The water temp on one of my twin big blocks (with about 60 hours on them since complete pro build) started to rise to close ~180F (they normally both run a max of 150). Oil temp did not rise substantially. I shut the engine down and motored in on one. I pulled the boat out of the water and put it on the hose (as I flush them after use using a Diamond Marine In-line flushing kit) and the temp rose as normal to about 150F (and did not continue to rise)...no issues.
Water flow from the exhaust was normal and equal to the other engine (1988 Formula 311 SR-1).
This seems to indicate that the raw water pumps (Hardin) are working normally.
I am thinking that the thermostat got stuck/jammed for some reason and ultimately cleared when I was on the hose...any other thoughts or ways to trouble shoot without just opening up the (what seems to be a pretty tight/reliable) system?
47440 thermostat housing

Running at idle on hose is not a good test to replicate actual running under load on the water. I'm guessing you don't have water pressure gauges??
I highly doubt the T stat would have got stuck closed from debris....stuck open possibly, but stuck closed is not likely
First thing I would do is pull off the water hose that feeds the oil cooler and check for debris or impeller pieces.

Roger! Is there a good diagram showing the flow pattern of this thermostat housing...so I can find the easiest hose to pull to look for debris?
I was hoping that the solid amount of water coming out of the exhaust on the hose would tell me that impeller is good.
Thanks!

I'm thinking its the hardin water pump impeller.
When running in the water the pump should move water thru the motor. If impeller is damaged , then no water. Connect it to the hose and if the impeller is damaged the hose water pressure will push it past the impeller

Roger. I am tending to agree...impeller. Since I have a flushing system with a T, the hose water normally exits the lower drive unit until I start the engine and then it is sucked through the impeller into the engine/exhaust and exits that way. I kinda was leaning away from the impeller being the culprit because this scenario seems to be consistent (hose water out through lower unit until engine starts and then out the exhaust). Maybe the impeller is damaged so that it does not work well enough to cool the engine BUT still appears to be working on the hose.
I believe the ss Hardin's make impeller changing/servicing easier.
Any impeller changing advice?
Thanks!!!

Always ck oil cooler for debris first. Frt of it first. Then I’d Check for Bravoitis too.
Bravoitis

Agree check oil cooler
To check impeller the easy way, Disconnect the hose going from the pump to the thermostat housing at the housing. Connect the hose on the drive and turn water ON. Without the motor running see if water starts coming out the disconnected hose.

If the impeller does need changing its easiest for me (hardin pump is heavy) is remove the pump and change it on the bench. I have the hardin pump and stopped using it after 2 seasons and went back to the Merc composite

Merc composite for weight mainly...or some other issue?

Its weight that had me place on the shelf. To change the impeller I have to hang upside down, there is no room. The added weight just makes it to hard for this old guy.

I agree with Griff. Check the oil cooler inlet first. Take both hoses off and check with a flashlight from the bottom up to see if there is debris blocking the capillary tubes. If there is debris, clear it and see what the debris is. If it's rubber impeller vane pieces, you will know. I found a complete vane in mine after a shop "allegedly" flushed all the hoses.

Most times on the hose, the temp will be close to normal, at least mine does. Curious, but what temp t-stat are you running? I run 160* in mine and the motor runs between 170-175. 180* isn't hot by any means. Personally, I like a little heat in the motor.