Merc 135 hot at idle, cool at WOT

[Cady8special]

Hey guys,

I'm new to this motor- 1999 135HP Blue Series Carbed V6 2.0. When I got it, I replaced Plugs, water pump and thermostats. I realized today that the water coming out of the tell tale is pretty hot at idle- about as hot as a faucet will go. If I rev it to about 1500 RPM's though, it actually cools down. Running WOT isn't a problem either- it's warm, but not hot. It only gets hot at idle. I know it calls for 145 degree thermostats, which is pretty high, but I wanted to see if this seems normal.

Any feedback would put my mind at ease.

 

[wired247]

Sounds normal.

 

[Cady8special]

Thanks for the response. So what is it, this motor is designed to run pretty hot then? I see Merc has two thermostats- the smaller outboards use a 120 degree, and the larger ones use a 143 degree. Flow is reduced by stats until 143 degrees, then they open all the way and poppet valve takes over from there?

 

[wired247]

143 isnt that hot. It is over kitchen sink hot but consider your average automotive engine runs 220 or so. When you dont have much flow through there the water gets hotter. Once you start running faster the pump is pumping more water. It relieves excess water flow with the poppet to keep the pressure down but that extra flow even without the poppet is keeping the temps down. I barely get mine up to 120 degrees at WOT but idling in gear I occasionally open the thermostats and it regulates at 145 ish when the stats open. This whole cooling system isnt set up like a car where all the flow goes through the thermostats. Just a portion goes through the thermostats so there is no bottom floor. Run in real cold water like I do in the winter and it can cause real damage if you arent careful.
I have to plug off some of my inlet ports on my lower unit as at 80-90 MPH I'm pushing 40+ PSI water pressure if I don't. It real hard to keep the temp up in that case.