96' Merc 90hp

[fish8flies]

Hello...New guy here. I've replaced the impeller on my 96' Merc 90hp. Sometimes the motor "pees" like a race horse. Sometimes it doesn't "pee" at all. There are no warning buzzer indications for overheating and the motor runs great. I double checked to ensure I installed the impeller correctly. I was told by a friend there is an internal ((motor)) thermostat and if the temp doesn't reach the required temp the thermostat won't open and the motor won't "pee". When connected to water hose ears, I can see water ejected through the exhaust ports in the prop so I know the impeller is working. If no thermostat, could there be a piece of trash preventing the motor from "peeing" properly? Thanks!