Had the lower unit housing replaced. Now there is exhaust coming out of the hole the water pressure tube goes into the lower unit, and pushing water and exhaust out the top of the neck where the water pump shaft comes out. The dealer mechanic replaced the powerhead gasket to fix the problem and that didn't do it. The problem didn't exist until the lower unit housing was replaced. Could there be something missing letting the exhaust into the water side of the neck.
There is a "damper, seal" in the lower unĂ*t near the water pump. I think you have forgot it from the new lower unit. If you leave it away, then exhaust and water comes up from the lower unit because it goes the easiest way.....You can look at yamahas part no. 688-45375-00, there is a guide to hold it in place.
1997 Yamaha ProV 200 exhaust leak
You can look at yamahas part no. 688-45375-00, there is a guide to hold it in place. 
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