Overheating at idle

[Big Dog]

I have a 1985 SeaRay with a 350 ci 260hp Mercruiser. I have a new water pump in lower unit and a new thermastat. Running up and down the river no problem. Stop and let the boat idle for 10 or so minutes and the temp starts to rise. Up to 3/4 on the gauge (no #'s on the gage). Speed the boat up and the temp comes down.

 

[Blue Thunder]

Re: Overheating at idle

I would check the engine water circulation pump. If the impeller is eroding you would need to have the rpm up to get reasonable flow.<br /><br />BT

 

[johnkc]

Re: Overheating at idle

remove the 2 hoses that are connected to the "T" connection on top of the thermostat hsg.you will see a long bolt with a plastic balls on each open end of the "T" fitting ,these balls can stick to the bolt in the open position and let too much water bypass causing a overheat at idle .you mention river which can have a lot of sand at times and the sand can stick these balls on the bolt .good luck ,john.

 

[Big Dog]

Re: Overheating at idle

John can this also cause one exhaust manifold to get hotter than the other. I forgot to mention that, but that is one of the other things that I have noticed.<br /><br />Big Dog

 

[johnkc]

Re: Overheating at idle

big dog ,is the manifold hotter than the other at idle or when running higher rpm's ,and is it the manifold and riser getting hot,or just the riser?if one of the balls are stuck shut and you are running higher rpm's, that could make one riser hotter than the other.

 

[Big Dog]

Re: Overheating at idle

One riser and manifold is always hotter than the other.(starboard side) Wether I am at an idle or have just come off plane it always seems to run hotter than the other. I am going to use an infrered thermometor on it this weekend to figure out how much hotter it is getting.

 

[Boatist]

Re: Overheating at idle

Big Dog<br />I do not know Merc very well but a hot manifold and riser usually means that side stopped up with rust and scale not allowing enough water flow. This would normally cause motor to over heat at high RPM and then cool down at adle. The info you got about circulation pump was very good. I will give you one other idea. The impeller may be bad, OMC at least the impeller functions two ways. At idle it pumps water as a displacement pump but at high RPMs it pumps water as a cintrifical pump. What can happen is Rubber impeller gets old and the rubber vanes do not spring back out against the outer walls of the pump so at low rpms water just flows around the vanes but still works ok at high rpms. Two thing cause this either someone ran motor with no water and vanes melted off from heat or the vanes have just been bent so long that they no longer straighted up.<br />If you have a water pressure guage should see the drop in pressure or a pee hole should also be able to see. If have niether then must pull hose coming from outdrive to engine and put in a bucket and start up with outdrive in a tank or in the water. Should get a good flow of water. Can not do this on muff as hose pressure may make it look like it is ok when it is not.<br />Good Luck