NS 9.8B 2S: water, not exhaust from idle exhaust port, poor idle

I have a Nissan 9.8B 2 stroke that now has water shooting out of the idle exhaust port in the back, instead of exhaust and a little bit of water. A full stream of water is also coming out of the cooling water check port on the side. Problem is that now the motor does not idle well. I am thinking the water pump is OK but am concerned about perhaps the thermostat is stuck open of I have some other water leak. Does anyone know what would cause water to start blasting out of the idle exhaust port instead of exhaust?

Odds are you need to get the motor running better. No, the thermostat is not to blame -- the same water exits regardless of whether the stat is opened or closed. At low RPMs, the hydraulic back-pressure in the leg causes the idle relief port to have greater flow of both exhaust and cooling water. At higher speeds, almost everything exits at the prop. You could also have a restriction in the leg or prop housing (think zebra mussels in fresh water), which could cause both poor idle and extra water at the idle port.

Just got an email back from Tohatsu and the smoke maybe caused by lower outside temp and higher air moisture level. My 9.8 would smoke on those damp days. We just had a warm snap here in MD and I ran the motor. No smoke and the temp out of the upper exhaust was only 98 degrees under load for 15 minutes. Water temp at pee hole was no higher. I am going to change the thermostat and do an impeller anyway on 9 year old engine.

You do realize that you are replying to a 3 year old post, right?
Yes, damp cold weather does create more steam.
If you have a 9 year old Tohatsu 9.8 in the US, it is a completely different model from the original poster's motor. His was a 2-stroke; yours is likely an MFS9.8A3 4-stroke.
Never change just an impeller; do the complete wp kit. Same labor, pumps better and lasts longer. In cold fresh water, with a 6-month Great Lakes season, we do that about every 3 years. If in silty water, it needs to be done more often. All outboard manufacturers recommend annual wp maintenance.