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Engine not cooling at speed


DH
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So last night I noticed at speeds higher than 30 or so the exhaust note would change and temp gauge would climb fast. Got the boat back to the hoist and started checking things. Boat is a 2000 Response with Vortec 310 engine.

- impeller is fine

- trans cooler is not blocked

- all hoses feeding the engine are not blocked

- raw water pick up is fine

 

At idle and low speed everything is fine and engine is cool, where do i go from here?

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Impeller is most likely cause, if that is a dead end, next time you run it put your hands on the exhaust manifold risers to make sure they are both about the same temp (or if you have one, check with a pyrometer, there are pretty nice laser ones available cheap). If not, something is blocking the flow other than the tstat or slipping impeller. Also check the housing to make sure all is good there, including any gaskets or o-rings, an impeller is a positive displacement pump so the water will go the easiest route. I have seen sand or even coarse weed debris tear up some of the plastic impeller housings.
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I thought it a little odd that it is hotter at higher speed, as the raw water intake can force water through the system and help cool the temps.

 

I was thinking some kind of blockage reducing water flow, as you need more at higher rpm. Hope it's just the impeller. That isnt too hard to replace I did pull out one once that looked good but was not functioning as noted above.

 

Would like to know what you find with the impeller.

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AB, since an impeller is positive displacement, if ti does not rotate, no water or very little water passes through it. You would have to remove it for the volume needed to cool the engine. The heat rejection rate climbs pretty fast with RPM, so hard to keep up.
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ive seen good impellers but no cooling from it ,the bearings can go in the pump leting the shaft move enough that it twists the impeller so water gets buy it at the top and bottom of housing letting it loose pressure.also check for old chunk of rubber in your lines right past the pump old piece could have moved ,well a shot in the dark

my 2 cents

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Good call @Paul_k !

Having a spare impeller around is always a good move. And, they are always just a few days away from ski-dim.

@D3-Skier - also a good idea is to brush some anti-sieze on the impeller shaft and hub before you slide the new one on. You'll be very thankful the next time you have to tug the sucker out of there again. These can be tough to get out sometimes.

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