1978 85hp Evinrude overheats one side on hose

MadtownChris

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Sep 16, 2013
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Hey folks,

I have a 1978 Evinrude v4 85hp motor and after storing my boat indoors for the icy winter, I changed plugs and restarted it today on the muffs. I let it warm up a while. It was working fine before I stored it.

In the past I thought one side of the cylinders seemed hotter than the other but not a whole lot so I kept touching both sides to double-check. Today one side got a lot hotter so much so that after about 5 minutes of running, the overheat alarm sounded. To the touch, the "cool" side got quite warm. The hot side was too hot to touch more than a second or two. It was HOT.

I have no idea when the impeller was last changed but the pee stream is strong.

Other related posts suggest changing the thermostat but the diagram on the Evinrude site makes it look like there's only one thermostat/valve so I'm confused why that would affect one side and not the other. Is that a simplified diagram and really there are two thermostats and valves?

Also, how in heck do you remove the thermostat housing? It sits sort of down in the bottom cowling bit. And is that spring inside the thermostat housing going to shoot all the parts into the bushes and be impossible to get back together?

Any other suggestions? Should I try it in the lake instead of hose?

Fortunately it looks too cold for boating for a while...

Thanks!

- Chris
 

emdsapmgr

Supreme Mariner
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Dec 9, 2005
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11,551
Re: 1978 85hp Evinrude overheats one side on hose

The 78 was the first year for the new dual thermostats. One for each head. If one sticks shut, it will overheat that side, even though you may have a strong telltale. When you get into the stat housing, replace both stats, and check the nylon pressure relief valves to be sure they are not heat-warped. That should fix the problem. There are springs inside, but since you have a flatback exhaust cover, should be little problem to disassemble and reassemble. That plastic valve body is sandwiched inside that housing. There are two tiny pinholes in it, one for each stat. Be sure they are both open, or it will overheat after reassembly....
 

Bosunsmate

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Apr 7, 2012
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Re: 1978 85hp Evinrude overheats one side on hose

I drilled those little pinpricks a little larger on a motor i worked on, figured i was making it a bit less easier for them to block up. As with everything there will be a trade off involved which would be that more water would pump through with thermostat closed. but i didnt think that would be too much of a negative.
Maybe there is more negative tradeoff i havent thought of

I drilled two holes through the back of the cowling to access the bolts and i just plug them after with a rubber plug, looks tidy and its very convienent.
Nothing should pop off too far. Just make sure you install thermostats facing the right way.
For some reason in my time it seems to be the starboard bank that heats up the most.
If the thermostat tests ok you might want to look at the water deflectors if your model has them.

Also you mention trying in the lake, thats a good idea after trying thermostats, you need good muffs on most motors to get a good pressure to the pump
 
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MadtownChris

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Re: 1978 85hp Evinrude overheats one side on hose

For some reason in my time it seems to be the starboard bank that heats up the most.

Interesting since that's the side that seemed slightly warmer last year and now is overheating.

I managed to get the two side bolts unscrewed without putting a hole in the cowling but there's no room to pull them out completely. I'm guessing they're out enough to remove the thermostat cover, do you think? I'd rather not make holes in the cowling...

Also the middle bolt is frozen :grumpy: I'm putting liquid wrench on it and have tapped it a bit. No luck. I'm going to try heat next but wondering if that might melt the relief valves though I'd guess I'd just replace those as well. Any other bolt removal suggestions?

- Chris
 

wayneo99

Petty Officer 2nd Class
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Oct 14, 2005
Messages
151
Re: 1978 85hp Evinrude overheats one side on hose

you can also take the hose off of the bottom of the head on each side and see what kind of water output your getting from the pump.
 

Bosunsmate

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Re: 1978 85hp Evinrude overheats one side on hose

Interesting since that's the side that seemed slightly warmer last year and now is overheating.

I managed to get the two side bolts unscrewed without putting a hole in the cowling but there's no room to pull them out completely. I'm guessing they're out enough to remove the thermostat cover, do you think? I'd rather not make holes in the cowling...

Also the middle bolt is frozen :grumpy: I'm putting liquid wrench on it and have tapped it a bit. No luck. I'm going to try heat next but wondering if that might melt the relief valves though I'd guess I'd just replace those as well. Any other bolt removal suggestions?

- Chris

Yep if you have backed them out once you get that middle one out you should be able to lift it out. Removing the hoses helps wiggle it loose.
Heat might work with some tapping. Ive had to drill that one out before.
The gaskets should be replaced there too.
You might be able to undo the bolt/screws holding up the lower cowling and drop it that way to get more access.
You need to remove the other lower cover also too to do that
Those screws are often seized in though
 
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MadtownChris

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Sep 16, 2013
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Re: 1978 85hp Evinrude overheats one side on hose

Hey guys,

As suggested, I detached the hoses and ran the motor in my new "bucket rig" (see picture like it? :)) just to ensure enough water was getting in.

I got a good stream of mildly warm water coming from both heads. Running in this configuration, both heads remained the same temperature (almost cold) and could have run forever.

I reattached hoses to the thermostat housing and same result as before - starboard hot quickly.

Seems like this confirms stuck starboard thermostat, right?

Thanks so much for the advice!

- Chris
IMG_20140503_135809548.jpg
 

boobie

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Nov 5, 2009
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20,826
Re: 1978 85hp Evinrude overheats one side on hose

You got it, pull the thermo's.
 
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