water temp issues

[boatissues]

97 7.4l mercruiser. (no idea what serial number is) Bravo1 outrdrive
Temp gauge was working fine, pulled the oil cooler and removed about 1 1/2 impeller blades worth of crap and reinstalled it. Ever since temp gauge will not register unless I shut it down and sit for about 5 minutes. Turn the key on and it goes to about 175. Start the engine and it dives to 40 in about 5 seconds and stays there.
New impeller, new OEM thermostat and gaskets, tried new sensor no difference. Have grounded the gauge and it pegs. It did finally go to about 105 the other day when I had made an extended run at 4K rpm (usually cruise at about 3k) Hit 105 and then dropped to 40, repeated until we stopped. Scratching my head over this one...

 

[tpenfield]

:welcome:

See if you have got any impeller fragments stuck in some of the chambers of the thermostat housing. Also, make sure the 'insert sleeve' (red arrow in pic) has not spun around from its proper position.

 

[boatissues]

Thanks for the answer. Yes I have done all of that. The weird thing is I have had that housing off four or five times and this has never happened before. All I did was remove the housing and put it back on after I cleaned out the oil cooler. I did reverse the wires with that and the over temp sensor which is on the opposite side of the housing but I straighten that issue out.

 

[Fun Times]

When you installed the thermostat housing, did the bottom gasket have brass rivets on both sides of the gasket?

What did you use to coat the temp senders threads? If the sending unit threads aren't properly grounded, the gauge could be reading intermittent false signals as the engine RPM changes.

Next you'll want to confirm if the engine water temp is actually fluctuating that much or is it just the gauge fluctuating that much?
An infrared temperature gun would be helpful in this case pointed all around the thermostat housing while watching the gauge too. https://www.harborfreight.com/Non-Contact-Infrared-Thermometer-With-Laser-Targeting-69465.html

The gasket could go on dry or at least don't cover the brass rivets with sealant,