Catalina 350 International Association

The engine had been overheating at 2300rpm or above for a couple years. The HX was cleaned and tested last year. Just replaced the fresh water pump a couple hours ago, along with 3 of the small hoses around it. Tried to drain as much coolant as possible by opening petcock at bottom of HX too, so I could start with fresh as possible coolant.

Filled with anti-freeze, started engine for a few minutes, refilled with coolant. Started again and it overheated, at 200 degrees, after 5-10 minutes. Water was flowing from the exhaust.

I'm guessing air lock, but I don't know how to purge it. I did open the petcock at the top of the cooling system while it was running, but no liquid came out.

What is the procedure to purge air from the system?

Thanks,

I've had the best luck purging air from the cooling system by removing the "radiator" cap and running the engine in gear at an idle while tied to the dock for 5-10 minutes.
One other thing to consider is that you may be getting a faulty reading from your gauge. Use an infrared thermometer and check a couple of readings on your engine block to verify the 200 degree reading your gauge is showing. You may have a loose/poor ground on the the temperature sender rather than an air lock.

I agree with Ken's suggestion.

I had a bad gauge. High temp reading, sometimes pegging...but no steam coming out of the port.

Had an infra-red gun on board...engine temp 170ish.

Very unnerving problem, but it could be the gauge and not the engine.
Had to replace the temp gauge...not cheap.

Good luck.

Continue to burp it. And, have you opened the air release on the coolant line at the water heater?