Over heating in idle position.

[Retiredsgt]

Have 2005 mercury optimax 150. Had it in shop four times and owner of ranger shop can't figure it out. When idling temp goes up and alarm goes off. But when you get the boat on plane, temp drops to normal. He replaced impeller and still does same. I've flushed engine to see if it was clogged or something. Just wondering from anyone for some tech support. Local ranger shop here mechanics have quit so owner is trying to do work himself.

 

[galamb]

Replaced the thermostat(s)?

At higher rpms the thermostats are pretty much bypassed in favour of the poppet valve which could account for the normal temp when running at higher rpms.

If the stats are stuck closed it wouldn't allow enough water to pass to keep things cool when they are the only thing controlling the water flow.

 

[Docksidemarineservices]

Poppit valve.

 

[Retiredsgt]

Thanks for all your help going to order thermostats and poppet value and replace. Thx.