Autozone website shows their TU22 is now made by Rostra Powertrain Controls and I have no idea if it's accurate or not.
As far as the accuracy of your IR thermometer, boil some water, it will be 212 degrees. Your IR may be a few degrees high or low but shouldn't be off much. The best place to measure the coolant temp is with the cap off the radiator but only do this after letting the car warm up WITHOUT the radiator cap on. Once you know the actual coolant temp coming into the radiator, you can get the temp of the upper and lower hoses to get an idea of the temp drop after going through the radiator and factor in any difference for the rubber insulating the actual temp. I recall around a 20 degree drop after going through the radiator.
According to Jim, even the NOS sending units were not accurate so the best bet is checking the resistance of whatever you use to know what you're working with.
Steve -