1999 Yamaha 90 - 2 stroke 3cyl

[bronco74]

While recently running the outboard on the hose hook up, I noticed that the water pump stream coming out the side of the motor was flowing, but not very strong. After idling for 10 minutes giving it gas every so often, I also noticed that the two back exhaust/water ports never released water. The stream of water began to warm up and the lower unit started to get mildly hot. At this point I killed the motor. I purchased a new impeller kit from yamaha, and still have not installed. I am wondering if the problem could be a broken thermostat, preventing the outflow from the two back ports. The local yamaha dealer said that the impeller probably needed changing, but never mentioned the possibility of a broken thermostat, which would circulate water through the engine block. Could this be a possibility? Thanks in advance.