2011 mercury 150 OptiMaxx pro running hot at idle.

[caphab]

Asking for some help! Engine runs hot at low speeds but run cold at high speeds? Thermostats or poppet valve?

[JL]

Always START at the SOURCE. That would be the water pump. Check for torn gaskets and worn impeller first. Your thermostats are bypassed at higher rpms by the poppet valve.

[JL]

Also, need more info. What temps do you have at idle, WOT, etc? What is your water pressure? Do you have a strong telltale stream? The MORE info the better.