Overheating problem

[bass-kitcase]

I just replaced the water pump impeller on a 1974 85hp Evenrude using a complete kit. I've had this motor for over 30 years and have never had an over heating problem. There wasn't much left of the old impeller but the housing looked ok. I cleaned eveything up and installed all new seals and impeller then reinstalled the gearcase to the motor.
When I took it for a test drive I ran it at a low RPM for several minutes. When I kiked up the R's the overheating horn began to sound. I backed off the throttle and within a few seconds the horn stopped. I tried it again and the same thing happened. It cools ok at low but not at high RPM's. Can you tell me what the problem is?

 

[jasper60103]

Re: Overheating problem

Did you notice if water was pumping? It may have sucked up a chunk of impeller.

 

[bass-kitcase]

Re: Overheating problem

The old impeller was badley worn but it was still in tact. Small amounts of water is being discharged form the exhaust relief ports when at idle. It never did discharge a large amount of water even at higher RPMs.

 

[emdsapmgr]

Re: Overheating problem

The water discharge through the exhaust relief ports is a good sign. Indicates the thermostat (vernatherm) has opened. At idle, all water flows from the block through that thermostat. Evidently this is functioning correctly. When on plane, the single pressure relief valve will open, flooding the powerhead with extra water to keep it cool at high rpms. It is possible that the nylon pressure relief valve has become heat-distorted and it may not be fully opening when on plane. If it can't flow this extra water volume, it will overheat when planing. Pull the thermostat housing apart and have a look.

 

[kenmyfam]

Re: Overheating problem

There wasn't much left of the old impeller but the housing looked ok.

Lump of old impeller stuck somewhere in the system restricting the flow ??
How was it before the impeller change ??? Was the impeller change just routine maintenance or is that why the impeller was changed ???

 

[glennaussie]

Re: Overheating problem

> Hi I need some help I have a v 120 Johnson and the alarm goes off at full throtle, I have replaced the themo stats, and the impellers. All I do to stop this is turn the engine off and restart. I runs great and there is plenty of water coming out of the tell tail. I have a built in bow tank and a emergency 5 gallon tank, this problem occurs on both tanks???? I am running out of ideas!!! Can you help me? I think the year of the engine is about 1990.
> Thanks
> Glenn

 

[reeldutch]

Re: Overheating problem

the cylinder heads have water deflectors in them wich overtime will swell up and restrict the waterflow causing engine to overheat.
just a tought
for the 74 85hp

the 120 should open its own post