2012 204fsx mercury 4.3 alpha gen ii temp gauge

[204FSX MIke 0]

Just purchased this used 204fsx. 204 hrs. Runs amazing. Just noticed that the cooling temp gauge is not working. Worked the other day. What should I check? I’m pretty tech savvy so should be able to trouble shoot with your help. 
merc 4.3L is not getting hot. Exhaust is cool to the touch...  hope someone can shed some light. First step I’m thinking is to check the temp sender wire. But not sure where it is?  
thanks all. 
happy boating 

[204FSX MIke 0]

Well the temp gauge drops to 100 degrees when running over 3000 Rpm. When I slow down and just cruise it pops back up to around 160 degrees.  Any idea what’s going on?

[Kelly from Jax 0]

I've got a similar problem so I'm following this thread.   My mechanic is going to hook up some analog gauges while I run to figure out what is going on.   Either a bad sensor or the raw water impeller is damage to where when you are running a idle it isn't spinning and providing cool water through the engine.      I get a solid alarm when it happens...

[204FSX MIke 0]

Thanks Kelly! I appreciate your response and look forward to your reply. 
My temp alarm is not sounding off so I am not over heating. 

Have a great day!

[GetAway 0]

When was the last time the impeller was changed? I had an impeller that was 2 years old and my temp gauge dropped to 120 no alarms. I have the 2000 Volvo 5.7 G??. I changed my impeller with a new Volvo unit and my temps returned to 158 - 160 and stays there at idle and 3000 - 3500 rpms. I've read the unit should be changed every 2 years.

[Kelly from Jax 0]

The mechanic hasn't come to see the boat yet.   We've been getting storms every afternoon.   Hopefully next week.