BBC 454 - 330hp with closed cooling systems. Never an overheat before and with 142-degree thermos it always runs at 150-ish at idle. Oil temp never goes over 150 at idle.

Last weekend Stbd motor overheated at dock - idle 1,000 rpms.

Checked water intake - no obstructions.
Checked raw water pump output - no problems
Checked coolant in heat exchange - full and no air bubbles
Pulled thermostadt and it still overheats at dock

popped hoses off and back flushed exchanger and block and all raw hoses. Nothing noticeable came out
Now at idle it runs up to 180 (still with no thermo) and stays there.

What would you check next?
Does removing thermo make the blocks run warmer than with them in??? Wierd
Do water circulation pumps wear out?

Depending on what I hear back from guys I may get the heat exchangers serviced next but one is fine and seems like they would get crusty at the same rate.

Thanks Folks for inputs!

Swamp

I'm thinking Bravoits, pull the hose and fitting of the transom and see what you find

Yes water recirculating pumps do wear out.

I dont have bravos - forgot to mention - arneson drives and Gill exhausts

Without thermo it should go back to 150-degrees assuming water flow and exchangers are okay - correct?

Agree with Glennamy, the circulating water pump vanes may have come loose from the shaft or broke off. A motor without a thermostat can overheat from getting air pockets. The pockets can form because the thermostat helps keep them out. That said, most would not overheat at idle, and normally temp will be a bit lower then 140.

Does sound like a restriction somewhere.

Put a bucket to catch the water coming out of the exhaust to see how much is coming out. Should fill a 5 gal bucket fairly quick sitting in the water.

Did you suck some trash up in your pickups?

found issue - loose fitting at oil filter due to a broken clamp - was sucking air! Ugh simple stuff when you can actually get to it and see it. Thanks for inputs all.

Aww , fond memories ! lol (Thankfully not the OP's problem though)

Just 2 weeks ago was working with our mechanic replacing burnt out trim sending units and in the process the oil transfer nipple broke due to ageing . It was bound to happen sooner or later so with the boat out of the water it was good timing. In the process we removed the water transfer elbow for better access to the oil nipple/elbow and also discovered Bravoitis , but not yet severe. Also good timing ? Everything is good now and no worries about that problem coming back for a very long time.
btw, ya gotta be a contortionist to do these procedures and that was in an engine room that the mechanic says is a dream compared to most others.