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Overheating Motor.


IWANNABROCK

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Hi,I have a 30hp johson 84 model. at low revs it runs fine but when you have throttle open around half or so for a few minutes it stops pumping water,slow down it pumps again and no worries.I changed impeller,that still didnt fix it,I changed head gasket,fitted a thermostat as it didnt have one,took for a run today,same thing. :ranting2: any ideas as what I should do next?

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Did the new impeller go in the right way round?

At low speed the impeller works by displacement..ie water is pushed around the system. At high speed the impeller works more like a pump with the impeller blades more flinging the water thru. If you put the blades backwards it may not work as it should and behave more like a worn out one.

Just a thought....

C.

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Guest Aussie007

u said it didnt have a thermostat that raises questions have u tryed taking the thermostat back out to see if it'll pump water at idle and running speed?

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u said it didnt have a thermostat that raises questions have u tryed taking the thermostat back out to see if it'll pump water at idle and running speed?

It had same issue,that is why I checked thermostat etc. any more ideas? pulled head back off,there is no build up at all in there or block,dont know what next.

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Guest Aussie007

take it into huey see if he can pressure test the cooling system? could be a blockage could be the impeller when running the engine at a higher RPM when it stops pumping water is it over heating?

Edited by gazza
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More questions than answers from me. They might sound dumb or simple as I don't know what you have checked etc etc so ignore them if you have done em all.

Are you sure it is overheating. Guages lie , back of hand or steam doesn't doesn't. Check temp sender and alarm sensors. Prove you have a problem. If you have the manual and a water pressure gauge you could check pressure and pay determin if it is a restriction or not.

Lean or seizing motors or ones with fresh rings can run hotter than normal especially if just honed not bored. Check the colour of carbon on your plugs .

Does it do the same on the muffs as it does in the water? If it doesn't do it on muffs at 2500 revs ( use a muffler plug or the neighbours will kill ya) . Could mean that the motor is mounted too high on the transom ( right shaft length) and therefore not enough water getting to pump while on the plane. Look near motor for anything that trails in the water at the transom that could either be airating the water or diverting the laminer flow from hull to gearbox. tranducers , berly pots etc.

Have seen rubber seals (gromet) cracked where the pipe runs into the top of the water pump. Same at the entry into the block of that tube.

Have seen cracked water pump housings from being over tightened that leak only under the higher pressure at higher revs. Same with gaskets or o-rngs left out sucking air.

Have seen impellers slip on shafts if key is not in.

Look at the full schematic diagram for the water pump and make sure all the parts are there and base plate is not warped atc.

Have seen a baby pippi in the inlet grille or in the pipe below the water pump act like the perfect flap valve and allow low pressure but at higher pressure or vacume it sealed the inlet tube near perfectly. Same for shell above the waterpump in tube to teh block.

Water wont go in if water can't get out. Have seen all sorts including cotton wrags, old gaskets , plastic , razor blades left in water galleries after rebuilds blocking exit from block into the exhaust housing.

Have seen new gaskets after rebuilds have the wrong or no holes that don't line up water galleries but look perfect from the outside as it is a different head etc etc.

Have seen thermostats in backwards and thermostats that don't bypass at higher throttle settings or sererate bypass valves that stick.

Have seen screens on gearbox put on backwards so they try and suck water out of pump not inject it in.

Have seen huge carbon build up in exhausts cause motors to run hot due to exhaust restriction and it can also block water outflow.

Have seen gearboxes repainted several times and the paint build up shrinking water inlet holes or total painting over of plastic grilles or masking tape still on.

Edited by pelican
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It is overheating as it steams up and you can see that water stops coming out of tell tale. when it first did it I changed impeller,then I took for a run still did it,so I changed head gasket,fitted a thermostat as it didnt have one,still same issues. it is fine on the muffs,at slow speeds it is fine,as soon as you start to get around half throttle or so, you can see the water stops flowing out of tell tale.then back off it starts pumping again and motor cools down again.I pulled head back off,there is no corrosion in head or block.

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It is overheating as it steams up and you can see that water stops coming out of tell tale. when it first did it I changed impeller,then I took for a run still did it,so I changed head gasket,fitted a thermostat as it didnt have one,still same issues. it is fine on the muffs,at slow speeds it is fine,as soon as you start to get around half throttle or so, you can see the water stops flowing out of tell tale.then back off it starts pumping again and motor cools down again.I pulled head back off,there is no corrosion in head or block.

Was it a brand new impeller? Did you use the original woodruff key or the plastic wedges supplied with a new impeller? Sure its the right one?

C.

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Was it a brand new impeller? Did you use the original woodruff key or the plastic wedges supplied with a new impeller? Sure its the right one?

C.

Yes was a brand new impeller,used the plastic key which came with it,the one from old one was plastic as well. it certainly seems like right one.

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Weird eh...really sounds like the impeller is slipping at high rpm and giving similar syptoms to a worn one. Gotta say though I'm tending towards what Pelican says........ someting is being sucked up and blocking the pump intake. Looks like a strip down again.

C.

Edited by chrisg
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Hi Jewy, have a look and make sure there is no way the water pump can suck in exhaust gas at higher speed. If I was you I would remove the gearbox again and pay close attention to the gaskets and o-rings in the pump. Best thing might be to fit a complete water pump kit and that way everything is new. Also with the gearbox off you can put a hose on the water tube and make sure there is no blockages in the powerhead. Your engine wants a thermostat to allow it to reach correct temp at low speed and run as designed.

Cheers,

Huey.

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