I have replace my 160 degree Thermostat with the same temp thermostat when I had cleaned my water inlet housing on my open cooling system and now my temp gauge spikes when I am running from 650 rpm - 4000 rpm the gauge moves from 140-185 constantly and I don't remember It doing this before. What can I do to solve this problem. ( HELP)

Been there had that happen...Water temp gauge moves back and forth like windshield wippers...
Bad impeller with chunks of the old impeller blocking water flow. What you are seeing is steam pockets being created from lack of flow. fix ASAP before you warp your heads....

you can drill two 1/8" holes in the thermostat to relieve the air pockets and even out the temp spikes

I had those holes Wally His problem is different, Bad impeller with chunks of the old impeller built up most likely in oil cooler will make the temp gauge move up and down like wind shield wipers. And I mean literally as fast too.

Thanks guys took off the raw water pump and check the impeller it was ok, took off all the hose and check for blockage all cleared, replace thermostat and problem solved. After checking thermostat it was a after market brand not the Merc brand if that makes a difference. Once again thanks for all your help and
( ALOHA )

had these issues too think we are fixed i hope