1977 Johnson 115hp overheating issue

[Highlife]

Need help here, had the impeller replaced this spring and now the engine overheats when in the water within 5 minutes or so, however I can run it on muffs for 20 minutes or longer and no issues. Took it back to the guy and he replaced the Vernatherm t-stat (don't know if he used an old one or new one) and still overheats when in the lake but never does when on muffs. Took it back to the guy and we pulled the lower unit so he could show me that the impeller and housing is all new and going the right direction. What could be causing the overheat issue? I know that the overheat temp sensor is working because when the alarm does go off the water coming out of the exhaust relief ports is steaming. Any thoughts?

 

[crb478]

Blocked passage in the heads or water lines between veritherm and the rest of the system. Try blowing all the passages out with water pressure and air.

 

[Highlife]

Can i remove both hoses that go to the thermo housing cover and blow backwards? or go up thru the exhaust relief holes?

 

[emdsapmgr]

The engine will idle normally all day when the vernatherm opens. Your description of the water out of the exhaust relief ports indicates the stat is opening. (it may idle all day fine) When the boat planes out, there is a nylon pressure-relief valve in the thermostat housing that also has to open for extra cooling water flow. If that does not open properly it will overheat when on plane. You may want to open up the stat housing again and check to see if that nylon valve is heat distorted (not functioning as designed.)

 

[F_R]

Screens in place in the water intake? Did he get both water tubes properly inserted in the pump grommets? Good seal on the pump housing? I suspect the trouble is air is getting into the pump inlet side or coming down the return pipe to the pump. Something is keeping that pump from working (it doesn't have to work with the muffs on---the water hose supplies the cooling water under that condition, not the pump)

 

[Highlife]

Thanks guys.. i will pull the t-stat housing and check the nylon valve. He did also install a telltale in the outer exhaust cover like on a 1978 motor and this shows that water is being circulated but the motor just doesn't seem to be cooling properly. I did notice when we took the lower unit off that the gasket/ seal for the exhaust is torn on the lower unit, could the exhaust heat be causing the water pump housing to warp a little thus losing the seal? The part that makes me mad is the motor ran just fine before i decided to replace the impeller this spring? Tried to do preventive maintenance and now end up with issues.

 

[Highlife]

Removed the t-stat housing and it appears to have an old vernatherm still in it, not a new one like he said he put in. Also checked the nylon pressure-relief valve like emdsapmrg said to and it appears to be a normal color and does not appear to be discolored or distorted in any way, might order a new one also and spring along with a new vernatherm.

Is there anything else i should get?

Also blew air thru all the passages and didn't see or feel any blockages had good air flow thru them.

What else i'm i missing?

 

[emdsapmgr]

Put a new vernatherm in and see what happens. If you still have overheating problems, you might want to pull the covers off the heads and have a look at the internal cooling passages. What you see in there will also be inside the water jackets around each cyl. The torn rubber exhaust seal at the lower unit can have an adverse affect on the plastic water pump housing. It can leak hot exhaust toward the impeller housing. If the housing gets hot enough, for a long enough period of time, it can deform and cause cooling problems. You will want to replace that rubber exhaust tube seal.

 

[Highlife]

I will get the gasket also and will let you know what happens once they are all replaced. Thanks for all your help guys.