merc i/o overheats

[Rhino]

hi, My 140 hp 1972 merc 4 cyl overheats at low rpm. I've replaced impeller in outdrive,h2o supply hose from outdrive to engine,water pump,t-stat,riser. What next? temp is ok at 2000 or so but when at idle it redlines in a couple minutes.When out of the water with a hose on the outdrive and engine at idle, there is alot of steam coming out of the exhaust. Do I have a head gasket problem? Thanks, Rhino

 

[Trent]

Re: merc i/o overheats

Did you change the waterpump housing also? <br /><br />Also rusted manafolds and risers can cause restrictions in the water flow. But I would check the housing if you did not replace it. Also you can put a clear hose from the transom plate to the thermostat housing and look for bubbles. Run it at cruise speed and if you have bubbles your problem is still in the outdrive.

 

[Rhino]

Re: merc i/o overheats

Yes I did replace the waterpump housing in the outdrive. I'll try the clear hose and see what happens.

 

[Rhino]

Re: merc i/o overheats

Finally got time to do the hose trick. Had zero water flow from outdrive to engine. Found the impeller was totally chewed up. Could this have happened from putting it in with the fins in the backward direction last time I replaced the waterpump assmbly? I've replaced the impeller and have good water flow for now. What else can cause the impeller to get broken up?