Low temp indication. 5.0 MPI

[FliesAndFloats]

My 5.0 MPI has started indicating low temperature recently on the cluster. It used to indicate "straight up and down" around 170-175 and now sits between 130-140. Water temperature here is around 68-70F. I left it in saltwater for about four days prior to this and assumed the thermostat went bad and replaced it with a new 160 degree thermostat. That did not resolve the issue. I assume the sensor on the left of the housing with one lead attached to the threads is a standard NTC sensor. I have "messed around" with the lead on the sensor and the leads on the gauge with no intermittent correct readings. I am going to assume that the engine is reaching the correct operating temperature, but if it's the sensor that it may be affecting things like the mixture. It will have a surging feeling and quit when the gauge starts to move and I put it in gear, but is fine once it reaches that 130-140 indication.

Do my assumptions sound correct? What is the correct way to verify function of the sensor?

 

[Fun Times]

Hi there, what as the condition of the old thermostat?

First you'll need to confirm what the actual engine temperature is at the thermostat housing and surrounding areas. Using an infrared temperature gun is usually the easiest way to inspect the engine temperature. http://www.harborfreight.com/catalogsearch/result/index/?q=infrared+temp+gun

Did you replace the cork gasket found under the thermostat? And did you install the T/housing to intake manifold gasket dry plus did the gasket have the two brass rivets through the gasket?

 

[FliesAndFloats]

I replaced both gaskets. The old thermostat appeared fine. There wasn't any corrosion that I saw. It was black, I'm not sure if the old one was painted/anodized black, but it looked like it was supposed to be that color. I'll check with the laser thermometer next time I'm on the boat.