2001 merc 150efi 2 stroke tempature

[Starcraft94]

Just purchased 21’ boat with a 2001 merc 150efi 2 stroke. I changed both thermostats with oem thermostats.
Finally got boat on water and the temperature gauge(new) appears to be all over the map.
Up to about 225 and then anywhere from that to 150.
Is that common? I never had a gauge on the old Johnson 150 I had years ago.
There’s good water flow out of the discharge and the lower unit replaced last fall.
Thoughts/ideas appreciated.
I have a water pressure gauge coming in the mail so that will help diagnose

 

[Dukedog]

tha pressure gauge will tell all.. most temp gauges can and will be all over tha place.. jus make sure your over temp warnin' buzzer is workin'.. its a lot more dependable than any metal ta metal temp gauge sensor.. get yourself a temp gun and be sure if ya worried 'bout it................ jmo.