1983 Merc 470, Overheat on plane.

MKT210SF

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So I have had this boat 2 years now, got it for very cheap.

Has the 4 in heat ex-changer, updated manifold/riser.

Now she idles and runs slow great, the weird thing is the temp never goes above 130* at slow speeds.

The issue I am running into is a slow over heat while on a plane, it takes about 5-7 mins to get to 190-195 and thats where I back off and let her cool down.

Took the riser off to inspect looks just fine no chips coming off etc.

Manifold looks like its brand spanking new.

Drive impeller done last year.


Now I heated up my thermostat in water just today, It seems as if its not opening till about 190* and now fully opening till the water almost boils and its rated for 160*.

could this be causing my issue? Why wouldn't it over heat at idle?


Thanks in advance guys!
 

stonyloam

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Re: 1983 Merc 470, Overheat on plane.

Change the impeller, new OEM pump housing and impeller, DO IT NOW!!! Do not run the boat again till,you do it or you'll be sorrrrrry!!!!
Oh! Welcome to the 470 Club :facepalm::D
 
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MKT210SF

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Re: 1983 Merc 470, Overheat on plane.

That was done, last year right before the season. Stated above less than 40 hrs on it.
 

stonyloam

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Re: 1983 Merc 470, Overheat on plane.

Take the rear hose off the heat exchanger (that goes up to the exhaust elbow) stick a garden hose in it and turn it on, see if the water flows freely out of the exhaust with little backpressure. If it does, then chances are it is the impeller, even though it is relatively new. You could pop the front cover off of the exchanger to make sure there is nothing blocking the tubes. If the impeller pump housing was not changed, that could be it. If it is not loosing coolant and the exchanger tubes are not blocked and the elbow is not plugged, then the only thing left is the impeller and housing. There are lots of things that can cause an impeller to fail, age is only one of them. Good luck.
 

mr 88

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Re: 1983 Merc 470, Overheat on plane.

You have a closed sysem so there is anti freeze in the block that is being semi circulated by the water pump,so at a idle the anti freeze is keeping it from overheating.High speeds with a load is a different story.Get another thermostat and compare them in the boiling water to see if you got a junk one. Follow stonys advice as well.
 
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dubs283

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Re: 1983 Merc 470, Overheat on plane.

if the housing AND base assy and gasket weren't replaced with the impeller that is most likely the issue

at higher rpms a faulty gasket can allow exhaust gasses to enter the pump assy and cause aeration and overheat
 

stonyloam

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Re: 1983 Merc 470, Overheat on plane.

Yeah, guess my reading comprehension is not what it used to be:facepalm: of course the thermostat is a problem, should open at 158 and be fully open at 170. Thanks for the kick in the head mr88;) New (OEM is best) one is in order. Could be just enough coolant circulation to keep it from overheating at low speed, but restricted enough at high speed to overheat.
 
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Bt Doctur

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Re: 1983 Merc 470, Overheat on plane.

My guess is the WP base gasket is sucking in hot exaust on plane.
 

MKT210SF

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Re: 1983 Merc 470, Overheat on plane.

No exhaust bellows so there is no gas going down there, I think its the thermo. It should be open by 170 fully wouldn't open fully till the water boiled.
 

Bt Doctur

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Re: 1983 Merc 470, Overheat on plane.

Really , you had better look at the design of marine exaust . unless you are running thru hull exaust that hole in the drive is on the low pressure side when the prop is moving thru the water. Considering that "center section" has no water in it on plane hot exaust gas and water discharge fills the void. Kinda like blowing thru a straw The pump base is trying to suck in water ,and a leak will cause it to suck air or in this case hot exaust gas .
Tell you what, drop the lower unit, put muffs on the lower unit, turn on the water and tell us where the water is leaking from.
 
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MKT210SF

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Re: 1983 Merc 470, Overheat on plane.

Path of least resistance, that much pressure is not going to go down there, Just spoke with a buddy who is a marine mechanic, its the thermostat.
 
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MKT210SF

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Re: 1983 Merc 470, Overheat on plane.

Also any reason it would stay at 130 when going slow/neutral?
 

Bt Doctur

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Re: 1983 Merc 470, Overheat on plane.

Because the water pump is underwater at that speed. As you get to plane, the drive raises up to about the cavitation plate, the water pump is sitting above that so its out of the water. It is now subjected to hot water and hot exaust gas.
 

MKT210SF

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Re: 1983 Merc 470, Overheat on plane.

Hmm, Last season this was an occasional issue, that would only happen from time to time, I also did replace all the gaskets in that housing.
I would rather not have to short haul the boat for my first go at this issue really...
 

Bt Doctur

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Re: 1983 Merc 470, Overheat on plane.

But did you replace the wp base gasket? When you do work on a wp base don't make this mistake.
gasket2520backwards2520small.jpg
 
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Bt Doctur

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Re: 1983 Merc 470, Overheat on plane.

The red arrows show the low pressure side of the pump base, the suction side
UntitledW_zpsc8488a47.jpg
 

dubs283

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Re: 1983 Merc 470, Overheat on plane.

every alpha one that overheats on plane i have run into has nothing to do with a thermostat and everything to do with the gasket bt doc refers to
 
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