Temp gauge vs. laser thermometer at thermostat housing

[MalibuMike]

I posted a question last week about my gauge reading 180 degrees at while cruising at 3200 rpm with a closed water system. Someone mentioned that it would be a good idea to get a laser thermometer and aim it at the thermostat housing. Went to Harbor Freight and got a $30 laser gun. So I took some readings while cruising in the harbor at a very slow speed. Gauge was at 168 degrees while the laser was at 156 degrees next to the sensor ( see pics) and the highest temp I could find was 158 about 2 inches higher above the thermostat
Are gauges and sensors usually that far apart? 8 to 10 degrees seems to be a lot. How would you go about trouble shooting this to figure out how to a get a more accurate reading?

 

[Lou C]

Yes that’s what I have found.

 

[larson260]

Yes that’s what I have found.

I have found with the laser gun that the readings depend on the material that you are taking them off of. I am a plumber and found that I can't depend on the readings being accurate on the job, back to the thermometer. I took various readings on the boat and catalogued them with there locations as to have readings to go by in the future if I need to check things out. Don't care to go down in the engine compartment with a thermometer in my hand.

 

[Lou C]

On my engine I got the closest to the dash gauge reading from the intake manifold right under the thermostat housing, about 10-15* lower than the dash gauge.

 

[Bt Doctur]

What you do is locate a 5K pot put it in series with the wire to the gauge wire the other end to the sender. Get to operating temp ,Adjust pot until both reading match . Carefully remove pot and measure the ohms. insert this resistor in series with the gauge wire

Amazon.com

 

[Horigan]

What you do is locate a 5K pot put it in series with the wire to the gauge wire the other end to the sender. Get to operating temp ,Adjust pot until both reading match . Carefully remove pot and measure the ohms. insert this resistor in series with the gauge wire

Amazon.com

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I did this per Bt's suggestion on my boat and it worked pretty good. If you can find a smaller pot than 5K it will be easier to find the correct resistance value. The 5K pot is very sensitive around the 55 ohms I ended up with. Just removing it to measure it can change the value a bit.

 

[MalibuMike]

Too advanced for me but I appreciate the suggestions. I guess running 179 degrees is horrible and not an immediate worry. I do plan on cracking open at least one riser to asses the condition of the passages. If the exhaust is cooled by seawater, and the engine cooled by coolant, Im guessing that the exhaust temps shouldnt affect the engine block and coolant temps. Am I right or wrong? This has got me thinking now......with basically two cooling systems, what is more likely to cause higher temps?