Temperature Gauge is Erratic

I have a 355 that the water temp will show about 140 and then it will show 190 for a few minutes and then go back, almost instantly to 140. It happens too quickly to be reading the real fluid temperature. I put a laser thermometer on the car and it looks like it is running about 190. I think the problem is likely with the sending unit but I was curious if anyone else has seen this in the past before I just start throwing parts at it. Also, where is the petcock to drain the cooling system? I looked (but not that hard) over the weekend and it didn't jump out at me.

 

I had a similar problem with my 308QV, replaced the sending unit and all is good now. As far as the drain plug for the cooling systems don't know for a 355 but would assume somewhere at the bottom of the radiator.

 

If you just changed fluid, you might have an air pocket, if not, I'd replace the pressure cap, then I'd make sure it's not a ground or other electrical problem in that circuit, then I'd look at the sender unit. Similar discussion in the 456 area to check for.

 

Could be the sender, could also be the plug to the sender. So clean the plug connecting pins if you decide to replace the sender. I would also clean and check the connector on the back of the gauge as well.